Otology & Neurotology 32:1506Y1512 Ó 2011, Otology & Neurotology, Inc. Electrocochleography as a Diagnostic and Intraoperative Adjunct in Superior Semicircular Canal Dehiscence Syndrome *Meredith E. Adams, †Paul R. Kileny, †Steven A. Telian, †Hussam K. El-Kashlan, †Katherine D. Heidenreich, †Gregory R. Mannarelli, and †H. Alexander Arts *Department of OtolaryngologyYHead and Neck Surgery, University of Minnesota, Minneapolis, Minnesota; and ÞDepartment of OtolaryngologyYHead and Neck Surgery, University of Michigan, Ann Arbor, Michigan, U.S.A. Objective: To determine the electrocochleographic characteristics of ears with superior semicircular canal dehiscence (SSCD) and to examine its use for intraoperative monitoring in canal occlusion procedures. Study Design: Case series. Setting: Academic medical center. Patients: Thirty-three patients (45 ears) had clinical and computed tomographic evidence of SSCD; 8 patients underwent intraoperative electrocochleography (ECoG) during superior canal occlusion; 9 patients underwent postoperative ECoG after SSCD occlusion. Interventions: Diagnostic, intraoperative, and postoperative extratympanic ECoG; middle fossa or transmastoid occlusion of the superior semicircular canal. Main Outcome Measure: Summating potential (SP) to action potential (AP) ratio, as measured by ECoG, and alterations in SP/AP during canal exposure and occlusion. Results: Using computed tomography as the standard, elevation of SP/AP on ECoG demonstrated 89% sensitivity and 70% specificity for SSCD. The mean SP/AP ratio among ears with SSCD was significantly higher than that among unaffected ears (0.62 versus 0.29, p G 0.0001). During occlusion procedures, SP/AP increased on exposure of the canal lumen (mean change T standard deviation, 0.48 T 0.30). After occlusion, SP/AP dropped below the intraoperative baseline in most cases (mean change, j0.23 T 0.52). All patients experienced symptomatic improvement. All patients who underwent postoperative ECoG 1 to 3 months after SSCD repair maintained SP/AP of 0.4 or lesser. Conclusion: These findings expand the differential diagnosis of abnormal ECoG. In conjunction with clinical findings, ECoG may support a clinical diagnosis of SSCD. Intraoperative ECoG facilitates dehiscence documentation and allows the surgeon to confirm satisfactory canal occlusion. Key Words: ElectrocochleographyV Semicircular canal dehiscenceVSummating potential. Semicircular canal dehiscence syndrome results from the absence of bone overlying the superior (superior semicircular canal dehiscence [SSCD]) or posterior semicircular canal. The constellation of associated auditory and vestibular symptoms includes sound- and pressure-evoked vertigo and oscillopsia, conductive hearing loss (CHL), conductive hyperacusis, autophony, pulsatile tinnitus, and aural fullness (1Y4). Intraoperative visualization of a dehiscence in symptomatic patients provides the greatest degree of diagnostic certainty. Pending direct surgical confirmation, SSCD is best identified on high-resolution temporal bone computed tomography (CT) reformatted in the planes parallel and orthogonal to the semicircular canal (5). Because radiographically apparent dehiscences may be asymptomatic, it also is important to obtain physiologic confirmation that the SSCD is affecting inner ear function (6). Clinical findings that have been advocated for this purpose include the following: 1) nystagmus in the plane of the affected canal in response to sound and pressure stimuli (2), 2) reduced thresholds for vestibularevoked myogenic potentials (VEMPs) (7), and 3) CHL (air-bone gap, Q10 dB) with preserved acoustic reflexes (8). Surgical plugging of the dehiscent canal can provide substantial relief to patients with disabling symptoms. The symptomatic improvement is thought to result from Otol Neurotol 32:1506Y1512, 2011. Address correspondence and reprint requests to Meredith E. Adams, M.D., Department of OtolaryngologyYHead and Neck Surgery, University of Minnesota, 420 Delaware St SE, MMC 396, Minneapolis, MN 55455; E-mail: firstname.lastname@example.org No sources of support require acknowledgment. The authors have no financial conflicts of interest to report. 