Cerebral Osmolytes and Plasma Osmolality in Pregnancy and Preeclampsia: A Proton Magnetic Resonance Spectroscopy Study

Cerebral Osmolytes and Plasma Osmolality in Pregnancy and Preeclampsia: A Proton Magnetic... Abstract BACKGROUND Cerebral complications contribute substantially to mortality in preeclampsia. Pregnancy calls for extensive maternal adaptations, some associated with increased propensity for seizures, but the pathophysiology behind the eclamptic seizures is not fully understood. Plasma osmolality and sodium levels are lowered in pregnancy. This could result in extrusion of cerebral organic osmolytes, including the excitatory neurotransmitter glutamate, but this remains to be determined. The hypothesis of this study was that cerebral levels of organic osmolytes are decreased during pregnancy, and that this decrease is even more pronounced in women with preeclampsia. METHODS We used proton magnetic resonance spectroscopy to compare levels of cerebral organic osmolytes, in women with preeclampsia (n = 30), normal pregnancy (n = 32), and nonpregnant controls (n = 16). Cerebral levels of organic osmolytes were further correlated to plasma osmolality and plasma levels of glutamate and sodium. RESULTS Compared to nonpregnant women, women with normal pregnancy and preeclampsia had lower levels of the cerebral osmolytes, myo-inositol, choline and creatine (P = 0.001 or less), and all these metabolites correlated with each other (P < 0.05). Women with normal pregnancies and preeclampsia had similar levels of osmolytes, except for glutamate, which was significantly lower in preeclampsia. Cerebral and plasma glutamate levels were negatively correlated with each other (P < 0.008), and myo-inositol, choline and creatine levels were all positively correlated with both plasma osmolality and sodium levels (P < 0.05). CONCLUSIONS Our results indicate that pregnancy is associated with extrusion of cerebral organic osmolytes. This includes the excitatory neurotransmitter glutamate, which may be involved in the pathophysiology of seizures in preeclampsia. blood pressure, cerebral osmolytes, eclampsia, glutamate, hypertension, preeclampsia, proton magnetic resonance spectroscopy Cerebral complications in preeclampsia/eclampsia contribute substantially to maternal deaths worldwide.1 Although seizures during pregnancy are described in the literature since Hippocrates and the most effective treatment, magnesium sulfate (MgSO4), has been in practice for almost a century, our knowledge of how and when eclampsia arises is surprisingly scarce. Eclampsia and severe preeclampsia are described as a form of posterior reversible encephalopathy syndrome.2 The predominant theory of the etiology includes breakdown of cerebral autoregulation due to a rapid rise in blood pressure (BP), with ensuing damage to the blood–brain barrier and vasogenic edema as a result.3 However, the fact that a large proportion of women with eclampsia have moderately elevated, or even normal BP, prior to seizures, questions this theory.4 Pregnancy causes fundamental changes in hemodynamics and body fluid hemostasis,5,6 including reductions in plasma osmolality and plasma sodium levels. These changes appear directly after conception, reaches a steady state at gestation week 10, and are achieved by pregnancy-induced changes in the osmotic thresholds for thirst and arginine and vasopressin secretion.7,8 A growing body of evidence shows that the increase in vasopressin is higher in pregnancies that subsequently develops preeclampsia and it has been proposed as a possible predictive biomarker.9,10 When osmolality falls, there is a risk of edema when osmotic forces cause water to enter the cells. This is prevented by mechanisms called regulatory volume decrease, intricate systems inherent in all animal species. The ability to preserve a stable volume despite changes in osmolality of the surrounding environment, is especially important for brain cells with the potentially life-threatening effects of cerebral edema.11 Studies using magnetic resonance spectroscopy (MRS) have shown that neurons in a chronic state of hypoosmolality protect themselves from swelling by efflux of organic osmolytes (amino acids, polyalcohols, sugars, and methylamines), including glutamate, the most important neurotransmitter.12 Plasma hypoosmolality is also known to increase susceptibility to seizures.13 No prior study has used MRS to investigate levels of cerebral organic osmolytes in women with preeclampsia and normal pregnancy. To our knowledge, there are only 2 publications with brain proton MRS in preeclampsia or eclampsia.14,15 These studies are limited by small sample sizes, and the latter focused on posteclampsia status, rather than the changes leading up to eclampsia. Using proton MRS (1H-MRS) where the spectrum obtained contains most organic osmolytes of interest, such as N-acetyl-aspartate (NAA), glutamate, myo-inositol, choline, creatine, and taurine, our aim was to compare levels of cerebral organic osmolytes between women with preeclampsia, normal pregnancy, and nonpregnant women and assess if organic osmolytes correlate to plasma osmolality and plasma sodium levels. Our hypotheses were that levels of cerebral organic osmolytes are decreased during pregnancy, that this decrease is more pronounced in preeclampsia and that levels of cerebral organic osmolytes correlate to plasma osmolality and plasma sodium levels. MATERIAL AND METHODS Study design and population The study participants in this cross-sectional study were recruited in Uppsala, Sweden during 2013–2016. Uppsala University Hospital is a tertiary referral centre with approximately 4,000 deliveries per year. The study population is in part described previously.16 Only women with singleton pregnancies and a gestational age between 22 + 0 and 41 + 6 according to an early second-trimester ultrasound dating were eligible. General exclusion criteria in the present study were chronic hypertension, diabetes mellitus, pre-existing renal disease, or standard contraindications for magnetic resonance imaging, e.g., claustrophobia metallic implants or pacemaker. We recruited 3 groups of women: women with preeclampsia (n = 30), normal pregnant women (n = 32), and nonpregnant women (n = 16). Women with preeclampsia were recruited from the obstetric ward or outpatient obstetric clinic. Preeclampsia was defined as de novo hypertension after 20 weeks of gestation in combination with proteinuria, according to the recommendations of the International Society for studies on Hypertension in Pregnancy (ISSHP).17 Hypertension was defined as systolic BP of ≥140 mm Hg and/or diastolic BP of ≥90 mm Hg measured on 2 subsequent occasions, at least 6 hours apart, and proteinuria defined as ≥2+ on a dipstick or ≥300 mg/24 hours in a urine collection. Preeclampsia was defined as severe when systolic BP was ≥160 mm Hg and/or diastolic BP ≥110 mm Hg; or if HELLP-syndrome was present.18 Women with preeclampsia had to be sufficiently clinically stable to be transported to the MR facility. None of the women was treated with MgSO4 or had developed eclampsia, neither before inclusion nor after. Every woman diagnosed with preeclampsia, either admitted or monitored in outpatient care, which came to the principal investigator’s (M.N.) knowledge, was approached regarding study participation. Only a small fraction of respondents refrained participation in the study, most often due to claustrophobia. The normal pregnant group was recruited through information posters at antenatal outpatient clinics in Uppsala and at Uppsala University. Women in the normal pregnant group were matched for gestational age at examination to women in the preeclampsia group. After examination, women in the normal pregnant group were monitored and those who developed preeclampsia, delivered preterm (<37 weeks) or did not deliver an infant of normal birth weight (±2 SDs of the mean birth weight for gestational age and sex19) were excluded. The nonpregnant group was recruited through Facebook and local networks and included both nulliparous and parous women. In addition to the general exclusion criteria, women with prior history of preeclampsia or gestational hypertension were not included either in the normal pregnant or nonpregnant study groups. All participants underwent BP measurement, blood and urine sampling, and MRS examination within a 4-hour period. Systolic and diastolic BPs were measured in supine position in the right arm after a 15-minute rest. A manual sphygmomanometer (Boso, Germany or Welch Allyn) with appropriate cuff size depending on arm circumference was used. Fresh midstream urine samples were collected and analyzed with a Combur 9 test (Roche) or Clinitek Status Analyzer (Siemens). Blood samples