The Superior Thalamic Vein and its Variations: A Proposed Classification

The Superior Thalamic Vein and its Variations: A Proposed Classification Abstract BACKGROUND The superior thalamic vein (STV) was first described comprehensively by Ferner in 1958 as the most prominent thalamic vein; it originates from the central superior portion of the thalamus, coursing medially to the third ventricular thalamic surface, where it turns posteriorly to parallel the internal cerebral vein (ICV) before ending into its posterior portion. Since historical anatomic and angiographic studies in the pre-computed tomography (CT)/magnetic resonance imaging era, the STV has not been investigated. OBJECTIVE To describe the anatomic course of the STV with its variations, and to propose a classification system based on its draining pattern. METHODS We retrospectively screened our imaging database for 50 patients who had a CT-angiography with predefined parameters. The images were independently reviewed by 3 neurosurgeons and 1 neuroradiologist to classify the STV into 4 types: type 1A—drainage into the anterior portion of the ICV, type 1B—drainage into the posterior portion of the ICV, type 2—drainage into the vein of Rosenthal, type 3—drainage into a medial (3A) or lateral (3B) atrial vein, and type 4—drainage into the vein of Galen. RESULTS In 50 patients, we could identify 96 STVs. In 2 hemispheres, the STV was doubled. The 92 single STVs were classified as type 1A in 25 hemispheres (27.2%), type 1B in 45 (48.9%), type 2 in 12 (13.0%), type 3A in 8 (8.7%), type 3B in 1 (1.1%), and type 4 in 1 (1.1%). CONCLUSION The draining pattern of the STV varies widely from the initial description. Thalamic veins, Thalamus, Superior thalamic vein, Cerebral deep venous system ABBREVIATIONS ABBREVIATIONS CT computed tomography CTA computed tomography angiography HU hounsfi-eld units ICV internal cerebral vein MRI magnetic resonance imaging STV superior thalamic vein Surgical procedures for thalamic lesions such as resections of tumors and cavernomas, stereotactic biopsies, or placement of electrodes for deep brain stimulation have been increasingly performed in recent years.1-3 Advances in neuroimaging have allowed for a better understanding of the surgical anatomy of the thalamus and its vasculature and encouraged neurosurgeons to target this critical brain region. While much attention has been given to the intrinsic anatomy of thalamic nuclei with their connections and pattern of the arterial supply, the veins of the thalamus remain underinvestigated. While anatomic studies on brain sections by Schlesinger in 19764 gave a detailed picture of the intrinsic thalamic venous anatomy, radioanatomic investigations of the thalamic veins and their draining pattern have not been performed since angiographic studies in the pre-computed tomography (CT) and magnetic resonance imaging (MRI) era.5-11 These angiographic studies were remarkably accurate and detailed given the scarce technological resources at that time; however, they were hampered by the resolution and the limitation to lateral projections.6 The superior thalamic vein (STV), which was referred to as vena principalis thalami in the anatomic descriptions by Schlesinger,4 was the largest and most consistently identified vein of the thalamus. With modern imaging modalities readily available, we recognized the anatomic course of the STV and its junction into the next vein to be highly variable from the historical descriptions. We therefore thought to further evaluate its anatomic variations, as a precise knowledge and understanding is crucial when operating in such a critical area. We present here our proposed classification of the STV based on its draining pattern. METHODS CT-angiography Computed tomography angiography (CTA) was performed on 1 of 2 64-slice CT scanners according to standardized protocols. After intravenous administration of 70 mL of nonionic iodinated contrast medium (iomeron 400; 400 mg of iodine per milliliter) at a rate of 4 mL/s by using a power injector, followed by a 40-mL saline flush, CTA was initiated by using bolus tracking (CareBolus; Siemens, Munich, Germany) with an attenuation threshold of 150 hounsfield units (HU) within a circular region of interest in the lumen of the aorta ascendance. The scanning protocol was as follows: 64 × 0.6 collimation, 1.2 pitch, 0.5 s rotational time, 120 kV tube voltage, and automated tube current modulation with 200 mAs as reference tube current. CTA data sets were reconstructed to 1 mm of section thickness and stored for later interactive multiplanar and 3-dimensional evaluation. To ensure an adequate enhancement of the thalamic veins, we selected only CTA data sets that showed a mean attenuation of ≥150 HU within the internal cerebral vein (ICV). Patient Selection After approval of the Ethics Committee of the Medical University Vienna, our imaging database was screened for patients who had a CTA between January 2015 and January 2016 with the above outlined criteria and had no pathology in or around the thalamus. As this was performed retrospectively, no additional patient consent was obtained. Fifty patients (28 females, 22 males; mean age 52, range from 23 to 84) were included. Evaluation The bilateral STVs were evaluated in the axial, coronal, and sagittal planes as well as 3-dimensional images independently by 1 experienced neuroradiologist (staff), 2 neurosurgeons (staff, chief resident), and 1 medical student. Drawings of the relevant venous anatomy of every patient were