1506 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. ELECTROCOCHLEOGRAPHY IN SSCD correction of abnormal flow of endolymph within the canal (6). VEMP thresholds may normalize after canal occlusion, potentially confirming successful SSCD closure (7). Postoperative correction of CHL and sound- and pressure-evoked nystagmus has not been consistently demonstrated (2). None of these tests can be performed while a patient is under general anesthesia, precluding intraoperative physiologic confirmation of SSCD closure. Although poorly understood, abnormalities of the cochlear summating potential (SP) have previously been identified in inner ear disorders such as Ménière’s disease and perilymphatic fistula. These SP abnormalities presumably reflect altered electrophysiologic and/or hydrodynamic function in the inner ear. As such, it might be anticipated that similar abnormalities may be found in SSCD. Preliminary findings of elevated SP to action potential (AP) ratios that consistently normalized after canal occlusion in a small cohort of patients with SSCD have been previously reported (9). The aim of the present study was to further investigate the use of electrocochleography (ECoG) in the clinical diagnosis, surgical management, and postoperative assessment of SSCD. In this study, additional experience with ECoG in SSCD patients, as well as a novel use of intraoperative ECoG, will be described. METHODS A review was performed of existing clinical data of patients who underwent a comprehensive neurotologic evaluation for SSCD at a tertiary referral academic medical center from January 2006 to April 2011. Inclusion criteria for the study were as follows: 1) presence of one or more auditory or vestibular symptoms characteristic of SSCD syndrome (sound- and pressure-evoked vertigo and oscillopsia, CHL, conductive hyperacusis, autophony, pulsatile tinnitus, and aural fullness); and 2) SSCD identified in at least 1 ear on CT. Audiometric testing, cervical VEMP testing, and ECoG were performed as part of the evaluation. Testing for tone-evoked nystagmus was not performed. There were 33 patients who qualified for inclusion on the basis of these criteria. Eleven of these patients were previously described (9). Twelve of these 33 patients underwent surgical canal occlusion. The study protocol was approved by the institutional review board. SSCD was identified in all patients by high-resolution noncontrast computed tomographic imaging. Images were obtained in the axial plane at 0.625-mm intervals and reformatted in the planes parallel and orthogonal to the superior semicircular canal (5). Air- and bone-conduction audiometry was performed in the standard fashion using calibrated equipment and a soundattenuated booth (ANSI-1969). CHL was defined as an air-bone gap of 10 dBnHL or greater at 1 frequency. Cervical VEMPs were obtained using clicks presented to the test ear at a rate of 4.7 per second (9). The range of normal values for click-evoked cervical VEMP threshold has been reported to be 80 to 100 dBnHL or 75 to 100 dBnHL, depending on test performance (10). Thresholds less than 80 dBnHL were considered to be abnormal for purposes of this study. ECoG was performed using a hydrogel-tipped tympanic membrane electrode (TM-ECochGtrode, Bio-Logic Systems/ Natus Medical, Mundelein, IL, USA) introduced onto the surface of the tympanic membrane under microscopic visualization and stabilized by the foam tip of the insert audio transducer. The patients were supine, with head elevated approximately 1507 30 degrees. Stimuli consisted of unfiltered alternating polarity clicks of 100-Ks duration, presented at an intensity of 85 dBnHL. Two replications of averaged responses elicited by 1,500 clicks presented at a rate of 11.7 per second were obtained. Responses were band-pass filtered (20Y1,500 Hz) and averaged, and the SP/AP ratio was calculated. SP/AP ratio of greater than 0.4 was defined as abnormal for purposes of this study, based on commonly used standards for clinical testing (11,12). Selected highly symptomatic patients underwent surgical repair of their SSCD. In all cases, the semicircular canal was occluded using either fascia/bone pate or bone wax. One patient was occluded via a transmastoid approach, and the others were occluded via a middle cranial fossa (MCF) approach. Patients were positioned supine, with head turned toward the contralateral ear. Intraoperative tympanic membrane ECoG was used for monitoring purposes during canal occlusion procedures. Stimuli