were centrifuged 10 minutes at 1,500 g and then immediately stored as aliquots in −70 °C until analysis. Magnetic resonance spectroscopy MR examinations were performed using a 1.5 T scanner (Achieva, Philips Healthcare, Best, The Netherlands). The whole body coil was used for excitation, and an 8-element receiver head coil served for signal reception. T1- and T2-weighted turbo spin echo images were obtained in the axial, sagittal, and coronal orientations (spatial resolution 1 × 1 × 4 mm3, interslice gap 0 mm). Single-voxel spectra were acquired using the point-resolved spectroscopy (PRESS) sequence (spectral bandwidth 1,000 Hz, 1,024 points, TR/TE 5,000/30 ms, 16 phase cycle steps). Volume of interest (voxel) with a size of 20 × 20 × 20 mm3 was positioned in the posterior midline, at the junction between the parietal and occipital lobes (Figure 1). The position was chosen to include areas mostly affected when vasogenic edema is found in eclampsia and severe preeclampsia. Magnetic field homogeneity was improved by iterative first-order shimming. Sixteen non–water-suppressed and 128 water-suppressed scans were performed in consecutive acquisition in 12 minutes. Intensities of spectral lines were estimated by fitting using the LCModel.20 Levels of the following 1H-metabolites were measured: NAA, choline, creatine, glutamate, myo-inositol, and taurine. Quantitation error of each metabolite concentration estimate was assessed from the SD (Cramér-Rao Lower Bound) expressed in percent of the estimated concentration.20 Cramér-Rao Lower Bound below 20% is commonly used as a criterion for estimates of acceptable reliability. Figure 1. View largeDownload slide T2- (left) and T1-weighted (right) turbo spin echo images with the typical voxel position in the posterior midline at the parieto-occipital fissure. Figure 1. View largeDownload slide T2- (left) and T1-weighted (right) turbo spin echo images with the typical voxel position in the posterior midline at the parieto-occipital fissure. Plasma samples Plasma analyzes were routine tests and performed at the accredited laboratory in the Department of Clinical Chemistry and Pharmacology, Uppsala University Hospital. Plasma osmolality was assessed using a Fiske Osmometer, model 210. Plasma sodium was analyzed on an Architect c16000 with a reagent from Abbott Laboratories. Plasma glutamate was analyzed with a glutamate assay kit from Sigma-Aldrich (MAK004). Statistics Clinical characteristics, median levels of cerebral 1H-metabolites, plasma osmolality and plasma levels of glutamate and sodium were compared between the women with preeclampsia, women with normal pregnancies and nonpregnant women by Kruskal–Wallis test. Pair-wise comparisons between the study groups were made with Mann–Whitney U-test. Adjustment for maternal age and body mass index (BMI) was made with analysis of covariance after testing for adequate normal distribution for the variables. Correlations between the different cerebral osmolytes and between cerebral osmolytes and plasma variables were assessed with Spearman’s correlation test. All significance tests were 2-tailed. P <0.05 was considered to denote a statistically significant difference. Analyses were performed using IBM SPSS Statistics 24 (IBM SPSS, Chicago, IL). Ethics The study was approved by the Regional Ethical Review Board in Uppsala, Sweden, and informed consent was obtained from each woman participating in the study. Reference number: 2012–087, 23 May 2012. RESULTS Of the 78 women included in the study, 3 were excluded; 1 woman with preeclampsia could not complete the 1H-MRS due to discomfort, and 2 women in the normal pregnancy group later developed preeclampsia. Thus, the final analysis was based on the spectra of 75 women (29 women with preeclampsia, 30 with normal pregnancies, and 16 nonpregnant women). Further, the levels of taurine could not be calculated in 8 women with normal pregnancies and in 1 nonpregnant woman. For cerebral glutamate, 2 outliers were excluded, 1 in the preeclampsia group, and 1 in the normal pregnant group. Both were >3 SDs lower than the median. Including these in the analysis did not alter statistical significance. None of the MR images showed edema compatible with posterior reversible encephalopathy syndrome. Table 1 depicts the clinical characteristics of the study population. There were significant differences between study groups for age, BMI, and parity. Among the women with preeclampsia, 10 had developed severe preeclampsia at the time of examination, with a recorded systolic BP ≥160 and/or diastolic BP ≥110.18 Antihypertensive treatment, usually monotherapy with labetalol, was used by 23 (79 %) women with preeclampsia. There was no difference in gestational age at the time of the magnetic resonance imaging scanning between women with preeclampsia and those with a normal pregnancy. Table 1. Clinical characteristics of the study population Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Data are presented as median (interquartile range). BMI measured in the first trimester in the 2 pregnant study groups. Abbreviation: BMI, body mass index. aP value according to Kruskal–Wallis, comparison of all groups. View Large Table 1. Clinical characteristics of the study population Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Data are presented as median (interquartile range). BMI measured in the first trimester in the 2 pregnant study groups. Abbreviation: BMI, body mass index. aP value according to Kruskal–Wallis, comparison of all groups. View Large Cerebral levels of 1H-metabolites in each study group are depicted in Figure 2a–f. We found significant differences on group level for glutamate (P = 0.001), myo-inositol, choline, and creatine (all P < 0.001). Normal pregnancy was characterized by lower cerebral levels of myo-inositol, choline, and creatine (P = 0.001 or less), whereas cerebral levels of NAA, taurine, and glutamate did not differ from the nonpregnant women. These results remained after adjustment for maternal age and BMI. Figure 2. View largeDownload slide (a–f) Boxplots of 1H-MRS metabolites in women with preeclampsia (n = 29), normal pregnancies (n = 30), and in nonpregnant women (n = 16). (mmol/l = millimol per liter). Figure 2. View largeDownload slide (a–f) Boxplots of 1H-MRS metabolites in women with preeclampsia (n = 29), normal pregnancies (n = 30), and in nonpregnant women (n = 16). (mmol/l = millimol per liter). Women with preeclampsia had lower cerebral glutamate levels, both in comparison with women with normal pregnancies and in comparison with the nonpregnant controls (P = 0.009 and 0.001, respectively). Cerebral levels of myo-inositol, choline, and creatine were lower in women with preeclampsia than in the nonpregnant women (P < 0.001), but did not differ from women with healthy pregnancies, Figure 2. These results also remained significant after adjustment for age and BMI. Cramér-Rao Lower Bound was lower than 20% for all metabolites except taurine. Plasma levels of organic osmolytes are demonstrated in Table 2. Women with preeclampsia had higher levels of plasma glutamate in comparison with women with normal pregnancies and nonpregnant women (both P < 0.001). This difference persisted after adjustment for maternal age and BMI. No difference in plasma glutamate levels was noted between women with normal pregnancy and nonpregnant women. Compared to the nonpregnant group, the 2 pregnant groups had lower osmolality and plasma sodium levels (P < 0.001). Compared to normal pregnancy, women with preeclampsia had higher osmolality, but lower levels of sodium (P = 0.02 and P = 0.04, respectively). There were no differences in levels of osmolytes between those who were on antihypertensive treatment and those who did not receive medication. Table 2. Plasma levels of glutamate, osmolality and sodium Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Data are presented as median (interquartile range). aP < 0.05 compared to normal pregnant, Mann–Whitney U-test. bP ≤ 0.001 compared to normal pregnant, Mann–Whitney U-test. cP < 0.05 compared to nonpregnant, Mann–Whitney U-test. dP ≤ 0.001 compared to nonpregnant, Mann–Whitney U-test. eP value according to Kruskal–Wallis. View Large Table 2. Plasma levels of glutamate, osmolality and sodium Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Data are presented as median (interquartile range). aP < 0.05 compared to normal pregnant, Mann–Whitney U-test. bP ≤ 0.001 compared to normal pregnant, Mann–Whitney U-test. cP < 0.05 compared to nonpregnant, Mann–Whitney U-test. dP ≤ 0.001 compared to nonpregnant, Mann–Whitney U-test. eP value according to Kruskal–Wallis. View Large The results of the correlation analyses, based on all 3 study groups, are presented in Table 3. There