performed. Where disagreement regarding the anatomic course occurred, consensus was obtained by an individual case discussion. We classified the STV according to its anatomic course and junction into the next vein. For the purpose of this classification, we divided the ICV into an anterior portion, defined as the part from the origin at the Foramen of Monro to the beginning of the vein's lateral convexity, and a posterior portion, defined as the part from the start of the convexity to the junction into the great vein of Galen.6 Type 1A The STV has a comparably short course. It runs either medially in a slightly rostrally convex curve or straight posteromedially towards the third ventricle, where it joins the ICV in its anterior portion, immediately after exiting the thalamus. A strict course perpendicular to the ICV was not observed. Type 1B This type corresponds to the original description by Ferner.7 The STV runs anteromedially towards the third ventricle before turning posteriorly at an almost 90° angle and continuing along the ventricular surface of the thalamus. In axial view, this appears as a hook-like curve, which is typical and similar for types 1B, 2, 3, and 4. In type 1B, the STV parallels the ICV backwards, where it joins the ICV in its posterior portion. Type 2 In this group, the STV has a similar course as type 1B, but continues its course to the level of the pulvinar thalami, where it turns laterally and downwards to join the vein of Rosenthal. Type 3 This type has the typical hook-like appearance before coursing posteriorly to end either in a medial atrial vein (3A) or a lateral atrial vein (3B) at the level of the posterior thalamus. Type 4 Similar to types 1B, 2, and 3, type 4 curves backward parallel to the ICV to enter the great vein of Galen. Statistical Analysis The differences in anatomic variations regarding laterality and gender were assessed using student's t-test, with a value of P < .05 considered statistically significant. Data were analyzed using SPSS version 23.0 (SPSS, IBM, Armonk, New York) RESULTS In 50 patients we identified 96 STVs. In 2 hemispheres, the STV was doubled (4 STVs), resulting in 92 single STVs. In the remaining 6 thalami, an anterior thalamic vein replaced the STV. Only in 1 thalamus was a simultaneous drainage via the STV together with an accessory anterior thalamic vein identified. The observed anterior thalamic veins coursed towards the Foramen of Monro and drained into the septal vein in 4, the thalamostriate vein in 2, and a caudate vein in 1 hemisphere. The following categorization is based on the single STVs (n = 92). Type 1 In 70 out of 92 hemispheres (76.1%), the STV was classified as type 1 with an equal distribution between left (n = 36) and right (n = 35) and male (20) and female (27) patients. In only 23 patients (46%) was a type 1 draining pattern of the STV depicted bilaterally. Type 1A In 25 out of 92 hemispheres (27.2%), the STV was classified as type 1A (left n = 17, right n = 8; male 11 and female 12; Figure 1). In only 2 patients (4%), the STV had a type 1A course bilaterally. FIGURE 1. View largeDownload slide STV type 1A—drainage into the anterior portion of the ICV. FIGURE 1. View largeDownload slide STV type 1A—drainage into the anterior portion of the ICV. Type 1B In 45 hemispheres (48.9%; left 19; right 26; male 14, female 22), the STV was classified as type 1B (Figure 2). In 9 patients (18%), a bilateral type 1B was seen. FIGURE 2. View largeDownload slide STV type 1B—drainage into the posterior portion of the ICV. FIGURE 2. View largeDownload slide STV type 1B—drainage into the posterior portion of the ICV. Type 2 Twelve thalami (13.0%; left n = 7, right n = 5) were drained by a type 2 STV (Figure 3). Only 1 patient (2%) had a type 2 drainage bilaterally. FIGURE 4. View largeDownload slide STV type 2—drainage into the vein of Rosenthal. FIGURE 4. View largeDownload slide STV type 2—drainage into the vein of Rosenthal. Type 3 Nine hemispheres (9.8%; left n = 5, right n = 4) showed a type 3 STV with 2 patients (4%) showing it bilaterally (Figure 4). In both patients, the STV drained into a medial atrial vein (type 3A). From the remaining 5 hemispheres, 4 STVs were classified as type 3A and 1 as type 3B. FIGURE 3. View largeDownload slide STV type 3—drainage into the medial (A) atrial vein. FIGURE 3. View largeDownload slide STV type 3—drainage into the medial (A) atrial vein. Type 4 In only 1 hemisphere (1.1%), the STV drained directly into the vein of Galen (Figure 5). FIGURE 5. View largeDownload slide STV type 4—direct drainage into the vein of Galen. FIGURE 5. View largeDownload slide STV type 4—direct drainage into the vein of Galen. Doubled STV (n = 4) In 2 thalami, the STV was doubled. In the first case, an anterior STV ran straight dorsally crossing a posterior STV ending in the vein of Rosenthal. The second posterior STV coursed medially towards the ventricle in a posterolateral curve and joined the ICV in its posterior portion. In the second case, an anterior STV coursed medially in the standard fashion, but joined the STV from the contralateral side before ending in the ICV in the posterior portion. The posterior STV ran dorsally in a lateral bend and joined the ICV in its posterior portion. No statistical difference in the STV anatomy was found with regard to the side (left or right) of the thalamus or gender of the patients. DISCUSSION To the best of our knowledge, our study represents the first detailed description of the anatomic course of the STV based on imaging in the