consisted of alternating polarity clicks presented at 90 dBnHL. Responses were recorded using a Nicolet Viasys Endeavor neurodiagnostic system (software version 184.108.40.206; Madison, WI, USA), a unit commonly used for multimodal intraoperative monitoring. SP and AP amplitudes were measured repeatedly throughout the procedures, with particular attention to measurement at critical time points: 1) after the induction of general anesthesia before the surgical incision (intraoperative baseline); 2) during dural elevation and dehiscence exposure (or canal fenestration); and 3) after canal occlusion, before emergence from anesthesia. Postoperative testing was performed between 4 and 12 weeks after canal occlusion. All preoperative and postoperative electrophysiologic testing was performed and interpreted by the same clinical auditory neurophysiologist. No attempt was made to blind the neurophysiologist to the clinical impression, individual CT results, or other clinical findings. Intraoperative testing was performed by audiologists experienced in intraoperative monitoring supervised by the same individual. The averaged SP/AP ratios of affected and unaffected ears were compared using 2-sample t tests after confirming that assumptions for pooled variance were met. The averaged VEMP thresholds of affected and unaffected ears were compared using 2-sample t tests assuming unequal variance (SDSCD/SDnormal = 2). Statistical analysis was performed with Microsoft Excel 2010 (Microsoft Corporation, Redmond, WA, USA). RESULTS Diagnostic ECoG Thirty-three patients with computed tomographic evidence of SSCD underwent diagnostic ECoG. There were 24 women (73%). The average patient age was 49 T 12 (mean T standard deviation [SD]) years. Of the 33 patients, 12 had bilateral SSCD, and 21 had unilateral SSCD, for a total of 45 affected ears and 21 unaffected ears. Unilateral SSCD was found in 13 left ears and 8 right ears. ECoG, VEMP, and audiometric testing were performed on all 66 ears, with the exception of a single unaffected ear that did not undergo ECoG. The mean SP/AP ratio among ears with CT-documented SSCD was significantly higher than that among unaffected ears (Table 1). The ranges of SP/AP were 0.29 to 1.48 for affected ears and 0.07 to 0.65 for unaffected ears. The mean VEMP threshold among ears with SSCD was significantly lower than that among unaffected ears (Table 1). Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. 1508 M. E. ADAMS ET AL. TABLE 1. Mean summating potential to action potential ratio on electrocochleography and vestibular-evoked myogenic potential thresholds in affected and unaffected ears Summating potential to action potential ratio on electrocochleography Vestibular-evoked myogenic potential threshold (dB nHL) Superior semicircular canal dehiscence Unaffected ears p value 0.62 T 0.21 0.29 T 0.17 G0.0001 70 T 8 82 T 4 G0.0001 Values are mean T standard deviation. p values are for 2-sample t tests. All 21 patients with unilateral SSCD had an elevated SP/AP (90.4) in the affected ear. All 12 patients with bilateral SSCD had an elevated SP/AP in at least 1 ear. Seven of the 12 patients with bilateral SSCD had bilateral SP/AP elevation, with the higher SP/AP value in the more symptomatic ear or nonlateralizing symptoms. In the remaining 5 patients with bilateral SSCD, a negative ECoG (SP/AP, e0.4) was documented in 1 ear. Three of these 5 patients were asymptomatic with regard to the ear with the normal SP/AP result; 1 patient had otosclerosis documented intraoperatively after the ECoG and no other audiovestibular symptoms in that ear, and 1 patient had already undergone round- and oval window plugging in the ‘‘false-negative’’ ear (which was still the more symptomatic ear) before being referred to our center. By nature of the study design, identifying only patients with SSCD in at least 1 ear, all 6 of the ‘‘false-positive’’ ECoG results were found in unaffected ears of patients with a contralateral SSCD. Two of the 6 ears had CHL (Q10 dB), and a different 2 of 6 had a VEMP threshold less than 80 dB, but there were no lateralizing auditory or vestibular symptoms attributed to the ears with the falsepositive ECoG results. Table 2 summarizes the performance statistics for ECoG, VEMP, and CHL in this series. We were interested in comparing sensitivity and specificity of ECoG