were significant correlations between cerebral levels of the osmolytes glutamate, myo-inositol, choline, and creatine. The cerebral levels of taurine correlated with levels of choline, but not to the other cerebral osmolytes. There was a significant negative correlation between cerebral and plasma glutamate levels. Plasma osmolality correlated with cerebral levels of myo-inositol, choline, and creatine, but not with cerebral levels of glutamate and taurine. The plasma levels of sodium correlated with plasma osmolality, plasma glutamate, and all cerebral metabolites except taurine. Table 3. Correlations (Spearman) between the cerebral levels of the organic osmolytes glutamate, myo-inositol, choline, and creatine and the plasma (P) levels of glutamate, osmolality, and sodium for all 3 study groups Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** *P < 0.001; **P < 0.01; ***P < 0.05. View Large Table 3. Correlations (Spearman) between the cerebral levels of the organic osmolytes glutamate, myo-inositol, choline, and creatine and the plasma (P) levels of glutamate, osmolality, and sodium for all 3 study groups Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** *P < 0.001; **P < 0.01; ***P < 0.05. View Large DISCUSSION Main findings This is to our awareness the first report of cerebral osmolytes in pregnancy and in women with preeclampsia. Our findings indicate that pregnant women have lower levels of cerebral osmolytes than nonpregnant women. Preeclampsia is further characterized by reduced levels of glutamate, which in turn acts both as an osmolyte and as an excitatory neurotransmitter. It has been well established for decades that plasma osmolality and plasma sodium in pregnancy are significantly reduced,21 and in our study, we expand this knowledge by also demonstrating that the levels of cerebral organic osmolytes are correlated with plasma osmolality and sodium levels in pregnancy. Ultimately, this finding suggests that altered circulating fluid metabolism is the underlying mechanism to the reduced levels of cerebral osmolytes we report. Strengths and limitations Strengths of our study are the usage of an advanced technique familiar in other research areas, but innovative in pregnancy, the relatively large study population and the standardized protocols for collection of BP and plasma samples in close proximity with the 1H-MRS examination. The region of interest was positioned over both parietal and occipital lobes, areas mostly affected when vasogenic edema is found in eclampsia and severe preeclampsia.2 However, we have only measured in 1 voxel, and there might be regional differences in levels of the osmolytes, which we have failed to register. The voxel was positioned so as to avoid cerebrospinal fluid and vessels, but the MRS technique cannot separate the contributions from intracellular and interstitial compartments, which would have been of interest. Imaging at higher field strength (3 Tesla instead of 1.5 Tesla) would improve signal-to-noise ratio and spectral resolution, but there is at present no consensus regarding its safe use during pregnancy. When presenting data from 1H-MRS, it has been customary to use a metabolite concentration ratio to creatine, a metabolite considered stable. This is to avoid partial volume effects due to contamination of metabolite free cerebrospinal fluid. Since we detect differences in creatine levels as a part of the same biological phenomenon, a ratio against creatine would be misleading in our case. Since we did not perform correction for partial volume effect of cerebrospinal fluid, our metabolite concentrations could be underestimated. Interpretations This is the first study using 1H-MRS to measure cerebral organic osmolytes in pregnancy and preeclampsia in women. This technique has, however, been previously used to examine the maternal human brain in a few publications.14,15 In a study of 10 women with eclampsia, studied 3–5 days after their seizures, a decreased NAA peak in comparison to healthy age-matched nonpregnant women was reported.15 A more recent publication reported normal NAA levels in women with preeclampsia, which we also did.14 This study by Rutherford et al. found lower concentrations of choline and creatine in pregnancy, parallel to our results. They further found that choline was significantly higher in preeclampsia compared to normal pregnancy, a finding we came close to replicate, since the unadjusted P value is 0.056 in our data. Their interpretation of this result is that it reflects an ischemia in preeclampsia since higher choline is also found in patients with unilateral severe carotid stenosis.22 In our study, we could further show decreased levels of cerebral glutamate and myo-inositol levels, compounds not reported from earlier 1H-MRS examinations in pregnancy. Glutamate, myo-inositol, choline, and creatine are all described as cerebral organic osmolytes11 and has been shown to decrease in conditions with hyponatremia.12,23,24 This is called regulatory volume decrease and is part of the mechanisms by which cells defend themselves from edema when extracellular osmolality decreases.11 In cases of acute hyponatremia, water enters the cells by osmotic forces, but when the decline is slower, the cells defend themselves, firstly by extrusion of electrolytes. Since altered ion gradients can affect cellular functions, a more stable and long-lasting solution is the efflux of organic osmolytes.25 Our study is the first in humans and pregnancy, but decreased levels of organic osmolytes has previously been demonstrated in pregnant rats.26 We demonstrated lower cerebral levels of glutamate, myo-inositol, choline, and creatine in both women with preeclampsia and women with normal pregnancies. All 4 metabolites correlated with each other, indicating that their decline was part of the same biological mechanism (Table 3). For myo-inositol, choline, and creatine, there was a clear correlation with plasma osmolality and sodium. This strengthens the argument that the observed decrease in these compounds is indeed a response to the lower plasma osmolality associated with pregnancy. Cerebral levels of glutamate were, however, not correlated with either plasma osmolality or sodium. If this is due to a separate biological mechanism or simply caused by a lack of statistical power, remains to be explained. Interestingly, glutamate, i.e., the most common excitatory neurotransmitter in the brain, was lower in women with preeclampsia than in women with normal pregnancies. This is an intriguing finding considering the incompletely chartered pathophysiology of seizures in preeclampsia. There has been speculation that extrusion of glutamate from cells raise extracellular levels of the neurotransmitter and thereby causing hyperexcitability.11 Plasma levels of glutamate also differed between women with preeclampsia and normal pregnancy, with the highest concentrations seen in preeclampsia, in line with findings from previous studies.27,28 There is evidence that plasma glutamate levels negatively correlates with worse outcomes in stroke and traumatic brain injury.29 Glutamate can also be released from aggregating platelets and polymorphonuclear leukocytes, which may influence permeability of endothelial cells, including brain endothelium,30,31 and pregnancy alters expression of certain glutamate receptors on peripheral blood mononuclear cells.32 An increased endothelial permeability has been proposed to play a crucial role in the pathophysiology of posterior reversible encephalopathy syndrome, a clinical-radiological entity seen in eclampsia and severe preeclampsia.33 Although our results with lower cerebral glutamate, myo-inositol, choline, and creatine are consistent with what has previously been shown in hyponatremia12,34 our detected differences in cerebral organic osmolytes are small, and the clinical relevance is not clear. In the previous studies, myo-inositol was 40–49% lower as compared to 20% in our population.12,34 All women in our study had plasma osmolality and sodium levels within normal range, although the pregnant women had lower levels. It remains to be established if and how these reductions in cerebral organic osmolytes contribute to the hyperexcitability observed in pregnancy, especially preeclampsia, and if these changes can explain how eclamptic seizures can arise without any prodromal symptoms or hypertension. DISCLOSURE The authors report no conflict of interest. ACKNOWLEDGMENT Funding for the present study has been received from Uppsala-Örebro Regional Research Council (project number RFR-479351) and the Swedish Research Council (project number 2014–3561). REFERENCES 1. Zeeman GG . Neurologic complications of pre-eclampsia . Semin Perinatol 2009 ; 33 : 166 – 172 . Google