CT/MRI era. On angiographic studies, the STV was firstly identified by Johanson in 19548 and then comprehensively described by Ferner in 19587 as a union of several slender veins in the central superior portion of the thalamus, which courses medially to the surface of the thalamus, where it emerges from the taenia thalami 5 mm above the pineal gland on lateral projections. Here, it turns posteriorinferiorly at an almost 90° angle to follow the ICV in close relation for some 2 cm and to end into its posterior part.7,8 Based on our multiplanar and 3-dimensional CTA investigations, we could demonstrate that (1) 51.1% of the cases were different from the purportedly typical draining pattern of the STV, and (2) 27.2% of the STV draining into the ICV differed from the original description and ended in the anterior portion instead of the posterior portion of the ICV (Figure 6). FIGURE 6. View largeDownload slide Three-dimensional CTA showing (1) septal vein, (2) thalamostriate vein, (3) internal cerebral vein, (4) left type 1B STV, (5) right type 1A STV FIGURE 6. View largeDownload slide Three-dimensional CTA showing (1) septal vein, (2) thalamostriate vein, (3) internal cerebral vein, (4) left type 1B STV, (5) right type 1A STV The venous drainage of the thalamus is often falsely attributed to the thalamostriate vein. However, while the term thalamostriate vein implies a role in the venous drainage of the thalamus, multiple investigators in the past had already demonstrated that the true thalamic veins very rarely drain into the thalamostriate vein, but that the superior and medial portions of the thalamus were typically drained by the ICV and the inferior and posterior parts of the thalamus by the basal or posterior mesencephalic veins.7-11 With regard to the venous drainage of the superior and medial portions of the thalamus into the ICV, Beau and Rabischong11 stated that this was accomplished by a posteriomedial and anteromedial thalamic vein. As later classified by Giudicelli et al,9 these would correspond to the STV draining into the posterior portion of the ICV and the anterior thalamic vein draining into the anterior portion of the ICV.9 Based on our observations, this purportedly typical venous draining pattern of the superior and medial thalamus into the ICV was not uniform. Only 64.3% followed the originally described course and entered the ICV in its posterior portion (type 1B), 35.7%, however, differed from this original description and drained into the anterior portion (type 1A). This short course of the STV into the anterior part of the ICV has not been mentioned in previous classifications. One potential explanation could have been that, in contrast to all other types, this type 1A did not have the hook-like appearance at its origin. According to the classification suggested by Giudicelli et al,9 one could consider this type 1A as an anterior thalamic vein.9 The true anterior thalamic vein, however, courses towards the Foramen of Monro, where it enters the ICV directly at its origin or into subependymal veins like the thalamostriate or septal veins.9 As a consequence of this distinction of a type 1A STV, a true anterior thalamic vein, which was previously acknowledged as a constant finding, was observed in only 7% of our cases.6 If present, it replaced the STV (85.7%) rather than being only an accessory. Our analysis further showed that the percentage of patients with an STV not draining into the ICV was much higher than previously described, reaching 23.9% of our cases (type 2-4). The STV could course along the entire third ventricular surface of the thalamus and around the thalamus to finally course downwards and laterally to enter the vein of Rosenthal (type 2). This draining pattern, which was previously described as exceptional, was present in 13.0% of our cases. This type could be misinterpreted as being part of the posterior thalamic veins, which, however, by definition only drained the posterolateral part of the thalamus.9 Less frequently, but still in 9.6% of the cases, the STV followed the typical course backward, but instead of ending in the ICV it drained into an atrial vein (type 3). According to the classification of the atrial veins by Stein and Rosenbaum,6 we observed drainage into a medial atrial vein in 8.7% of the cases (type 3A) and into a lateral atrial vein in 1.1% (type 3B). In general, these findings would argue against the historical apprehension that the deep venous system is less variable than the superficial venous system in humans.7 Accordingly, it was our observation that the draining pattern of the STV could vary significantly between both thalami in the same patient. In only 15 patients (30%), the STV ended symmetrically in the same segment of its terminate vein. For the most common type 1 STV, only 46% of all patients had the same draining pattern bilaterally. Therefore, our findings contribute to the general understanding of the venous thalamic draining system and allow for a classification of the normal anatomy of the STV. This seems relevant as the STV is the largest and most consistently identified thalamic vein on preoperative imaging, and is thereby of great importance when targeting the thalamus surgically. Schneider et al,3 for instance, recently identified the STV as the major obstacle and pitfall in trajectory planning for targeting the lateral habenular complex in deep brain stimulation. In the presence of a prominent STV, it constituted an impassable obstacle in the majority of the cases through a standard