versus VEMP for the diagnosis and confirmation of SSCD. For purposes of these calculations, we used the presence of a dehiscence on CT as the ‘‘gold standard’’ because most TABLE 2. a CT CHL VEMP ECoG Summary statistics for tests for superior semicircular canal dehiscence TP TN FP FN Sensitivity (%) Specificity (%) 46 43 39 40 20 11 16 14 0 8 4 6 0 2 7 5 96 84 89 58 80 70 Table modeled after (14). CHL indicates conductive hearing loss with air bone gap of 10 dB or higher; CT, computed tomography; ECoG, electrocochleography with SP/AP greater than 0.4; FN, false negative; FP, false positive; TN, true negative; TP, true positive; VEMP, vestibular-evoked myogenic potential threshold less than 80 dB nHL. a CT is the clinical gold standard for this study, recognizing that rare false-positive studies are likely. dehiscences were not confirmed by direct visualization in the operating room. Although it is well established that a proportion of dehiscences present on CT are not symptomatic (13,14), in the present study, all dehiscences present on CT also were associated with a CHL of at least 10 dB at 1 frequency and/or an abnormally low VEMP threshold. These latter 2 findings suggest that all of the dehiscences seen on CT were physiologically active. Using these assumptions, ECoG had comparable sensitivity and specificity to VEMP if a VEMP threshold less than 80 dBnHL is considered abnormal. If VEMP threshold less than 75 dBnHL is considered abnormally low, VEMP sensitivity decreases to 69%, and specificity increases to 100%. ECoG did not enlarge the patient group, with objective findings attributable to SSCD, but not all patients had all 3 objective findings. Intraoperative ECoG Twelve patients underwent superior canal occlusion for symptomatic SSCD. Nine patients were scheduled FIG. 1. Intraoperative SP/AP amplitude ratios, in response to click stimuli, are plotted at 3 time points during SSCD occlusion procedures: at the start of the operative procedure before dural elevation or canal fenestration (baseline), after the dural elevation when the dehiscence is exposed or after the canal is fenestrated (exposure), and after canal occlusion (occlusion). (*) Patient with bilateral SSCD and preoperative SP/AP of 0.59 in operated ear. Although the SP/APBASE = SP/APOCC = 1.0, the patient reported postoperative improvement in lateralizing audiovestibular symptoms and postoperative SP/AP of 0.26. (**) Large SSCD with preoperative SP/AP of 0.7 and SP/APEXP of 1.0. SP/AP decreased to below 0.4 with initial canal occlusion with bone wax. Moments after packing, AP amplitude decreased, and SP increased, prompting repacking, but SP/AP remained at 1.0. Postoperatively, the patient was acutely vertiginous and was confirmed to have a new unilateral vestibular paresis without sensorineural hearing loss. Sound-evoked vestibular symptoms in the operated ear resolved and postoperative SP/AP was borderline normal at 0.4. Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. ELECTROCOCHLEOGRAPHY IN SSCD 1509 for intraoperative ECoG; however, in 1 case, ECoG could not be monitored after dural elevation because of fluid entering the middle ear through multiple large tegmen dehiscences. Of the 8 fully monitored cases, 7 were performed by the MCF approach, and one by a transmastoid approach. A definitive dehiscence was identified at the time of surgery in all cases. Although continuous ECoG was performed throughout the cases, the value of SP/AP was specifically established at 3 points during the procedures for this study: 1) intraoperative baseline before the surgical incision (SP/APBASE); 2) upon exposure of the dehiscence or fenestration of the canal (SP/APEXP); and 3) after canal occlusion (SP/APOCC). Data for SP/ APEXP were available for 7 of the 8 cases. Data for SP/ APBASE and SP/POCC were available for all 8 cases. SP/APBASE was elevated in all patients (Fig. 1). During all 7 procedures for which data for SP/APEXP was available, SP/AP increased from the SP/APBASE upon exposure of the dehiscence or fenestration of the canal (SP/APEXP, mean change T SD, 0.48 T 0.30). After canal occlusion, SP/APOCC dropped below SP/APEXP in all 8 cases (mean change T SD, j0.66 T 0.34). SP/APOCC dropped below SP/APBASE in 6 of the 8 cases and dropped to be equal to SP/APBASE in 1 case (mean change, j0.23 T 0.52). SP/APOCC was higher than