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Bilateral Dehiscence of the Bony Cochlear Basal Turn

Bilateral Dehiscence of the Bony Cochlear Basal Turn ObjectiveTo identify bony labyrinth defects as causing symptoms that might otherwise be difficult to understand.DesignImaging investigation on cases of lowered vestibular evoked myogenic potentials (VEMP) threshold.SettingAcademic tertiary case-referral medical center.PatientsOne patient with continuous bilateral tinnitus.InterventionsHigh-resolution computed tomography of petrous bone was performed after audiometry, impedance, and VEMP recording.ResultsWe found a previously unreported dehiscence of the bony plate between the basal turn of the cochlea and the carotid canal.ConclusionsA VEMP study could prompt diagnostic imaging tests that might otherwise be neglected. Moreover, it is important to consider the possibility of a bony dehiscence involving not just the semicircular canals.Defects of bony labyrinth causing cochleovestibular symptoms have been reported by Minor et al,who first described a dehiscence of the superior semicircular canal (SCD) that caused symptoms primarily derived from Tullio phenomenon. This defect is characterized by an abnormally lowered detection threshold of vestibular evoked myogenic potentials (VEMPs), which may result from a reduced impedance of the inner ear.The study of VEMPshas therefore become almost routine in our evaluation of otoneurologic patients, allowing us to record a significantly reduced detection threshold in 12 patients, who subsequently underwent high-resolution computed tomography and showed the following results: an SCD was found in 7 cases; a labyrinthine fistula in 3, indicating alterations possibly different from SCD; the 11th case remains unexplained; and the last case, herein described, involved the exceptional finding of a bilateral dehiscence of the bony basal turn of the cochlea contiguous with the carotid canal.REPORT OF A CASEA 63-year-old man presented with complaints of bilateral, continuous, nonpulsatile tinnitus for the last 7 years. Although the tinnitus was bilateral, it was more severe on the right side than on the left. No other symptoms were reported. A history of underwater diving, halted 6 years before presentation for reasons other than otolaryngologic symptoms, suggested the possibility of barotraumatic fistula.Otoscopy findings were normal. At clinical examination, the only noteworthy finding was subjective decreased intensity of the tinnitus after rotating the neck to the left side and exerting pressure on the left jugular vein. Pure tone audiometry (PTA) showed a mild, sensorineural, bilateral hearing loss in the 4- to 8-kHz range, substantially consistent with the hearing loss associated with normal aging (Figure 1). Stapedial reflex was present. Vestibular investigation (electronystagmography and electrooculography) showed a normal pattern on both bithermal caloric test and rotatory test. Even echo Doppler findings of the supra-aortic vessels were within normal limits.Figure 1.Audiogram of a 63-year-old man shows bilateral sensorineural hearing loss substantially consistent with that expected for a patient of this age.In contrast, auditory brainstem response showed a bilateral increased value for fifth-wave latency and first-to-fifth wave interval, which prompted a magnetic resonance imaging study of the posterior cranial fossa using paramagnetic contrast enhancement, which did not show any alteration. The VEMPs recording was carried out with a tone burst of 500 Hz lasting 2 milliseconds with a repetition frequency of 4 Hz for 100 times. Within these parameters, a clearly detectable response would be expected at 95 dB normal hearing level (NHL), while no response would be detected at less than 80 dB NHL. These values, which proved reliable in our laboratory, are generally accepted as the standard values under the described conditions. In this case, a bilateral lowering of the detection threshold was found, P13-N23 complex being evokable by a 65- and 70-dB NHL stimulus on the right and left sides, respectively.This finding called into question a perilymphatic posttraumatic fistula or SCD and suggested the need for high-resolution computed tomography, which was performed on a Multislice light-speed General Electric Medical Systems scanner (Buc, France). Helical slices with 1.25-mm collimation of the x-ray beam and 0.3-mm incremental reconstructions in the axial plane were obtained, yielding voxels measuring 1.25 × 1.25 × 1.25 mm. Images were then reformatted at 0.3-mm increments into a plane corresponding to the basal turn of the cochlea, starting from both transverse and coronal projection. This investigation allowed us to identify a bilateral dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal (Figure 2and Figure 3). This dehiscence presented a certain degree of asymmetry, the greater extension being detectable on the right side. Given the patient’s good hearing function and the risks linked with anatomic location of the dehiscence, surgery was not recommended.Figure 2.Transverse high-resolution