Scholar CrossRef Search ADS PubMed 2. Brewer J , Owens MY , Wallace K , Reeves AA , Morris R , Khan M , LaMarca B , Martin JN Jr . Posterior reversible encephalopathy syndrome in 46 of 47 patients with eclampsia . Am J Obstet Gynecol 2013 ; 208 : 468.e1 – 468.e6 . Google Scholar CrossRef Search ADS 3. Bartynski WS . Posterior reversible encephalopathy syndrome, part 2: controversies surrounding pathophysiology of vasogenic edema . AJNR Am J Neuroradiol 2008 ; 29 : 1043 – 1049 . Google Scholar CrossRef Search ADS PubMed 4. Cooray SD , Edmonds SM , Tong S , Samarasekera SP , Whitehead CL . Characterization of symptoms immediately preceding eclampsia . Obstet Gynecol 2011 ; 118 : 995 – 999 . Google Scholar CrossRef Search ADS PubMed 5. Sanghavi M , Rutherford JD . Cardiovascular physiology of pregnancy . Circulation 2014 ; 130 : 1003 – 1008 . Google Scholar CrossRef Search ADS PubMed 6. Tkachenko O , Shchekochikhin D , Schrier RW . Hormones and hemodynamics in pregnancy . Int J Endocrinol Metab 2014 ; 12 : e14098 . Google Scholar CrossRef Search ADS PubMed 7. Davison JM , Gilmore EA , Dürr J , Robertson GL , Lindheimer MD . Altered osmotic thresholds for vasopressin secretion and thirst in human pregnancy . Am J Physiol 1984 ; 246 : F105 – F109 . Google Scholar PubMed 8. Lindheimer MD , Davison JM . Osmoregulation, the secretion of arginine vasopressin and its metabolism during pregnancy . Eur J Endocrinol 1995 ; 132 : 133 – 143 . Google Scholar CrossRef Search ADS PubMed 9. Zulfikaroglu E , Islimye M , Tonguc EA , Payasli A , Isman F , Var T , Danisman N . Circulating levels of copeptin, a novel biomarker in pre-eclampsia . J Obstet Gynaecol Res 2011 ; 37 : 1198 – 1202 . Google Scholar CrossRef Search ADS PubMed 10. Santillan MK , Santillan DA , Scroggins SM , Min JY , Sandgren JA , Pearson NA , Leslie KK , Hunter SK , Zamba GK , Gibson-Corley KN , Grobe JL . Vasopressin in preeclampsia: a novel very early human pregnancy biomarker and clinically relevant mouse model . Hypertension 2014 ; 64 : 852 – 859 . Google Scholar CrossRef Search ADS PubMed 11. Verbalis JG . Brain volume regulation in response to changes in osmolality . Neuroscience 2010 ; 168 : 862 – 870 . Google Scholar CrossRef Search ADS PubMed 12. Videen JS , Michaelis T , Pinto P , Ross BD . Human cerebral osmolytes during chronic hyponatremia. A proton magnetic resonance spectroscopy study . J Clin Invest 1995 ; 95 : 788 – 793 . Google Scholar CrossRef Search ADS PubMed 13. Andrew RD . Seizure and acute osmotic change: clinical and neurophysiological aspects . J Neurol Sci 1991 ; 101 : 7 – 18 . Google Scholar CrossRef Search ADS PubMed 14. Rutherford JM , Moody A , Crawshaw S , Rubin PC . Magnetic resonance spectroscopy in pre-eclampsia: evidence of cerebral ischaemia . BJOG 2003 ; 110 : 416 – 423 . Google Scholar CrossRef Search ADS PubMed 15. Sengar AR , Gupta RK , Dhanuka AK , Roy R , Das K . MR imaging, MR angiography, and MR spectroscopy of the brain in eclampsia . AJNR Am J Neuroradiol 1997 ; 18 : 1485 – 1490 . Google Scholar PubMed 16. Nelander M , Weis J , Bergman L , Larsson A , Wikström AK , Wikström J . Cerebral magnesium levels in preeclampsia; a phosphorus magnetic resonance spectroscopy study . Am J Hypertens 2017 ; 30 : 667 – 672 . Google Scholar CrossRef Search ADS PubMed 17. Tranquilli AL , Dekker G , Magee L , Roberts J , Sibai BM , Steyn W , Zeeman GG , Brown MA . The classification, diagnosis and management of the hypertensive disorders of pregnancy: a revised statement from the ISSHP . Pregnancy Hypertens 2014 ; 4 : 97 – 104 . Google Scholar CrossRef Search ADS PubMed 18. Tranquilli AL , Brown MA , Zeeman GG , Dekker G , Sibai BM . The definition of severe and early-onset preeclampsia. Statements from the International Society for the Study of Hypertension in Pregnancy (ISSHP) . Pregnancy Hypertens 2013 ; 3 : 44 – 47 . Google Scholar CrossRef Search ADS PubMed 19. Marsál K , Persson PH , Larsen T , Lilja H , Selbing A , Sultan B . Intrauterine growth curves based on ultrasonically estimated foetal weights . Acta Paediatr 1996 ; 85 : 843 – 848 . Google Scholar CrossRef Search ADS PubMed 20. Provencher SW . Estimation of metabolite concentrations from localized in vivo proton NMR spectra . Magn Reson Med 1993 ; 30 : 672 – 679 . Google Scholar CrossRef Search ADS PubMed 21. Davison JM . Renal haemodynamics and volume homeostasis in pregnancy . Scand J Clin Lab Invest Suppl 1984 ; 169 : 15 – 27 . Google Scholar CrossRef Search ADS PubMed 22. van der Grond J , Balm R , Kappelle LJ , Eikelboom BC , Mali WP . Cerebral metabolism of patients with stenosis or occlusion of the internal carotid artery. A 1H-MR spectroscopic imaging study . Stroke 1995 ; 26 : 822 – 828 . Google Scholar CrossRef Search ADS PubMed 23. Verbalis JG , Gullans SR . Hyponatremia causes large sustained reductions in brain content of multiple organic osmolytes in rats . Brain Res 1991 ; 567 : 274 – 282 . Google Scholar CrossRef Search ADS PubMed 24. Moritz ML , Ayus JC . The pathophysiology and treatment of hyponatraemic encephalopathy: an update . Nephrol Dial Transplant 2003 ; 18 : 2486 – 2491 . Google Scholar CrossRef Search ADS PubMed 25. Lang F , Busch GL , Ritter M , Völkl H , Waldegger S , Gulbins E , Häussinger D . Functional significance of cell volume regulatory mechanisms . Physiol Rev 1998 ; 78 : 247 – 306 . Google Scholar CrossRef Search ADS PubMed 26. Law RO . Effects of pregnancy on the contents of water, taurine, and total amino nitrogen in rat cerebral cortex . J Neurochem 1989 ; 53 : 300 – 302 . Google Scholar CrossRef Search ADS PubMed 27. Terán Y , Ponce O , Betancourt L , Hernández L , Rada P . Amino acid profile of plasma and cerebrospinal fluid in preeclampsia . Pregnancy Hypertens 2012 ; 2 : 416 – 422 . Google Scholar CrossRef Search ADS PubMed 28. Kenny LC , Broadhurst D , Brown M , Dunn WB , Redman CW , Kell DB , Baker PN . Detection and identification of novel metabolomic biomarkers in preeclampsia . Reprod Sci 2008 ; 15 : 591 – 597 . Google Scholar CrossRef Search ADS PubMed 29. Castellanos M , Sobrino T , Pedraza S , Moldes O , Pumar JM , Silva Y , Serena J , García-Gil M , Castillo J , Dávalos A . High plasma glutamate concentrations are associated with infarct growth in acute ischemic stroke . Neurology 2008 ; 71 : 1862 – 1868 . Google Scholar CrossRef Search ADS PubMed 30. Collard CD , Park KA , Montalto MC , Alapati S , Buras JA , Stahl GL , Colgan SP . Neutrophil-derived glutamate regulates vascular endothelial barrier function . J Biol Chem 2002 ; 277 : 14801 – 14811 . Google Scholar CrossRef Search ADS PubMed 31. Tremolizzo L , DiFrancesco JC , Rodriguez-Menendez V , Sirtori E , Longoni M , Cassetti A , Bossi M , El Mestikawy S , Cavaletti G , Ferrarese C . Human platelets express the synaptic markers VGLUT1 and 2 and release glutamate following aggregation . Neurosci Lett 2006 ; 404 : 262 – 265 . Google Scholar CrossRef Search ADS PubMed 32. Bhandage AK , Jin Z , Hellgren C , Korol SV , Nowak K , Williamsson L , Sundström-Poromaa I , Birnir B . AMPA, NMDA and kainate glutamate receptor subunits are expressed in human peripheral blood mononuclear cells (PBMCs) where the expression of GluK4 is altered by pregnancy and GluN2D by depression in pregnant women . J Neuroimmunol 2017 ; 305 : 51 – 58 . Google Scholar CrossRef Search ADS PubMed 33. Marra A , Vargas M , Striano P , Del Guercio L , Buonanno P , Servillo G . Posterior reversible encephalopathy syndrome: the endothelial hypotheses . Med Hypotheses 2014 ; 82 : 619 – 622 . Google Scholar CrossRef Search ADS PubMed 34. Häussinger D , Laubenberger J , vom Dahl S , Ernst T , Bayer S , Langer M , Gerok W , Hennig J . Proton magnetic resonance spectroscopy studies on human brain myo-inositol in hypo-osmolarity and hepatic encephalopathy . Gastroenterology 1994 ; 107 : 1475 – 1480 . Google Scholar CrossRef Search ADS PubMed © American Journal of Hypertension, Ltd 2018. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png American Journal of Hypertension Oxford University Press

Cerebral Osmolytes and Plasma Osmolality in Pregnancy and Preeclampsia: A Proton Magnetic Resonance Spectroscopy Study

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Oxford University Press
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© American Journal of Hypertension, Ltd 2018. All rights reserved. For Permissions, please email: journals.permissions@oup.com
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0895-7061
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1941-7225
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10.1093/ajh/hpy019
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Abstract