SP/APBASE in 1 case. The exceptions to the observed pattern are indicated in Figure 1 and discussed further in the legend. An illustrative ECoG recording from an MCF approach for occlusion of SSCD is provided in Figure 2. The contribution of SP changes versus AP changes to the alteration of SP/AP varied between patients and could not be precisely determined because of the test-retest variation that is inherent to the operative setting. Postoperative ECoG Postoperative ECoG was performed 4 to 12 weeks after SSCD occlusion in 9 of the operated cases. Occlusion of the semicircular canal effectively relieved the lateralizing preoperative symptoms in all patients who underwent postoperative testing. All cases had preoperative SP/AP greater than 0.5 and postoperative SP/AP of 0.4 or lesser. The average decrease from preoperative SP/AP was 0.47 T 0.36. DISCUSSION FIG. 2. Intraoperative ECoG recordings from a patient undergoing an MCF approach for occlusion of SSCD. The preoperative SP/AP was 0.66. The intraoperative baseline SP/AP was 0.49. During dural elevation, upon complete exposure of the SSCD, the SP/AP increased to 1.0. When plugging with bone pate and fascia did not result in a normal SP/AP, the surgeon suspected incomplete occlusion. The dehiscence was packed with additional bone pate, resulting in a normal SP/AP of 0.17. The patient experienced excellent postoperative symptom resolution. x axis: time, 1 ms/division. y axis: amplitude, 0.5 KV/division. In this study, SP/AP elevation was detected in the dehiscent ear in every patient with unilateral SSCD and at least 1 ear of every patient with bilateral SSCD. Overall, the sensitivity of ECoG for detecting SSCD was 89%, and specificity was 70% when using CT as the gold standard for diagnosis. In cases of bilateral SSCD in which symptoms lateralized to 1 ear, the SP/AP tended to be higher in the more symptomatic ear. The SP/AP was found to normalize in those patients who underwent ECoG after superior canal occlusion. It was possible to detect physiologic changes in the inner ear in ‘‘real time’’ by using intraoperative ECoG. Several limitations of the study design necessitate caution when interpreting preoperative ECoG test performance characteristics. First, this is a retrospective study without a normal control group. Because of the high number of bilateral cases in this series, test performance characteristics were calculated based on a small group of negative control ears in patients with computed tomographic evidence of SSCD in the contralateral ear. Multiplanar reformatted CT is reported to have a 100% negative predictive value for SSCD, making it unlikely that the ‘‘negative’’ ears in our study have undiagnosed SSCD Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. 1510 M. E. ADAMS ET AL. (13,14). A normal control group would allow us to determine if an asymptomatic patient with SSCD would have SP/AP elevation and would help to establish if the threshold value of SP/AP (0.4), selected based on experience with endolymphatic hydrops (ELH), is appropriate for third-window lesions. Additionally, some ears considered to have SSCD in this series may represent false positives because CT was used as the gold standard. The radiographic prevalence of SSCD in asymptomatic patients on CT reformatted in the plane of the superior canal has been estimated to be 4.0%, a value that exceeds the 0.5% prevalence on histopathologic examination (13,15). Although not part of the inclusion criteria, all of the patients in this series were symptomatic and had at least 1 objective finding of SSCD (CHL, low VEMP threshold), potentially reducing the likelihood of false positives. Still, the presence of false positives may explain the low specificity for ECoG that was observed. Additional studies to define the clinical usefulness of ECoG in the diagnosis of SSCD are warranted. ECoG is the measurement of electrical potentials generated by the cochlea and auditory nerve in response to acoustic stimulation. The recording electrode is placed as close to the cochlea as possible, and the epoch of interest is the first 3 ms after stimulus presentation. In this study, we obtained consistent results in both the outpatient clinic and the operating room with a commercially available tympanic membrane electrode, obviating the need for transtympanic needle placement. Depending