computed tomographic image of the patient’s right side, the side most affected by hearing loss. The arrow indicates a dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal.Figure 3.Coronal high-resolution computed tomographic image reformatted into a plane corresponding to the basal turn of the cochlea of the patient’s right ear, the ear most affected by hearing loss. The arrow indicates the site of the dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal.COMMENTAfter a thorough review of the English-language literature, we believe that this is the first reported case of bony dehiscence of the cochlear partition of the labyrinth. The bilateral nature of the defect may support the hypothesis of a developmental abnormality, even if of a mild degree, as already has been suggested for SCD.The present case presents some peculiar elements. First, symptoms related to Tullio phenomenon were lacking, possibly owing to the location of the dehiscence in the cochlear area.Second, the substantial absence of cochleovestibular functional alterations clearly associated with the dehiscence might be misleading. Actually, the sensorineural hearing loss for high frequencies detected by PTA, although characterized by a mild asymmetry, did not substantially differ from the expected audiologic condition of a 63-year-old man. On the other hand, it cannot be excluded that a certain degree of hearing impairment may somehow be linked to this bony dehiscence. Attempts to explain the possible functional implications of SCD itself have not reached an unequivocal statement.It is even more difficult to explain the origin of auditory brainstem response anomalies, which might be associated with the carotid pulse, possibly leading to decreased detectability of the electrical activity of the cochlea. The strict contiguity between carotid artery and cochlea might also explain the onset or increased severity of tinnitus by some arterial wall alterations linked to age. However, the cause of the nonpulsatile pattern of the tinnitus in the present case remains unclear.Finally, the reduced threshold of VEMPs may be explained by the “third window effect,” as suggested by Minor et al.In the present case, the anatomic location of the dehiscence could reasonably be expected to result in a less pronounced fall in impedance than occurs with SCD, thus explaining the finding of less dramatically lowered threshold values. The same less pronounced increase of immittance could be responsible for the lack of any conductive defects, which contrasts with SCD syndrome usual findings.Despite the nonspecific pattern of the symptoms, which might understandably create some difficulties in reaching a satisfactory diagnosis, an accurate electrophysiologic study played a fundamental role in correct clinical evaluation. This leads us to argue that VEMPs might represent a useful diagnostic tool, even in cases that seem not to have vestibular involvement. If nothing else, a VEMP study could point the way toward diagnostic imaging tests, which might otherwise be neglected.Although many aspects remain to be clarified, it seems reasonable to expect that many abnormalities similar to the one described herein might be detected, and so possibly treated, using a more detailed diagnostic approach, which could be enhanced by an ever more focused application and interpretation of electrophysiology. Finally, it is important also to keep in mind that a bony dehiscence might be detected involving the entire otic capsule and not just the semicircular canals.Correspondence:Giovanni Carlo Modugno, MD, Unita Operativa di Otorinolaringoiatria, Policlinico S. Orsola Malpighi, Padiglione 5, Via Massarenti No. 9, Bologna 40138, Italy (giovanni.modugno@unibo.it).Submitted for Publication:March 12, 2004; final revision received July 30, 2004; accepted August 12, 2004.REFERENCESLBMinorDSolomonJSZinreichDSZeeSound- and/or pressure-induced vertigo due to bone dehiscence of the superior semicircular canal.Arch Otolaryngol Head Neck Surg19981242492589525507KBrantbergJBergeniusATribukaitVestibular-evoked myogenic potentials in patients with dehiscence of the superior semicircular canal.Acta Otolaryngol199911963364010586994GMHalmagyiJGColebatchISCurthoysNew tests of vestibular function.Baillieres Clin Neurol199434855007874404TPHirvonenNWegSJZinreichLBMinorHigh-resolution CT findings suggest a developmental abnormality underlying superior canal dehiscence syndrome.Acta Otolaryngol200312347748112797581HSohmerSFreemanRPerezSemicircular canal fenestration: improvement of bone- but not air-conducted auditory thresholds.Hear Res200418710511014698091LBMinorJPCareyPDCremerLRLustigSOStreubelMJRuckensteinDehiscence of bone overlying the superior canal as a cause of apparent conductive hearing loss.Otol Neurotol20032427027812621343KMCoxDJLeeJPCareyLBMinorDehiscence of bone overlying the superior semicircular canal as a cause of an air-bone gap on audiometry: a case study.Am J Audiol200312111612894862 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png JAMA Otolaryngology - Head & Neck Surgery American Medical Association