trajectory. Based on these findings, they suggested an alternative, steeper trajectory to the lateral habenular complex.3 For thalamic tumors, a detailed analysis of the STV anatomy might contribute to a better understanding of the origin and growth pattern of the tumor with implications for the choice of the best surgical strategy (Figure 7). For instance, in tumors arising from the ventral part of the thalamus, the STV is typically displaced dorsally and superiorly, in tumors originating in the posterior portion of the thalamus the STV could be displaced anteriorly or could be incorporated by the tumor. FIGURE 7. View largeDownload slide A, T1-weighted image showing a contrast enhancing tumor occluding the third ventricle. B, CTA in axial view depicting the right (arrow) and left (asterisk) STV. The right STV is displaced laterally by the tumor indicating that the origin of the tumor lies within the most medial part of the right thalamus. The STV marks the contrast enhancing border of the tumor and was used as an intraoperative landmark for where to stop the resection. C, T1-weighted image after transcallosal resection of the contrast enhancing part of the tumor showing the right STV. Histology revealed a diffuse glioma of the midline H3K27M. FIGURE 7. View largeDownload slide A, T1-weighted image showing a contrast enhancing tumor occluding the third ventricle. B, CTA in axial view depicting the right (arrow) and left (asterisk) STV. The right STV is displaced laterally by the tumor indicating that the origin of the tumor lies within the most medial part of the right thalamus. The STV marks the contrast enhancing border of the tumor and was used as an intraoperative landmark for where to stop the resection. C, T1-weighted image after transcallosal resection of the contrast enhancing part of the tumor showing the right STV. Histology revealed a diffuse glioma of the midline H3K27M. Moreover, in pineal region tumors, a meticulous analysis and understanding of the STV contributes to the decision on the best surgical approach. As we could show in our analysis, the STV could course all the way back to the vein of Galen complex (type 2 and 4). Thereby, the STV could be surgically relevant not only for pineal tumors protruding into the third ventricle, but also for tumors predominately located posteriorly to the thalamus. Limitations Our analysis and classification of the STV anatomy is limited by the small sample size and the technical limitations of CTA, which was the single imaging modality we used. We cannot exclude that other imaging modalities, such as 3-dimensional reconstruction of angiographic data with digital subtraction, could have potentially influenced our results. Furthermore, even though we tried to minimize the confounding factor from the interobserver variability by having a consensus decision from 3 different raters, we have to admit that assignment to a certain type was sometimes challenging. CONCLUSION The draining pattern of the STV is variable and can be classified into 4 types. The hook-like appearance on axial views facilitates proper identification of the STV. Careful analysis of its anatomic course might help in understanding the origin and growth pattern of thalamic lesions and could serve as an intraoperative landmark. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Puget S , Crimmins DW , Garnett MR et al. Thalamic tumors in children: a reappraisal . J Neurosurg . 2007 ; 106 : 354 - 362 . Google Scholar PubMed 2. Rangel-Castilla L , Spetzler RF . The 6 thalamic regions: surgical approaches to thalamic cavernous malformations, operative results, and clinical outcomes . J Neurosurg . 2015 ; 123 : 676 - 685 . Google Scholar CrossRef Search ADS PubMed 3. Schneider TM , Beynon C , Sartorius A , Unterberg AW , Kiening KL . Deep brain stimulation of the lateral habenular complex in treatment-resistant depression: traps and pitfalls of trajectory choice . Neurosurgery . 2013 ; 72 ( 2 Suppl Operative ): ons184 - ons193 . Google Scholar PubMed 4. Schlesinger B , ed. The Upper Brainstem in the Human: Its Nuclear Configuration and Vascular Supply . Berlin, Heidelberg : Springer Verlag ; 1976 . Google Scholar CrossRef Search ADS 5. Schlesinger B. The venous drainage of the brain, with special reference to the Galenic system . Brain . 1939 ; 62 : 274 - 291 . Google Scholar CrossRef Search ADS 6. Stein RL , Rosenbaum AE . Deep supratentorial veins. Section I. Normal deep cerebral venous system . In: Newton TH , Potts DG , eds. Radiology of the Skull and Brain: Angiography . Vol 2 . St. Louis, London : Mosby ; 1974 : 1903 - 1998 . 7. Ferner H . Anatomische und phlebographische Studien der inneren Hirnvenen des Menschen . Z Anat Entwicklungsgesch . 1958 ; 120 : 481 - 491 . Google Scholar CrossRef Search ADS PubMed 8. Johanson C. The central veins and deep dural sinuses of the brain; an anatomical and angiographic study . Acta Radiol Suppl . 1954 ; 107 : 8 - 184 . Google Scholar PubMed 9. Giudicelli G , Salamon G . The veins of the thalamus . Neuroradiology . 1970 ; 1 : 92 - 98 . Google Scholar CrossRef Search ADS 10. Stephens RB , Stilwell DL , eds. Arteries and Veins of the Human Brain . Springfield IL : Charles C Thomas ; 1969 . 11. Beau A , Rabischong P . Les veins internes du cerveau . C R Assoc Anat . 1959 ; 102 : 175 - 181 . Copyright © 2017 by the Congress of Neurological Surgeons 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 Operative Neurosurgery Oxford University Press

The Superior Thalamic Vein and its Variations: A Proposed Classification