Abstract BACKGROUND Cerebral complications contribute substantially to mortality in preeclampsia. Pregnancy calls for extensive maternal adaptations, some associated with increased propensity for seizures, but the pathophysiology behind the eclamptic seizures is not fully understood. Plasma osmolality and sodium levels are lowered in pregnancy. This could result in extrusion of cerebral organic osmolytes, including the excitatory neurotransmitter glutamate, but this remains to be determined. The hypothesis of this study was that cerebral levels of organic osmolytes are decreased during pregnancy, and that this decrease is even more pronounced in women with preeclampsia. METHODS We used proton magnetic resonance spectroscopy to compare levels of cerebral organic osmolytes, in women with preeclampsia (n = 30), normal pregnancy (n = 32), and nonpregnant controls (n = 16). Cerebral levels of organic osmolytes were further correlated to plasma osmolality and plasma levels of glutamate and sodium. RESULTS Compared to nonpregnant women, women with normal pregnancy and preeclampsia had lower levels of the cerebral osmolytes, myo-inositol, choline and creatine (P = 0.001 or less), and all these metabolites correlated with each other (P < 0.05). Women with normal pregnancies and preeclampsia had similar levels of osmolytes, except for glutamate, which was significantly lower in preeclampsia. Cerebral and plasma glutamate levels were negatively correlated with each other (P < 0.008), and myo-inositol, choline and creatine levels were all positively correlated with both plasma osmolality and sodium levels (P < 0.05). CONCLUSIONS Our results indicate that pregnancy is associated with extrusion of cerebral organic osmolytes. This includes the excitatory neurotransmitter glutamate, which may be involved in the pathophysiology of seizures in preeclampsia. blood pressure, cerebral osmolytes, eclampsia, glutamate, hypertension, preeclampsia, proton magnetic resonance spectroscopy Cerebral complications in preeclampsia/eclampsia contribute substantially to maternal deaths worldwide.1 Although seizures during pregnancy are described in the literature since Hippocrates and the most effective treatment, magnesium sulfate (MgSO4), has been in practice for almost a century, our knowledge of how and when eclampsia arises is surprisingly scarce. Eclampsia and severe preeclampsia are described as a form of posterior reversible encephalopathy syndrome.2 The predominant theory of the etiology includes breakdown of cerebral autoregulation due to a rapid rise in blood pressure (BP), with ensuing damage to the blood–brain barrier and vasogenic edema as a result.3 However, the fact that a large proportion of women with eclampsia have moderately elevated, or even normal BP, prior to seizures, questions this theory.4 Pregnancy causes fundamental changes in hemodynamics and body fluid hemostasis,5,6 including reductions in plasma osmolality and plasma sodium levels. These changes appear directly after conception, reaches a steady state at gestation week 10, and are achieved by pregnancy-induced changes in the osmotic thresholds for thirst and arginine and vasopressin secretion.7,8 A growing body of evidence shows that the increase in vasopressin is higher in pregnancies that subsequently develops preeclampsia and it has been proposed as a possible predictive biomarker.9,10 When osmolality falls, there is a risk of edema when osmotic forces cause water to enter the cells. This is prevented by mechanisms called regulatory volume decrease, intricate systems inherent in all animal species. The ability to preserve a stable volume despite changes in osmolality of the surrounding environment, is especially important for brain cells with the potentially life-threatening effects of cerebral edema.11 Studies using magnetic resonance spectroscopy (MRS) have shown that neurons in a chronic state of hypoosmolality protect themselves from swelling by efflux of organic osmolytes (amino acids, polyalcohols, sugars, and methylamines), including glutamate, the most important neurotransmitter.12 Plasma hypoosmolality is also known to increase susceptibility to seizures.13 No prior study has used MRS to investigate levels of cerebral organic osmolytes in women with preeclampsia and normal pregnancy. To our knowledge, there are only 2 publications with brain proton MRS in preeclampsia or eclampsia.14,15 These studies are limited by small sample sizes, and the latter focused on posteclampsia status, rather than the changes leading up to eclampsia. Using proton MRS (1H-MRS) where the spectrum obtained contains most organic osmolytes of interest, such as N-acetyl-aspartate (NAA), glutamate, myo-inositol, choline, creatine, and taurine, our aim was to compare levels of cerebral organic osmolytes between women with preeclampsia, normal pregnancy, and nonpregnant women and assess if organic osmolytes correlate to plasma osmolality and plasma sodium levels. Our hypotheses were that levels of cerebral organic osmolytes are decreased during pregnancy, that this decrease is more pronounced in preeclampsia and that levels of cerebral organic osmolytes correlate to plasma osmolality and plasma sodium levels. MATERIAL AND METHODS Study design and population The study participants in this cross-sectional study were recruited in Uppsala, Sweden during 2013–2016. Uppsala University Hospital is a tertiary referral centre with approximately 4,000 deliveries per year. The study population is in part described previously.16 Only women with singleton pregnancies and a gestational age between 22 + 0 and 41 + 6 according to an early second-trimester ultrasound dating were eligible. General exclusion criteria in the present study were chronic hypertension, diabetes mellitus, pre-existing renal disease, or standard contraindications for magnetic resonance imaging, e.g., claustrophobia metallic implants or pacemaker. We recruited 3 groups of women: women with preeclampsia (n = 30), normal pregnant women (n = 32), and nonpregnant women (n = 16). Women with preeclampsia were recruited from the obstetric ward or outpatient obstetric clinic. Preeclampsia was defined as de novo hypertension after 20 weeks of gestation in combination with proteinuria, according to the recommendations of the International Society for studies on Hypertension in Pregnancy (ISSHP).17 Hypertension was defined as systolic BP of ≥140 mm Hg and/or diastolic BP of ≥90 mm Hg measured on 2 subsequent occasions, at least 6 hours apart, and proteinuria defined as ≥2+ on a dipstick or ≥300 mg/24 hours in a urine collection. Preeclampsia was defined as severe when systolic BP was ≥160 mm Hg and/or diastolic BP ≥110 mm Hg; or if HELLP-syndrome was present.18 Women with preeclampsia had to be sufficiently clinically stable to be transported to the MR facility. None of the women was treated with MgSO4 or had developed eclampsia, neither before inclusion nor after. Every woman diagnosed with preeclampsia, either admitted or monitored in outpatient care, which came to the principal investigator’s (M.N.) knowledge, was approached regarding study participation. Only a small fraction of respondents refrained participation in the study, most often due to claustrophobia. The normal pregnant group was recruited through information posters at antenatal outpatient clinics in Uppsala and at Uppsala University. Women in the normal pregnant group were matched for gestational age at examination to women in the preeclampsia group. After examination, women in the normal pregnant group were monitored and those who developed preeclampsia, delivered preterm (<37 weeks) or did not deliver an infant of normal birth weight (±2 SDs of the mean birth weight for gestational age and sex19) were excluded. The nonpregnant group was recruited through Facebook and local networks and included both nulliparous and parous women. In addition to the general exclusion criteria, women with prior history of preeclampsia or gestational hypertension were not included either in the normal pregnant or nonpregnant study groups. All participants underwent BP measurement, blood and urine sampling, and MRS examination within a 4-hour period. Systolic and diastolic BPs were measured in supine position in the right arm after a 15-minute rest. A manual sphygmomanometer (Boso, Germany or Welch Allyn) with appropriate cuff size depending on arm circumference was used. Fresh midstream urine samples were collected and analyzed with a Combur 9 test (Roche) or Clinitek Status Analyzer (Siemens). Blood samples were centrifuged 10 minutes at 1,500 g and then immediately stored as aliquots in −70 °C until analysis. Magnetic resonance spectroscopy MR examinations were performed using a 1.5 T scanner (Achieva, Philips Healthcare, Best, The Netherlands). The whole body coil was used for excitation, and an 8-element receiver head coil