Bilateral Dehiscence of the Bony Cochlear Basal Turn

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Publisher
American Medical Association
Copyright
Copyright 2004 American Medical Association. All Rights Reserved. Applicable FARS/DFARS Restrictions Apply to Government Use.
ISSN
2168-6181
eISSN
2168-619X
DOI
10.1001/archotol.130.12.1427
pmid
15611405
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Abstract

ObjectiveTo identify bony labyrinth defects as causing symptoms that might otherwise be difficult to understand.DesignImaging investigation on cases of lowered vestibular evoked myogenic potentials (VEMP) threshold.SettingAcademic tertiary case-referral medical center.PatientsOne patient with continuous bilateral tinnitus.InterventionsHigh-resolution computed tomography of petrous bone was performed after audiometry, impedance, and VEMP recording.ResultsWe found a previously unreported dehiscence of the bony plate between the basal turn of the cochlea and the carotid canal.ConclusionsA VEMP study could prompt diagnostic imaging tests that might otherwise be neglected. Moreover, it is important to consider the possibility of a bony dehiscence involving not just the semicircular canals.Defects of bony labyrinth causing cochleovestibular symptoms have been reported by Minor et al,who first described a dehiscence of the superior semicircular canal (SCD) that caused symptoms primarily derived from Tullio phenomenon. This defect is characterized by an abnormally lowered detection threshold of vestibular evoked myogenic potentials (VEMPs), which may result from a reduced impedance of the inner ear.The study of VEMPshas therefore become almost routine in our evaluation of otoneurologic patients, allowing us to record a significantly reduced detection threshold in 12 patients, who subsequently underwent high-resolution computed tomography and showed the following results: an SCD was found in 7 cases; a labyrinthine fistula in 3, indicating alterations possibly different from SCD; the 11th case remains unexplained; and the last case, herein described, involved the exceptional finding of a bilateral dehiscence of the bony basal turn of the cochlea contiguous with the carotid canal.REPORT OF A CASEA 63-year-old man presented with complaints of bilateral, continuous, nonpulsatile tinnitus for the last 7 years. Although the tinnitus was bilateral, it was more severe on the right side than on the left. No other symptoms were reported. A history of underwater diving, halted 6 years before presentation for reasons other than otolaryngologic symptoms, suggested the possibility of barotraumatic fistula.Otoscopy findings were normal. At clinical examination, the only noteworthy finding was subjective decreased intensity of the tinnitus after rotating the neck to the left side and exerting pressure on the left jugular vein. Pure tone audiometry (PTA) showed a mild, sensorineural, bilateral hearing loss in the 4- to 8-kHz range, substantially consistent with the hearing loss associated with normal aging (Figure 1). Stapedial reflex was present. Vestibular investigation (electronystagmography and electrooculography) showed a normal pattern on both bithermal caloric test and rotatory test. Even echo Doppler findings of the supra-aortic vessels were within normal limits.Figure 1.Audiogram of a 63-year-old man shows bilateral sensorineural hearing loss substantially consistent with that expected for a patient of this age.In contrast, auditory brainstem response showed a bilateral increased value for fifth-wave latency and first-to-fifth wave interval, which prompted a magnetic resonance imaging study of the posterior cranial fossa using paramagnetic contrast enhancement, which did not show any alteration. The VEMPs recording was carried out with a tone burst of 500 Hz lasting 2 milliseconds with a repetition frequency of 4 Hz for 100 times. Within these parameters, a clearly detectable response would be expected at 95 dB normal hearing level (NHL), while no response would be detected at less than 80 dB NHL. These values, which proved reliable in our laboratory, are generally accepted as the standard values under the described conditions. In this case, a bilateral lowering of the detection threshold was found, P13-N23 complex being evokable by a 65- and 70-dB NHL stimulus on the right and left sides, respectively.This finding called into question a perilymphatic posttraumatic fistula or SCD and suggested the need for high-resolution computed tomography, which was performed on a Multislice light-speed General Electric Medical Systems scanner (Buc, France). Helical slices with 1.25-mm collimation of the x-ray beam and 0.3-mm incremental reconstructions in the axial plane were obtained, yielding voxels measuring 1.25 × 1.25 × 1.25 mm. Images were then reformatted at 0.3-mm increments into a plane corresponding to the basal turn of the cochlea, starting from both transverse and coronal projection. This investigation allowed us to identify a bilateral dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal (Figure 2and Figure 3). This dehiscence presented a certain degree of asymmetry, the greater extension being detectable on the right side. Given the patient’s good hearing function and the risks linked with anatomic