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Abstract

Abstract BACKGROUND The superior thalamic vein (STV) was first described comprehensively by Ferner in 1958 as the most prominent thalamic vein; it originates from the central superior portion of the thalamus, coursing medially to the third ventricular thalamic surface, where it turns posteriorly to parallel the internal cerebral vein (ICV) before ending into its posterior portion. Since historical anatomic and angiographic studies in the pre-computed tomography (CT)/magnetic resonance imaging era, the STV has not been investigated. OBJECTIVE To describe the anatomic course of the STV with its variations, and to propose a classification system based on its draining pattern. METHODS We retrospectively screened our imaging database for 50 patients who had a CT-angiography with predefined parameters. The images were independently reviewed by 3 neurosurgeons and 1 neuroradiologist to classify the STV into 4 types: type 1A—drainage into the anterior portion of the ICV, type 1B—drainage into the posterior portion of the ICV, type 2—drainage into the vein of Rosenthal, type 3—drainage into a medial (3A) or lateral (3B) atrial vein, and type 4—drainage into the vein of Galen. RESULTS In 50 patients, we could identify 96 STVs. In 2 hemispheres, the STV was doubled. The 92 single STVs were classified as type 1A in 25 hemispheres (27.2%), type 1B in 45 (48.9%), type 2 in 12 (13.0%), type 3A in 8 (8.7%), type 3B in 1 (1.1%), and type 4 in 1 (1.1%). CONCLUSION The draining pattern of the STV varies widely from the initial description. Thalamic veins, Thalamus, Superior thalamic vein, Cerebral deep venous system ABBREVIATIONS ABBREVIATIONS CT computed tomography CTA computed tomography angiography HU hounsfi-eld units ICV internal cerebral vein MRI magnetic resonance imaging STV superior thalamic vein Surgical procedures for thalamic lesions such as resections of tumors and cavernomas, stereotactic biopsies, or placement of electrodes for deep brain stimulation have been increasingly performed in recent years.1-3 Advances in neuroimaging have allowed for a better understanding of the surgical anatomy of the thalamus and its vasculature and encouraged neurosurgeons to target this critical brain region. While much attention has been given to the intrinsic anatomy of thalamic nuclei with their connections and pattern of the arterial supply, the veins of the thalamus remain underinvestigated. While anatomic studies on brain sections by Schlesinger in 19764 gave a detailed picture of the intrinsic thalamic venous anatomy, radioanatomic investigations of the thalamic veins and their draining pattern have not been performed since angiographic studies in the pre-computed tomography (CT) and magnetic resonance imaging (MRI) era.5-11 These angiographic studies were remarkably accurate and detailed given the scarce technological resources at that time; however, they were hampered by the resolution and the limitation to lateral projections.6 The superior thalamic vein (STV), which was referred to as vena principalis thalami in the anatomic descriptions by Schlesinger,4 was the largest and most consistently identified vein of the thalamus. With modern imaging modalities readily available, we recognized the anatomic course of the STV and its junction into the next vein to be highly variable from the historical descriptions. We therefore thought to further evaluate its anatomic variations, as a precise knowledge and understanding is crucial when operating in such a critical area. We present here our proposed classification of the STV based on its draining pattern. METHODS CT-angiography Computed tomography angiography (CTA) was performed on 1 of 2 64-slice CT scanners according to standardized protocols. After intravenous administration of 70 mL of nonionic iodinated contrast medium (iomeron 400; 400 mg of iodine per milliliter) at a rate of 4 mL/s by using a power injector, followed by a 40-mL saline flush, CTA was initiated by using bolus tracking (CareBolus; Siemens, Munich, Germany) with an attenuation threshold of 150 hounsfield units (HU) within a circular region of interest in the lumen of the aorta ascendance. The scanning protocol was as follows: 64 × 0.6 collimation, 1.2 pitch, 0.5 s rotational time, 120 kV tube voltage, and automated tube current modulation with 200 mAs as reference tube current. CTA data sets were reconstructed to 1 mm of section thickness and stored for later interactive multiplanar and 3-dimensional evaluation. To ensure an adequate enhancement of the thalamic veins, we selected only CTA data sets that showed a mean attenuation of ≥150 HU within the internal cerebral vein (ICV). Patient Selection After approval of the Ethics Committee of the Medical University Vienna, our imaging database was screened for patients who had a CTA between January 2015 and January 2016 with the above outlined criteria and had no pathology in or around the thalamus. As this was performed retrospectively, no additional patient consent was obtained. Fifty patients (28 females, 22 males; mean age 52, range from 23 to 84) were included. Evaluation The bilateral STVs were evaluated in the axial, coronal, and sagittal planes as well as 3-dimensional images independently by 1 experienced neuroradiologist (staff), 2 neurosurgeons (staff, chief resident), and 1 medical student. Drawings of the relevant venous anatomy of every patient were performed. Where disagreement regarding the anatomic course occurred, consensus