served for signal reception. T1- and T2-weighted turbo spin echo images were obtained in the axial, sagittal, and coronal orientations (spatial resolution 1 × 1 × 4 mm3, interslice gap 0 mm). Single-voxel spectra were acquired using the point-resolved spectroscopy (PRESS) sequence (spectral bandwidth 1,000 Hz, 1,024 points, TR/TE 5,000/30 ms, 16 phase cycle steps). Volume of interest (voxel) with a size of 20 × 20 × 20 mm3 was positioned in the posterior midline, at the junction between the parietal and occipital lobes (Figure 1). The position was chosen to include areas mostly affected when vasogenic edema is found in eclampsia and severe preeclampsia. Magnetic field homogeneity was improved by iterative first-order shimming. Sixteen non–water-suppressed and 128 water-suppressed scans were performed in consecutive acquisition in 12 minutes. Intensities of spectral lines were estimated by fitting using the LCModel.20 Levels of the following 1H-metabolites were measured: NAA, choline, creatine, glutamate, myo-inositol, and taurine. Quantitation error of each metabolite concentration estimate was assessed from the SD (Cramér-Rao Lower Bound) expressed in percent of the estimated concentration.20 Cramér-Rao Lower Bound below 20% is commonly used as a criterion for estimates of acceptable reliability. Figure 1. View largeDownload slide T2- (left) and T1-weighted (right) turbo spin echo images with the typical voxel position in the posterior midline at the parieto-occipital fissure. Figure 1. View largeDownload slide T2- (left) and T1-weighted (right) turbo spin echo images with the typical voxel position in the posterior midline at the parieto-occipital fissure. Plasma samples Plasma analyzes were routine tests and performed at the accredited laboratory in the Department of Clinical Chemistry and Pharmacology, Uppsala University Hospital. Plasma osmolality was assessed using a Fiske Osmometer, model 210. Plasma sodium was analyzed on an Architect c16000 with a reagent from Abbott Laboratories. Plasma glutamate was analyzed with a glutamate assay kit from Sigma-Aldrich (MAK004). Statistics Clinical characteristics, median levels of cerebral 1H-metabolites, plasma osmolality and plasma levels of glutamate and sodium were compared between the women with preeclampsia, women with normal pregnancies and nonpregnant women by Kruskal–Wallis test. Pair-wise comparisons between the study groups were made with Mann–Whitney U-test. Adjustment for maternal age and body mass index (BMI) was made with analysis of covariance after testing for adequate normal distribution for the variables. Correlations between the different cerebral osmolytes and between cerebral osmolytes and plasma variables were assessed with Spearman’s correlation test. All significance tests were 2-tailed. P <0.05 was considered to denote a statistically significant difference. Analyses were performed using IBM SPSS Statistics 24 (IBM SPSS, Chicago, IL). Ethics The study was approved by the Regional Ethical Review Board in Uppsala, Sweden, and informed consent was obtained from each woman participating in the study. Reference number: 2012–087, 23 May 2012. RESULTS Of the 78 women included in the study, 3 were excluded; 1 woman with preeclampsia could not complete the 1H-MRS due to discomfort, and 2 women in the normal pregnancy group later developed preeclampsia. Thus, the final analysis was based on the spectra of 75 women (29 women with preeclampsia, 30 with normal pregnancies, and 16 nonpregnant women). Further, the levels of taurine could not be calculated in 8 women with normal pregnancies and in 1 nonpregnant woman. For cerebral glutamate, 2 outliers were excluded, 1 in the preeclampsia group, and 1 in the normal pregnant group. Both were >3 SDs lower than the median. Including these in the analysis did not alter statistical significance. None of the MR images showed edema compatible with posterior reversible encephalopathy syndrome. Table 1 depicts the clinical characteristics of the study population. There were significant differences between study groups for age, BMI, and parity. Among the women with preeclampsia, 10 had developed severe preeclampsia at the time of examination, with a recorded systolic BP ≥160 and/or diastolic BP ≥110.18 Antihypertensive treatment, usually monotherapy with labetalol, was used by 23 (79 %) women with preeclampsia. There was no difference in gestational age at the time of the magnetic resonance imaging scanning between women with preeclampsia and those with a normal pregnancy. Table 1. Clinical characteristics of the study population Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Data are presented as median (interquartile range). BMI measured in the first trimester in the 2 pregnant study groups. Abbreviation: BMI, body mass index. aP value according to Kruskal–Wallis, comparison of all groups. View Large Table 1. Clinical characteristics of the study population Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P valuea Age, years 27 (7) 33 (6) 27 (12) 0.02 BMI, kg/m2 26 (6) 24 (5) 22 (5) 0.001 Prior births, n (%) 5 (17) 20 (67) 7 (44) <0.001 At examination:  Gestational age, days 246 (55) 246 (74) 0.93  Blood pressure, mm Hg    Systolic 150 (20) 110 (10) 110 (9) <0.001    Diastolic 95 (13) 70 (13) 70 (5) <0.001 Data are presented as median (interquartile range). BMI measured in the first trimester in the 2 pregnant study groups. Abbreviation: BMI, body mass index. aP value according to Kruskal–Wallis, comparison of all groups. View Large Cerebral levels of 1H-metabolites in each study group are depicted in Figure 2a–f. We found significant differences on group level for glutamate (P = 0.001), myo-inositol, choline, and creatine (all P < 0.001). Normal pregnancy was characterized by lower cerebral levels of myo-inositol, choline, and creatine (P = 0.001 or less), whereas cerebral levels of NAA, taurine, and glutamate did not differ from the nonpregnant women. These results remained after adjustment for maternal age and BMI. Figure 2. View largeDownload slide (a–f) Boxplots of 1H-MRS metabolites in women with preeclampsia (n = 29), normal pregnancies (n = 30), and in nonpregnant women (n = 16). (mmol/l = millimol per liter). Figure 2. View largeDownload slide (a–f) Boxplots of 1H-MRS metabolites in women with preeclampsia (n = 29), normal pregnancies (n = 30), and in nonpregnant women (n = 16). (mmol/l = millimol per liter). Women with preeclampsia had lower cerebral glutamate levels, both in comparison with women with normal pregnancies and in comparison with the nonpregnant controls (P = 0.009 and 0.001, respectively). Cerebral levels of myo-inositol, choline, and creatine were lower in women with preeclampsia than in the nonpregnant women (P < 0.001), but did not differ from women with healthy pregnancies, Figure 2. These results also remained significant after adjustment for age and BMI. Cramér-Rao Lower Bound was lower than 20% for all metabolites except taurine. Plasma levels of organic osmolytes are demonstrated in Table 2. Women with preeclampsia had higher levels of plasma glutamate in comparison with women with normal pregnancies and nonpregnant women (both P < 0.001). This difference persisted after adjustment for maternal age and BMI. No difference in plasma glutamate levels was noted between women with normal pregnancy and nonpregnant women. Compared to the nonpregnant group, the 2 pregnant groups had lower osmolality and plasma sodium levels (P < 0.001). Compared to normal pregnancy, women with preeclampsia had higher osmolality, but lower levels of sodium (P = 0.02 and P = 0.04, respectively). There were no differences in levels of osmolytes between those who were on antihypertensive treatment and those who did not receive medication. Table 2. Plasma levels of glutamate, osmolality and sodium Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Data are presented as median (interquartile range). aP < 0.05 compared to normal pregnant, Mann–Whitney U-test. bP ≤ 0.001 compared to normal pregnant, Mann–Whitney U-test. cP < 0.05 compared to nonpregnant, Mann–Whitney U-test. dP ≤ 0.001 compared to nonpregnant, Mann–Whitney U-test. eP value according to Kruskal–Wallis. View Large Table 2. Plasma levels of glutamate, osmolality and sodium Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Preeclampsia (n = 29) Normal pregnant (n = 30) Nonpregnant (n = 16) P value Glutamate (mmol/l) 0.22 (0.07)b,d 0.17 (0.08) 0.15 (0.04) <0.001 Osmolality (osm/kg) 289 (6.5)a,c 285 (7)d 292 (2.8) 0.001 Sodium (mmol/l) 138 (1.5)a,d 139 (4)d 146 (4.8) <0.001 Data are presented as median (interquartile range). aP < 0.05 compared to normal pregnant, Mann–Whitney U-test. bP ≤ 0.001 compared to normal pregnant, Mann–Whitney U-test. cP < 0.05 compared to