on the nature of the stimulus and the mode of stimulus delivery, the ECoG response may consist of the cochlear microphonic, the SP, and the cochlear nerve compound AP. The SP is a direct current potential that consists of a shift, positive or negative in polarity, from the baseline upon which the cochlear microphonic is superimposed (16). It is thought to reflect nonlinear properties of cochlear hydrodynamics and basilar membrane mechanics (17). Both inner and outer hair cells likely contribute to the production of the SP (18). In response to a click stimulus alternated in polarity, as used in this study, the SP appears as a deflection, or a shoulder preceding the onset of the AP, with the same polarity. The AP is the sum of the individual APs of synchronously firing auditory nerve fibers and is equivalent to Wave I of the auditory brainstem response. Although we can only speculate about the mechanism of SP/AP elevation in SSCD, we think that insights can be gleaned from studies of ECoG in presumed hydropic conditions of the inner ear, most notably Ménière’s disease and perilymph fistula (PLF), in which secondary ELH is thought to occur. An increase in the SP amplitude relative to the AP amplitude is widely thought to be the most useful ECoG measure for identifying ELH. Significant increases in SP amplitude result from experimental displacement of the basilar membrane toward the scala tympani, either by increasing hydrostatic pressure above the basilar membrane (19) or by low-frequency biasing (20). Clinical SP elevation may reflect hydromechanical changes within the scala media caused by a relative in- crease in endolymphatic fluid pressure (17), explaining the elevated SP/AP observed in two-thirds of patients with Ménière’s disease (21) and in animal models of hydrops (22). Moreover, elevation in SP/AP has been acutely induced by suctioning perilymph from surgically generated round window fistulas (PLFs) in guinea pigs. After the fistula healed, the SP/AP returned to normal (23). This finding has been replicated in other animal models and in humans undergoing surgical exploration for PLF. Arenberg et al. (24) found that 14 of 27 patients with surgically confirmed PLFs had SP/AP greater than 0.5 and that animals with an induced ‘‘active’’ PLF (i.e., leaking perilymph) had ECoG findings and histologic evidence of ELH. Gibson (25) monitored intraoperative SP/AP during stapedectomies and cochleosacculotomies. He found that suctioning perilymph from the vestibule or round window could induce a significant increase in the SP and decrease in the AP and that the changes could be reversed if the vestibule or basal turn refilled with perilymph by increasing intrathoracic pressure. A SSCD may induce hydrostatic changes similar to those described for PLF and ELH. The third-mobile window may reduce the pressure in the scala vestibuli and scala tympani as pressures equalize around the helicotrema, thus inducing an endolymphatic pressure differential. In the operative setting, exposure of the dehiscence or canal lumen, thus opening the perilymph compartment, would acutely lower the perilymphatic pressure. The pressure differential would lead to a temporary relative increase in endolymphatic pressure, driving the basilar membrane toward the scala tympani, resulting in an increase in the SP/AP. Satisfactory plugging of the dehiscence would be expected to promptly restore physiologic perilymph pressures. Further investigations will be needed to investigate this appealing but potentially oversimplified model. Based on our findings, we propose that SSCD and other third-window lesions be added to the differential diagnosis of an elevated SP/AP and that ECoG may be used to confirm a physiologically active SSCD. We found that the sensitivity of ECoG was comparable to VEMP in our series. Crane et al. (14) reported similar sensitivity and specificity values for VEMP (G85 dB defined as abnormal) in their series comparing diagnostic tests and 3-dimensional CT. The mean VEMP thresholds for both affected and unaffected ears were 10 to 20 dB lower in this study than in other reported series (2), suggesting institutional differences in test performance. Regardless of technique, VEMP requires active participation and continuous muscle contraction on the part of the patient. On the other hand, ECoG requires a potentially