was obtained by an individual case discussion. We classified the STV according to its anatomic course and junction into the next vein. For the purpose of this classification, we divided the ICV into an anterior portion, defined as the part from the origin at the Foramen of Monro to the beginning of the vein's lateral convexity, and a posterior portion, defined as the part from the start of the convexity to the junction into the great vein of Galen.6 Type 1A The STV has a comparably short course. It runs either medially in a slightly rostrally convex curve or straight posteromedially towards the third ventricle, where it joins the ICV in its anterior portion, immediately after exiting the thalamus. A strict course perpendicular to the ICV was not observed. Type 1B This type corresponds to the original description by Ferner.7 The STV runs anteromedially towards the third ventricle before turning posteriorly at an almost 90° angle and continuing along the ventricular surface of the thalamus. In axial view, this appears as a hook-like curve, which is typical and similar for types 1B, 2, 3, and 4. In type 1B, the STV parallels the ICV backwards, where it joins the ICV in its posterior portion. Type 2 In this group, the STV has a similar course as type 1B, but continues its course to the level of the pulvinar thalami, where it turns laterally and downwards to join the vein of Rosenthal. Type 3 This type has the typical hook-like appearance before coursing posteriorly to end either in a medial atrial vein (3A) or a lateral atrial vein (3B) at the level of the posterior thalamus. Type 4 Similar to types 1B, 2, and 3, type 4 curves backward parallel to the ICV to enter the great vein of Galen. Statistical Analysis The differences in anatomic variations regarding laterality and gender were assessed using student's t-test, with a value of P < .05 considered statistically significant. Data were analyzed using SPSS version 23.0 (SPSS, IBM, Armonk, New York) RESULTS In 50 patients we identified 96 STVs. In 2 hemispheres, the STV was doubled (4 STVs), resulting in 92 single STVs. In the remaining 6 thalami, an anterior thalamic vein replaced the STV. Only in 1 thalamus was a simultaneous drainage via the STV together with an accessory anterior thalamic vein identified. The observed anterior thalamic veins coursed towards the Foramen of Monro and drained into the septal vein in 4, the thalamostriate vein in 2, and a caudate vein in 1 hemisphere. The following categorization is based on the single STVs (n = 92). Type 1 In 70 out of 92 hemispheres (76.1%), the STV was classified as type 1 with an equal distribution between left (n = 36) and right (n = 35) and male (20) and female (27) patients. In only 23 patients (46%) was a type 1 draining pattern of the STV depicted bilaterally. Type 1A In 25 out of 92 hemispheres (27.2%), the STV was classified as type 1A (left n = 17, right n = 8; male 11 and female 12; Figure 1). In only 2 patients (4%), the STV had a type 1A course bilaterally. FIGURE 1. View largeDownload slide STV type 1A—drainage into the anterior portion of the ICV. FIGURE 1. View largeDownload slide STV type 1A—drainage into the anterior portion of the ICV. Type 1B In 45 hemispheres (48.9%; left 19; right 26; male 14, female 22), the STV was classified as type 1B (Figure 2). In 9 patients (18%), a bilateral type 1B was seen. FIGURE 2. View largeDownload slide STV type 1B—drainage into the posterior portion of the ICV. FIGURE 2. View largeDownload slide STV type 1B—drainage into the posterior portion of the ICV. Type 2 Twelve thalami (13.0%; left n = 7, right n = 5) were drained by a type 2 STV (Figure 3). Only 1 patient (2%) had a type 2 drainage bilaterally. FIGURE 4. View largeDownload slide STV type 2—drainage into the vein of Rosenthal. FIGURE 4. View largeDownload slide STV type 2—drainage into the vein of Rosenthal. Type 3 Nine hemispheres (9.8%; left n = 5, right n = 4) showed a type 3 STV with 2 patients (4%) showing it bilaterally (Figure 4). In both patients, the STV drained into a medial atrial vein (type 3A). From the remaining 5 hemispheres, 4 STVs were classified as type 3A and 1 as type 3B. FIGURE 3. View largeDownload slide STV type 3—drainage into the medial (A) atrial vein. FIGURE 3. View largeDownload slide STV type 3—drainage into the medial (A) atrial vein. Type 4 In only 1 hemisphere (1.1%), the STV drained directly into the vein of Galen (Figure 5). FIGURE 5. View largeDownload slide STV type 4—direct drainage into the vein of Galen. FIGURE 5. View largeDownload slide STV type 4—direct drainage into the vein of Galen. Doubled STV (n = 4) In 2 thalami, the STV was doubled. In the first case, an anterior STV ran straight dorsally crossing a posterior STV ending in the vein of Rosenthal. The second posterior STV coursed medially towards the ventricle in a posterolateral curve and joined the ICV in its posterior portion. In the second case, an anterior STV coursed medially in the standard fashion, but joined the STV from the contralateral side before ending in the ICV in the posterior portion. The posterior STV ran dorsally in a lateral bend and joined the ICV in its posterior portion. No statistical difference in the STV anatomy was found with regard to the side (left or right) of the thalamus or gender of the patients. DISCUSSION To the best of our knowledge, our study represents the first detailed description of the anatomic course of the STV based on imaging in the CT/MRI era. On angiographic studies, the STV was firstly identified by