nonpregnant, Mann–Whitney U-test. dP ≤ 0.001 compared to nonpregnant, Mann–Whitney U-test. eP value according to Kruskal–Wallis. View Large The results of the correlation analyses, based on all 3 study groups, are presented in Table 3. There were significant correlations between cerebral levels of the osmolytes glutamate, myo-inositol, choline, and creatine. The cerebral levels of taurine correlated with levels of choline, but not to the other cerebral osmolytes. There was a significant negative correlation between cerebral and plasma glutamate levels. Plasma osmolality correlated with cerebral levels of myo-inositol, choline, and creatine, but not with cerebral levels of glutamate and taurine. The plasma levels of sodium correlated with plasma osmolality, plasma glutamate, and all cerebral metabolites except taurine. Table 3. Correlations (Spearman) between the cerebral levels of the organic osmolytes glutamate, myo-inositol, choline, and creatine and the plasma (P) levels of glutamate, osmolality, and sodium for all 3 study groups Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** *P < 0.001; **P < 0.01; ***P < 0.05. View Large Table 3. Correlations (Spearman) between the cerebral levels of the organic osmolytes glutamate, myo-inositol, choline, and creatine and the plasma (P) levels of glutamate, osmolality, and sodium for all 3 study groups Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** Myo-Inositol Choline Creatine Taurine P-Glutamate P-Osmolality P-Sodium rho rho rho rho rho rho rho Glutamate 0.51* 0.25*** 0.49 a 0.11 −0.35** 0.08 0.22 Myo-Inositol – 0.43* 0.56* 0.08 −0.28*** 0.29*** 0.51* Choline – – 0.44* 0.27 −0.18 0.27*** 0.30*** Creatine – – – 0.04 0.16 0.28*** 0.30*** Taurine – – – – 0.02 0.15 0.02 P-Glutamate – – – – – 0 −0.54* P-Osmolality – – – – – – 0.31** *P < 0.001; **P < 0.01; ***P < 0.05. View Large DISCUSSION Main findings This is to our awareness the first report of cerebral osmolytes in pregnancy and in women with preeclampsia. Our findings indicate that pregnant women have lower levels of cerebral osmolytes than nonpregnant women. Preeclampsia is further characterized by reduced levels of glutamate, which in turn acts both as an osmolyte and as an excitatory neurotransmitter. It has been well established for decades that plasma osmolality and plasma sodium in pregnancy are significantly reduced,21 and in our study, we expand this knowledge by also demonstrating that the levels of cerebral organic osmolytes are correlated with plasma osmolality and sodium levels in pregnancy. Ultimately, this finding suggests that altered circulating fluid metabolism is the underlying mechanism to the reduced levels of cerebral osmolytes we report. Strengths and limitations Strengths of our study are the usage of an advanced technique familiar in other research areas, but innovative in pregnancy, the relatively large study population and the standardized protocols for collection of BP and plasma samples in close proximity with the 1H-MRS examination. The region of interest was positioned over both parietal and occipital lobes, areas mostly affected when vasogenic edema is found in eclampsia and severe preeclampsia.2 However, we have only measured in 1 voxel, and there might be regional differences in levels of the osmolytes, which we have failed to register. The voxel was positioned so as to avoid cerebrospinal fluid and vessels, but the MRS technique cannot separate the contributions from intracellular and interstitial compartments, which would have been of interest. Imaging at higher field strength (3 Tesla instead of 1.5 Tesla) would improve signal-to-noise ratio and spectral resolution, but there is at present no consensus regarding its safe use during pregnancy. When presenting data from 1H-MRS, it has been customary to use a metabolite concentration ratio to creatine, a metabolite considered stable. This is to avoid partial volume effects due to contamination of metabolite free cerebrospinal fluid. Since we detect differences in creatine levels as a part of the same biological phenomenon, a ratio against creatine would be misleading in our case. Since we did not perform correction for partial volume effect of cerebrospinal fluid, our metabolite concentrations could be underestimated. Interpretations This is the first study using 1H-MRS to measure cerebral organic osmolytes in pregnancy and preeclampsia in women. This technique has, however, been previously used to examine the maternal human brain in a few publications.14,15 In a study of 10 women with eclampsia, studied 3–5 days after their seizures, a decreased NAA peak in comparison to healthy age-matched nonpregnant women was reported.15 A more recent publication reported normal NAA levels in women with preeclampsia, which we also did.14 This study by Rutherford et al. found lower concentrations of choline and creatine in pregnancy, parallel to our results. They further found that choline was significantly higher in preeclampsia compared to normal pregnancy, a finding we came close to replicate, since the unadjusted P value is 0.056 in our data. Their interpretation of this result is that it reflects an ischemia in preeclampsia since higher choline is also found in patients with unilateral severe carotid stenosis.22 In our study, we could further show decreased levels of cerebral glutamate and myo-inositol levels, compounds not reported from earlier 1H-MRS examinations in pregnancy. Glutamate, myo-inositol, choline, and creatine are all described as cerebral organic osmolytes11 and has been shown to decrease in conditions with hyponatremia.12,23,24 This is called regulatory volume decrease and is part of the mechanisms by which cells defend themselves from edema when extracellular osmolality decreases.11 In cases of acute hyponatremia, water enters the cells by osmotic forces, but when the decline is slower, the cells defend themselves, firstly by extrusion of electrolytes. Since altered ion gradients can affect cellular functions, a more stable and long-lasting solution is the efflux of organic osmolytes.25 Our study is the first in humans and pregnancy, but decreased levels of organic osmolytes has previously been demonstrated in pregnant rats.26 We demonstrated lower cerebral levels of glutamate, myo-inositol, choline, and creatine in both women with preeclampsia and women with normal pregnancies. All 4 metabolites correlated with each other, indicating that their decline was part of the same biological mechanism (Table 3). For myo-inositol, choline, and creatine, there was a clear correlation with plasma osmolality and sodium. This strengthens the argument that the observed decrease in these compounds is indeed a response to the lower plasma osmolality associated with pregnancy. Cerebral levels of glutamate were, however, not correlated with either plasma osmolality or sodium. If this is due to a separate biological mechanism or simply caused by a lack of statistical power, remains to be explained. Interestingly, glutamate, i.e., the most common excitatory neurotransmitter in the brain, was lower in women with preeclampsia than in women with normal pregnancies. This is an intriguing finding considering the incompletely chartered pathophysiology of seizures in preeclampsia. There has been speculation that extrusion of glutamate from cells raise extracellular levels of the neurotransmitter and thereby causing hyperexcitability.11 Plasma levels of glutamate also differed between women with preeclampsia and normal pregnancy, with the highest concentrations seen in preeclampsia, in line with findings from previous studies.27,28 There is evidence that plasma glutamate levels negatively correlates with worse outcomes in stroke and traumatic brain injury.29 Glutamate can also be released from aggregating platelets and polymorphonuclear leukocytes, which may influence permeability of endothelial cells, including brain endothelium,30,31 and pregnancy alters expression of certain glutamate receptors on peripheral blood mononuclear cells.32 An increased endothelial permeability has been proposed to play a crucial role in the pathophysiology of posterior reversible encephalopathy syndrome, a clinical-radiological entity seen in eclampsia and severe preeclampsia.33 Although our results with lower cerebral glutamate, myo-inositol, choline, and creatine are consistent with what has previously been shown in hyponatremia12,34 our detected differences in cerebral organic osmolytes are small, and the clinical relevance is not clear. In the previous studies, myo-inositol was 40–49% lower as compared to 20% in our population.12,34 All women in our study had plasma osmolality and sodium levels within normal range, although the pregnant women had lower levels. It remains to be established