location of the dehiscence, surgery was not recommended.Figure 2.Transverse high-resolution computed tomographic image of the patient’s right side, the side most affected by hearing loss. The arrow indicates a dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal.Figure 3.Coronal high-resolution computed tomographic image reformatted into a plane corresponding to the basal turn of the cochlea of the patient’s right ear, the ear most affected by hearing loss. The arrow indicates the site of the dehiscence of the bony septum separating the basal turn of the cochlea from the contiguous carotid canal.COMMENTAfter a thorough review of the English-language literature, we believe that this is the first reported case of bony dehiscence of the cochlear partition of the labyrinth. The bilateral nature of the defect may support the hypothesis of a developmental abnormality, even if of a mild degree, as already has been suggested for SCD.The present case presents some peculiar elements. First, symptoms related to Tullio phenomenon were lacking, possibly owing to the location of the dehiscence in the cochlear area.Second, the substantial absence of cochleovestibular functional alterations clearly associated with the dehiscence might be misleading. Actually, the sensorineural hearing loss for high frequencies detected by PTA, although characterized by a mild asymmetry, did not substantially differ from the expected audiologic condition of a 63-year-old man. On the other hand, it cannot be excluded that a certain degree of hearing impairment may somehow be linked to this bony dehiscence. Attempts to explain the possible functional implications of SCD itself have not reached an unequivocal statement.It is even more difficult to explain the origin of auditory brainstem response anomalies, which might be associated with the carotid pulse, possibly leading to decreased detectability of the electrical activity of the cochlea. The strict contiguity between carotid artery and cochlea might also explain the onset or increased severity of tinnitus by some arterial wall alterations linked to age. However, the cause of the nonpulsatile pattern of the tinnitus in the present case remains unclear.Finally, the reduced threshold of VEMPs may be explained by the “third window effect,” as suggested by Minor et al.In the present case, the anatomic location of the dehiscence could reasonably be expected to result in a less pronounced fall in impedance than occurs with SCD, thus explaining the finding of less dramatically lowered threshold values. The same less pronounced increase of immittance could be responsible for the lack of any conductive defects, which contrasts with SCD syndrome usual findings.Despite the nonspecific pattern of the symptoms, which might understandably create some difficulties in reaching a satisfactory diagnosis, an accurate electrophysiologic study played a fundamental role in correct clinical evaluation. This leads us to argue that VEMPs might represent a useful diagnostic tool, even in cases that seem not to have vestibular involvement. If nothing else, a VEMP study could point the way toward diagnostic imaging tests, which might otherwise be neglected.Although many aspects remain to be clarified, it seems reasonable to expect that many abnormalities similar to the one described herein might be detected, and so possibly treated, using a more detailed diagnostic approach, which could be enhanced by an ever more focused application and interpretation of electrophysiology. Finally, it is important also to keep in mind that a bony dehiscence might be detected involving the entire otic capsule and not just the semicircular canals.Correspondence:Giovanni Carlo Modugno, MD, Unita Operativa di Otorinolaringoiatria, Policlinico S. Orsola Malpighi, Padiglione 5, Via Massarenti No. 9, Bologna 40138, Italy (giovanni.modugno@unibo.it).Submitted for Publication:March 12, 2004; final revision received July 30, 2004; accepted August 12, 2004.REFERENCESLBMinorDSolomonJSZinreichDSZeeSound- and/or pressure-induced vertigo due to bone dehiscence of the superior semicircular canal.Arch Otolaryngol Head Neck Surg19981242492589525507KBrantbergJBergeniusATribukaitVestibular-evoked myogenic potentials in patients with dehiscence of the superior semicircular canal.Acta Otolaryngol199911963364010586994GMHalmagyiJGColebatchISCurthoysNew tests of vestibular function.Baillieres Clin Neurol199434855007874404TPHirvonenNWegSJZinreichLBMinorHigh-resolution CT findings suggest a developmental abnormality underlying superior canal dehiscence syndrome.Acta Otolaryngol200312347748112797581HSohmerSFreemanRPerezSemicircular canal fenestration: improvement of bone- but not air-conducted auditory thresholds.Hear Res200418710511014698091LBMinorJPCareyPDCremerLRLustigSOStreubelMJRuckensteinDehiscence of bone overlying the superior canal as a cause of apparent conductive hearing loss.Otol Neurotol20032427027812621343KMCoxDJLeeJPCareyLBMinorDehiscence of bone overlying the superior semicircular canal as a cause of an air-bone gap on audiometry: a case study.Am J Audiol200312111612894862

Journal

JAMA Otolaryngology - Head & Neck Surgery – American Medical Association

Published: Dec 1, 2004

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