uncomfortable, albeit safe, tympanic membrane surface electrode placement. Institutional experience with more than 800 tympanic ECoG procedures suggests that the procedure is well tolerated by patients as young as 10 years old. In bilateral cases, Welgampola et al. (7) observed that VEMP thresholds were concordant with symptoms and may be useful in determining the more severely affected side. Similarly, this study found that the ECoG tended to be higher in the symptomatic ear. Further investigation is Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. ELECTROCOCHLEOGRAPHY IN SSCD necessary to determine if the SP/AP may provide physiologic evidence of the more affected ear when selecting the ear for surgery in ambiguous bilateral cases. The greatest use of ECoG in SSCD may be realized in the operating room. Unlike VEMP, CHL, and soundevoked nystagmus, changes in the SP/AP can be detected in a patient under general anesthesia. The ECoG surface electrode can be easily placed and secured before beginning the surgery. The potentials are monitored with standard clinical equipment and software already used during neurotologic surgery to monitor the auditory brainstem response (6). It seems that the monitoring audiologist is able to pinpoint the moment of dehiscence exposure without notification by the surgeon, simply by observing an increase in the SP/AP. We have used the same technology intraoperatively when treating posterior semicircular canal disease, with similar findings. Interestingly, the phenomenon of SP elevation at the moment of canal fenestration has been described once previously. Gibson (26) published ECoG tracings, documenting a sudden marked increase in SP amplitude when the posterior semicircular canal was inadvertently fenestrated during an endolymphatic sac decompression. He sealed the opening, rather than plugging the canal, and the SP remained abnormally elevated at the end of the procedure. Physiologic documentation of SSCD exposure may be particularly helpful in patients with multiple tegmen dehiscences, when the exact location of the semicircular canal lumen may not be immediately apparent. In such cases, it should be possible to avoid suction trauma to the membranous labyrinth while exploring the middle fossa floor, taking special precautions once the SP/AP rises. In addition, a decrease in the SP/AP after canal occlusion confirms satisfactory plugging of the true SSCD. Image guidance has been used at some centers for similar reasons (6). Although it does not allow for direct correlation to imaging findings, ECoG may be considered a simpler and more physiologic navigation system. In surgery for SSCD, the operating surgeon is challenged by balancing the need to protect the delicate membranous labyrinth, while sufficiently packing a microscopic canal lumen. With experience, surgeons may come to rely on the intraoperative normalization or decrease in SP/AP to assure that the canal is sufficiently plugged without overpacking. Normalization of SP/AP can provide useful feedback to the surgeon that the canal is occluded and that sensorineural hearing has been maintained. If the SP/AP fails to decrease after initial plugging, additional packing may be needed. In this study, all SSCDs were repaired with canal occlusion techniques. Pure resurfacing techniques may result in different SP/AP findings but would be expected to follow suit. Potential limitations of intraoperative ECoG include an inability to monitor potentials if fluid percolates from exposed air cells into the middle ear or if a patient with CSF otorrhea is undergoing concurrent encephalocele repair. In either case, the resultant CHL may prevent adequate ECoG monitoring. Likewise, patients with preexisting CHL may present special problems. 1511 In summary, results of this study suggest that an elevated SP/AP (90.4) on ECoG may be an additional electrophysiologic indicator of SSCD and that canal plugging results in postoperative normalization of SP/AP. Thus, the presence of an undiagnosed SSCD should be added to the differential diagnosis of an elevated SP/AP. SP/AP elevation occurs intraoperatively upon exposure of the canal lumen, either at the time of dural elevation overlying a dehiscence or at the time of a transmastoid semicircular canal fenestration. Thus, ECoG may be used in the operating room to document both dehiscence exposure and satisfactory canal occlusion. REFERENCES 1. Minor LB, Solomon D, Zinreich JS, Zee DS. Sound- and/or pressureinduced vertigo due to bone dehiscence of the superior semicircular canal. Arch Otolaryngol Head Neck Surg 1998;124:249Y58. 