Johanson in 19548 and then comprehensively described by Ferner in 19587 as a union of several slender veins in the central superior portion of the thalamus, which courses medially to the surface of the thalamus, where it emerges from the taenia thalami 5 mm above the pineal gland on lateral projections. Here, it turns posteriorinferiorly at an almost 90° angle to follow the ICV in close relation for some 2 cm and to end into its posterior part.7,8 Based on our multiplanar and 3-dimensional CTA investigations, we could demonstrate that (1) 51.1% of the cases were different from the purportedly typical draining pattern of the STV, and (2) 27.2% of the STV draining into the ICV differed from the original description and ended in the anterior portion instead of the posterior portion of the ICV (Figure 6). FIGURE 6. View largeDownload slide Three-dimensional CTA showing (1) septal vein, (2) thalamostriate vein, (3) internal cerebral vein, (4) left type 1B STV, (5) right type 1A STV FIGURE 6. View largeDownload slide Three-dimensional CTA showing (1) septal vein, (2) thalamostriate vein, (3) internal cerebral vein, (4) left type 1B STV, (5) right type 1A STV The venous drainage of the thalamus is often falsely attributed to the thalamostriate vein. However, while the term thalamostriate vein implies a role in the venous drainage of the thalamus, multiple investigators in the past had already demonstrated that the true thalamic veins very rarely drain into the thalamostriate vein, but that the superior and medial portions of the thalamus were typically drained by the ICV and the inferior and posterior parts of the thalamus by the basal or posterior mesencephalic veins.7-11 With regard to the venous drainage of the superior and medial portions of the thalamus into the ICV, Beau and Rabischong11 stated that this was accomplished by a posteriomedial and anteromedial thalamic vein. As later classified by Giudicelli et al,9 these would correspond to the STV draining into the posterior portion of the ICV and the anterior thalamic vein draining into the anterior portion of the ICV.9 Based on our observations, this purportedly typical venous draining pattern of the superior and medial thalamus into the ICV was not uniform. Only 64.3% followed the originally described course and entered the ICV in its posterior portion (type 1B), 35.7%, however, differed from this original description and drained into the anterior portion (type 1A). This short course of the STV into the anterior part of the ICV has not been mentioned in previous classifications. One potential explanation could have been that, in contrast to all other types, this type 1A did not have the hook-like appearance at its origin. According to the classification suggested by Giudicelli et al,9 one could consider this type 1A as an anterior thalamic vein.9 The true anterior thalamic vein, however, courses towards the Foramen of Monro, where it enters the ICV directly at its origin or into subependymal veins like the thalamostriate or septal veins.9 As a consequence of this distinction of a type 1A STV, a true anterior thalamic vein, which was previously acknowledged as a constant finding, was observed in only 7% of our cases.6 If present, it replaced the STV (85.7%) rather than being only an accessory. Our analysis further showed that the percentage of patients with an STV not draining into the ICV was much higher than previously described, reaching 23.9% of our cases (type 2-4). The STV could course along the entire third ventricular surface of the thalamus and around the thalamus to finally course downwards and laterally to enter the vein of Rosenthal (type 2). This draining pattern, which was previously described as exceptional, was present in 13.0% of our cases. This type could be misinterpreted as being part of the posterior thalamic veins, which, however, by definition only drained the posterolateral part of the thalamus.9 Less frequently, but still in 9.6% of the cases, the STV followed the typical course backward, but instead of ending in the ICV it drained into an atrial vein (type 3). According to the classification of the atrial veins by Stein and Rosenbaum,6 we observed drainage into a medial atrial vein in 8.7% of the cases (type 3A) and into a lateral atrial vein in 1.1% (type 3B). In general, these findings would argue against the historical apprehension that the deep venous system is less variable than the superficial venous system in humans.7 Accordingly, it was our observation that the draining pattern of the STV could vary significantly between both thalami in the same patient. In only 15 patients (30%), the STV ended symmetrically in the same segment of its terminate vein. For the most common type 1 STV, only 46% of all patients had the same draining pattern bilaterally. Therefore, our findings contribute to the general understanding of the venous thalamic draining system and allow for a classification of the normal anatomy of the STV. This seems relevant as the STV is the largest and most consistently identified thalamic vein on preoperative imaging, and is thereby of great importance when targeting the thalamus surgically. Schneider et al,3 for instance, recently identified the STV as the major obstacle and pitfall in trajectory planning for targeting the lateral habenular complex in deep brain stimulation. In the presence of a prominent STV, it constituted an impassable obstacle in the majority of the cases through a standard trajectory. Based on these findings, they suggested an alternative, steeper