if and how these reductions in cerebral organic osmolytes contribute to the hyperexcitability observed in pregnancy, especially preeclampsia, and if these changes can explain how eclamptic seizures can arise without any prodromal symptoms or hypertension. DISCLOSURE The authors report no conflict of interest. ACKNOWLEDGMENT Funding for the present study has been received from Uppsala-Örebro Regional Research Council (project number RFR-479351) and the Swedish Research Council (project number 2014–3561). REFERENCES 1. Zeeman GG . Neurologic complications of pre-eclampsia . Semin Perinatol 2009 ; 33 : 166 – 172 . Google Scholar CrossRef Search ADS PubMed 2. Brewer J , Owens MY , Wallace K , Reeves AA , Morris R , Khan M , LaMarca B , Martin JN Jr . Posterior reversible encephalopathy syndrome in 46 of 47 patients with eclampsia . Am J Obstet Gynecol 2013 ; 208 : 468.e1 – 468.e6 . Google Scholar CrossRef Search ADS 3. Bartynski WS . Posterior reversible encephalopathy syndrome, part 2: controversies surrounding pathophysiology of vasogenic edema . AJNR Am J Neuroradiol 2008 ; 29 : 1043 – 1049 . Google Scholar CrossRef Search ADS PubMed 4. Cooray SD , Edmonds SM , Tong S , Samarasekera SP , Whitehead CL . Characterization of symptoms immediately preceding eclampsia . Obstet Gynecol 2011 ; 118 : 995 – 999 . Google Scholar CrossRef Search ADS PubMed 5. Sanghavi M , Rutherford JD . Cardiovascular physiology of pregnancy . Circulation 2014 ; 130 : 1003 – 1008 . Google Scholar CrossRef Search ADS PubMed 6. Tkachenko O , Shchekochikhin D , Schrier RW . Hormones and hemodynamics in pregnancy . Int J Endocrinol Metab 2014 ; 12 : e14098 . Google Scholar CrossRef Search ADS PubMed 7. Davison JM , Gilmore EA , Dürr J , Robertson GL , Lindheimer MD . Altered osmotic thresholds for vasopressin secretion and thirst in human pregnancy . Am J Physiol 1984 ; 246 : F105 – F109 . Google Scholar PubMed 8. Lindheimer MD , Davison JM . Osmoregulation, the secretion of arginine vasopressin and its metabolism during pregnancy . Eur J Endocrinol 1995 ; 132 : 133 – 143 . Google Scholar CrossRef Search ADS PubMed 9. Zulfikaroglu E , Islimye M , Tonguc EA , Payasli A , Isman F , Var T , Danisman N . Circulating levels of copeptin, a novel biomarker in pre-eclampsia . J Obstet Gynaecol Res 2011 ; 37 : 1198 – 1202 . Google Scholar CrossRef Search ADS PubMed 10. Santillan MK , Santillan DA , Scroggins SM , Min JY , Sandgren JA , Pearson NA , Leslie KK , Hunter SK , Zamba GK , Gibson-Corley KN , Grobe JL . Vasopressin in preeclampsia: a novel very early human pregnancy biomarker and clinically relevant mouse model . Hypertension 2014 ; 64 : 852 – 859 . Google Scholar CrossRef Search ADS PubMed 11. Verbalis JG . Brain volume regulation in response to changes in osmolality . Neuroscience 2010 ; 168 : 862 – 870 . Google Scholar CrossRef Search ADS PubMed 12. Videen JS , Michaelis T , Pinto P , Ross BD . Human cerebral osmolytes during chronic hyponatremia. A proton magnetic resonance spectroscopy study . J Clin Invest 1995 ; 95 : 788 – 793 . Google Scholar CrossRef Search ADS PubMed 13. Andrew RD . Seizure and acute osmotic change: clinical and neurophysiological aspects . J Neurol Sci 1991 ; 101 : 7 – 18 . Google Scholar CrossRef Search ADS PubMed 14. Rutherford JM , Moody A , Crawshaw S , Rubin PC . Magnetic resonance spectroscopy in pre-eclampsia: evidence of cerebral ischaemia . BJOG 2003 ; 110 : 416 – 423 . Google Scholar CrossRef Search ADS PubMed 15. Sengar AR , Gupta RK , Dhanuka AK , Roy R , Das K . MR imaging, MR angiography, and MR spectroscopy of the brain in eclampsia . AJNR Am J Neuroradiol 1997 ; 18 : 1485 – 1490 . Google Scholar PubMed 16. Nelander M , Weis J , Bergman L , Larsson A , Wikström AK , Wikström J . Cerebral magnesium levels in preeclampsia; a phosphorus magnetic resonance spectroscopy study . Am J Hypertens 2017 ; 30 : 667 – 672 . Google Scholar CrossRef Search ADS PubMed 17. Tranquilli AL , Dekker G , Magee L , Roberts J , Sibai BM , Steyn W , Zeeman GG , Brown MA . The classification, diagnosis and management of the hypertensive disorders of pregnancy: a revised statement from the ISSHP . Pregnancy Hypertens 2014 ; 4 : 97 – 104 . Google Scholar CrossRef Search ADS PubMed 18. Tranquilli AL , Brown MA , Zeeman GG , Dekker G , Sibai BM . The definition of severe and early-onset preeclampsia. Statements from the International Society for the Study of Hypertension in Pregnancy (ISSHP) . Pregnancy Hypertens 2013 ; 3 : 44 – 47 . Google Scholar CrossRef Search ADS PubMed 19. Marsál K , Persson PH , Larsen T , Lilja H , Selbing A , Sultan B . Intrauterine growth curves based on ultrasonically estimated foetal weights . Acta Paediatr 1996 ; 85 : 843 – 848 . Google Scholar CrossRef Search ADS PubMed 20. Provencher SW . Estimation of metabolite concentrations from localized in vivo proton NMR spectra . Magn Reson Med 1993 ; 30 : 672 – 679 . Google Scholar CrossRef Search ADS PubMed 21. Davison JM . Renal haemodynamics and volume homeostasis in pregnancy . Scand J Clin Lab Invest Suppl 1984 ; 169 : 15 – 27 . Google Scholar CrossRef Search ADS PubMed 22. van der Grond J , Balm R , Kappelle LJ , Eikelboom BC , Mali WP . Cerebral metabolism of patients with stenosis or occlusion of the internal carotid artery. A 1H-MR spectroscopic imaging study . Stroke 1995 ; 26 : 822 – 828 . Google Scholar CrossRef Search ADS PubMed 23. Verbalis JG , Gullans SR . Hyponatremia causes large sustained reductions in brain content of multiple organic osmolytes in rats . Brain Res 1991 ; 567 : 274 – 282 . Google Scholar CrossRef Search ADS PubMed 24. Moritz ML , Ayus JC . The pathophysiology and treatment of hyponatraemic encephalopathy: an update . Nephrol Dial Transplant 2003 ; 18 : 2486 – 2491 . Google Scholar CrossRef Search ADS PubMed 25. Lang F , Busch GL , Ritter M , Völkl H , Waldegger S , Gulbins E , Häussinger D . Functional significance of cell volume regulatory mechanisms . Physiol Rev 1998 ; 78 : 247 – 306 . Google Scholar CrossRef Search ADS PubMed 26. Law RO . Effects of pregnancy on the contents of water, taurine, and total amino nitrogen in rat cerebral cortex . J Neurochem 1989 ; 53 : 300 – 302 . Google Scholar CrossRef Search ADS PubMed 27. Terán Y , Ponce O , Betancourt L , Hernández L , Rada P . Amino acid profile of plasma and cerebrospinal fluid in preeclampsia . Pregnancy Hypertens 2012 ; 2 : 416 – 422 . Google Scholar CrossRef Search ADS PubMed 28. Kenny LC , Broadhurst D , Brown M , Dunn WB , Redman CW , Kell DB , Baker PN . Detection and identification of novel metabolomic biomarkers in preeclampsia . Reprod Sci 2008 ; 15 : 591 – 597 . Google Scholar CrossRef Search ADS PubMed 29. Castellanos M , Sobrino T , Pedraza S , Moldes O , Pumar JM , Silva Y , Serena J , García-Gil M , Castillo J , Dávalos A . High plasma glutamate concentrations are associated with infarct growth in acute ischemic stroke . Neurology 2008 ; 71 : 1862 – 1868 . Google Scholar CrossRef Search ADS PubMed 30. Collard CD , Park KA , Montalto MC , Alapati S , Buras JA , Stahl GL , Colgan SP . Neutrophil-derived glutamate regulates vascular endothelial barrier function . J Biol Chem 2002 ; 277 : 14801 – 14811 . Google Scholar CrossRef Search ADS PubMed 31. Tremolizzo L , DiFrancesco JC , Rodriguez-Menendez V , Sirtori E , Longoni M , Cassetti A , Bossi M , El Mestikawy S , Cavaletti G , Ferrarese C . Human platelets express the synaptic markers VGLUT1 and 2 and release glutamate following aggregation . Neurosci Lett 2006 ; 404 : 262 – 265 . Google Scholar CrossRef Search ADS PubMed 32. Bhandage AK , Jin Z , Hellgren C , Korol SV , Nowak K , Williamsson L , Sundström-Poromaa I , Birnir B . AMPA, NMDA and kainate glutamate receptor subunits are expressed in human peripheral blood mononuclear cells (PBMCs) where the expression of GluK4 is altered by pregnancy and GluN2D by depression in pregnant women . J Neuroimmunol 2017 ; 305 : 51 – 58 . Google Scholar CrossRef Search ADS PubMed 33. Marra A , Vargas M , Striano P , Del Guercio L , Buonanno P , Servillo G . Posterior reversible encephalopathy syndrome: the endothelial hypotheses . Med Hypotheses 2014 ; 82 : 619 – 622 . Google Scholar CrossRef Search ADS PubMed 34. Häussinger D , Laubenberger J , vom Dahl S , Ernst T , Bayer S , Langer M , Gerok W , Hennig J . Proton magnetic resonance spectroscopy studies on human brain myo-inositol in hypo-osmolarity and hepatic encephalopathy . Gastroenterology 1994 ; 107 : 1475 – 1480 . Google Scholar CrossRef Search ADS PubMed © American Journal of Hypertension, Ltd 2018. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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American Journal of HypertensionOxford University Press

Published: Feb 3, 2018

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