2. Minor LB. Clinical manifestations of superior semicircular canal dehiscence. Laryngoscope 2005;115:1717Y27. 3. Gopen Q, Zhou G, Poe D, Kenna M, Jones D. Posterior semicircular canal dehiscence: first reported case series. Otol Neurotol 2010;31: 339Y44. 4. Zhou G, Gopen Q, Poe DS. Clinical and diagnostic characterization of canal dehiscence syndrome: a great otologic mimicker. Otol Neurotol 2007;28:920Y6. 5. Belden CJ, Weg N, Minor LB, Zinreich SJ. CT evaluation of bone dehiscence of the superior semicircular canal as a cause of sound- and/or pressure-induced vertigo. Radiology 2003;226: 337Y43. 6. Carey JP, Migliaccio AA, Minor LB. Semicircular canal function before and after surgery for superior canal dehiscence. Otol Neurotol 2007;28:356Y64. 7. Welgampola MS, Myrie OA, Minor LB, Carey JP. Vestibularevoked myogenic potential thresholds normalize on plugging superior canal dehiscence. Neurology 2008;70:464Y72. 8. Minor LB, Carey JP, Cremer PD, Lustig LR, Streubel SO, Ruckenstein MJ. Dehiscence of bone overlying the superior canal as a cause of apparent conductive hearing loss. Otol Neurotol 2003;24:270Y8. 9. Arts HA, Adams ME, Telian SA, El-Kashlan H, Kileny PR. Reversible electrocochleographic abnormalities in superior canal dehiscence. Otol Neurotol 2009;30:79Y86. 10. Akin FW, Murnane OD, Proffitt TM. The effects of click and tone-burst stimulus parameters on the vestibular evoked myogenic potential (VEMP). J Am Acad Audiol 2003;14:500Y9; quiz 34Y5. 11. Margolis RH, Rieks D, Fournier EM, Levine SE. Tympanic electrocochleography for diagnosis of Meniere’s disease. Arch Otolaryngol Head Neck Surg 1995;121:44Y55. 12. Aso S, Watanabe Y, Mizukoshi K. A clinical study of electrocochleography in Meniere’s disease. Acta Otolaryngol 1991;111: 44Y52. 13. Cloutier JF, Belair M, Saliba I. Superior semicircular canal dehiscence: positive predictive value of high-resolution CT scanning. Eur Arch Otorhinolaryngol 2008;265:1455Y60. 14. Crane BT, Minor LB, Carey JP. Three-dimensional computed tomography of superior canal dehiscence syndrome. Otol Neurotol 2008;29:699Y705. 15. Carey JP, Minor LB, Nager GT. Dehiscence or thinning of bone overlying the superior semicircular canal in a temporal bone survey. Arch Otolaryngol Head Neck Surg 2000;126:137Y47. 16. Adams ME, Heidenreich KD, Kileny PR. Audiovestibular testing in patients with Meniere’s disease. Otolaryngol Clin North Am 2010;43:995Y1009. 17. Arenberg IK, Kobayashi H, Obert AD, Gibson WP. Intraoperative electrocochleography of endolymphatic hydrops surgery using clicks and tone bursts. Acta Otolaryngol Suppl 1993;504:58Y67. 18. Durrant JD, Wang J, Ding DL, Salvi RJ. Are inner or outer hair Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. 1512 19. 20. 21. 22. M. E. ADAMS ET AL. cells the source of summating potentials recorded from the round window? J Acoust Soc Am 1998;104:370Y7. Butler RA, Honrubia V. Responses of cochlear potentials to changes in hydrostatic pressure. J Acoust Soc Am 1963;35:1185Y92. Durrant JD, Gans D. Biasing of the summating potentials. Acta Otolaryngol 1975;80:13Y8. Ferraro JA, Arenberg IK, Hassanein RS. Electrocochleography and symptoms of inner ear dysfunction. Arch Otolaryngol 1985;111:71Y4. Jin XM, Guo YQ, Huangfu MS. Electrocochleography in an experimental animal model of acute endolymphatic hydrops. Acta Otolaryngol 1990;110:334Y41. 23. Campbell KC, Savage MM. Electrocochleographic recordings in acute and healed perilymphatic fistula. Arch Otolaryngol Head Neck Surg 1992;118:301Y4. 24. Arenberg IK, Ackley RS, Ferraro J, Muchnik C. ECoG results in perilymphatic fistula: clinical and experimental studies. Otolaryngol Head Neck Surg 1988;99:435Y43. 25. Gibson WP. Electrocochleography in the diagnosis of perilymphatic fistula: intraoperative observations and assessment of a new diagnostic office procedure. Am J Otol 1992;13:146Y51. 26. Gibson WP. The use of intraoperative electrocochleography in Meniere’s surgery. Acta Otolaryngol Suppl 1991;485:65Y73. Otology & Neurotology, Vol. 32, No. 9, 2011 Copyright © 2011 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.