trajectory to the lateral habenular complex.3 For thalamic tumors, a detailed analysis of the STV anatomy might contribute to a better understanding of the origin and growth pattern of the tumor with implications for the choice of the best surgical strategy (Figure 7). For instance, in tumors arising from the ventral part of the thalamus, the STV is typically displaced dorsally and superiorly, in tumors originating in the posterior portion of the thalamus the STV could be displaced anteriorly or could be incorporated by the tumor. FIGURE 7. View largeDownload slide A, T1-weighted image showing a contrast enhancing tumor occluding the third ventricle. B, CTA in axial view depicting the right (arrow) and left (asterisk) STV. The right STV is displaced laterally by the tumor indicating that the origin of the tumor lies within the most medial part of the right thalamus. The STV marks the contrast enhancing border of the tumor and was used as an intraoperative landmark for where to stop the resection. C, T1-weighted image after transcallosal resection of the contrast enhancing part of the tumor showing the right STV. Histology revealed a diffuse glioma of the midline H3K27M. FIGURE 7. View largeDownload slide A, T1-weighted image showing a contrast enhancing tumor occluding the third ventricle. B, CTA in axial view depicting the right (arrow) and left (asterisk) STV. The right STV is displaced laterally by the tumor indicating that the origin of the tumor lies within the most medial part of the right thalamus. The STV marks the contrast enhancing border of the tumor and was used as an intraoperative landmark for where to stop the resection. C, T1-weighted image after transcallosal resection of the contrast enhancing part of the tumor showing the right STV. Histology revealed a diffuse glioma of the midline H3K27M. Moreover, in pineal region tumors, a meticulous analysis and understanding of the STV contributes to the decision on the best surgical approach. As we could show in our analysis, the STV could course all the way back to the vein of Galen complex (type 2 and 4). Thereby, the STV could be surgically relevant not only for pineal tumors protruding into the third ventricle, but also for tumors predominately located posteriorly to the thalamus. Limitations Our analysis and classification of the STV anatomy is limited by the small sample size and the technical limitations of CTA, which was the single imaging modality we used. We cannot exclude that other imaging modalities, such as 3-dimensional reconstruction of angiographic data with digital subtraction, could have potentially influenced our results. Furthermore, even though we tried to minimize the confounding factor from the interobserver variability by having a consensus decision from 3 different raters, we have to admit that assignment to a certain type was sometimes challenging. CONCLUSION The draining pattern of the STV is variable and can be classified into 4 types. The hook-like appearance on axial views facilitates proper identification of the STV. Careful analysis of its anatomic course might help in understanding the origin and growth pattern of thalamic lesions and could serve as an intraoperative landmark. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Puget S , Crimmins DW , Garnett MR et al. Thalamic tumors in children: a reappraisal . J Neurosurg . 2007 ; 106 : 354 - 362 . Google Scholar PubMed 2. Rangel-Castilla L , Spetzler RF . The 6 thalamic regions: surgical approaches to thalamic cavernous malformations, operative results, and clinical outcomes . J Neurosurg . 2015 ; 123 : 676 - 685 . Google Scholar CrossRef Search ADS PubMed 3. Schneider TM , Beynon C , Sartorius A , Unterberg AW , Kiening KL . Deep brain stimulation of the lateral habenular complex in treatment-resistant depression: traps and pitfalls of trajectory choice . Neurosurgery . 2013 ; 72 ( 2 Suppl Operative ): ons184 - ons193 . Google Scholar PubMed 4. Schlesinger B , ed. The Upper Brainstem in the Human: Its Nuclear Configuration and Vascular Supply . Berlin, Heidelberg : Springer Verlag ; 1976 . Google Scholar CrossRef Search ADS 5. Schlesinger B. The venous drainage of the brain, with special reference to the Galenic system . Brain . 1939 ; 62 : 274 - 291 . Google Scholar CrossRef Search ADS 6. Stein RL , Rosenbaum AE . Deep supratentorial veins. Section I. Normal deep cerebral venous system . In: Newton TH , Potts DG , eds. Radiology of the Skull and Brain: Angiography . Vol 2 . St. Louis, London : Mosby ; 1974 : 1903 - 1998 . 7. Ferner H . Anatomische und phlebographische Studien der inneren Hirnvenen des Menschen . Z Anat Entwicklungsgesch . 1958 ; 120 : 481 - 491 . Google Scholar CrossRef Search ADS PubMed 8. Johanson C. The central veins and deep dural sinuses of the brain; an anatomical and angiographic study . Acta Radiol Suppl . 1954 ; 107 : 8 - 184 . Google Scholar PubMed 9. Giudicelli G , Salamon G . The veins of the thalamus . Neuroradiology . 1970 ; 1 : 92 - 98 . Google Scholar CrossRef Search ADS 10. Stephens RB , Stilwell DL , eds. Arteries and Veins of the Human Brain . Springfield IL : Charles C Thomas ; 1969 . 11. Beau A , Rabischong P . Les veins internes du cerveau . C R Assoc Anat . 1959 ; 102 : 175 - 181 . Copyright © 2017 by the Congress of Neurological Surgeons 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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Operative NeurosurgeryOxford University Press

Published: Jul 20, 2017

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