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Auditory Function in Patients with Surgically TreatedSuperior Semicircular Canal Dehiscence

*†Charles J. Limb, *John P. Carey, *Sharmila Srireddy, and *Lloyd B. Minor

*Department of OtolaryngologyYHead and Neck Surgery, Johns Hopkins Hospital, Baltimore, MD; andÞThe National Institute on Deafness and Other Communication Disorders, National Institutes of Health,

Bethesda, Maryland, U.S.A.

Objective: To characterize preoperative and postoperative au-diologic findings in patients with superior semicircular canaldehiscence syndrome.Study Design: Retrospective case review.Setting: Tertiary referral center.Patients: Patients with documented superior semicircularcanal dehiscence syndrome (according to history, vestibulartesting, and high-resolution computed tomography imaging)who underwent surgical repair of their dehiscence.Intervention: Middle fossa craniotomy for superior semicir-cular canal plugging and/or resurfacing.Main Outcome Measures: Audiologic testing both before andafter surgery with pure-tone threshold measurements of air andbone conduction.Results: Twenty-nine subjects underwent surgical repair of su-perior semicircular canal dehiscence. Overall, there were no sta-tistically significant differences by paired t test in hearing beforeor after surgery, in either air-conduction or bone-conductionthresholds, for 19 patients that had no previous surgical history.

At least partial closure of air-bone gap was achieved in five pa-tients. One patient with previous stapes surgery had significantlyworse hearing both before and after canal repair compared withthose without previous surgery. Two patients who had undergoneprevious middle fossa surgery with incomplete resolution ofsymptoms developed sensorineural hearing loss after revisionsurgery. Previous middle-ear exploration and tympanostomytube placement did not seem to affect audiologic outcomes. Sur-gical hearing results did not differ according to method of canalrepair (plugging versus resurfacing).Conclusion: Primary middle fossa repair of superior semicir-cular canal dehiscence is not associated with sensorineuralhearing loss and, in some cases, can lead to normalization ofconductive hearing loss. Revision middle fossa repair or pre-vious stapes surgery may be associated with postoperativesensorineural hearing loss. Key Words: Canal pluggingVCanalresurfacingVHearingVMiddle fossa approachVSuperior semi-circular canal dehiscence syndromeVSurgical repair.Otol Neurotol 27:969Y980, 2006.

In the syndrome of superior semicircular canal dehis-cence (SSCD), an absence of bone overlying the supe-rior semicircular canal leads to symptoms of oscillopsia,pressure-induced vertigo, and sound-induced vertigo(1Y3). In addition to vestibular deficits, many patientswith SSCD also note auditory deficits, the most notablebeing an increased air-bone gap (ABG) of the lowerfrequencies attributable to both a loss of air conductionand a gain in bone conduction (4Y7). These auditorydeficits have been postulated to be the consequence ofa Bthird mobile window^, in addition to the oval andround windows, created by the dehiscence (1,4,7), anidea that is supported by animal studies in chinchillas(8). This third window acts to either dissipate incoming

vibromechanical energy, causing a loss of air-conductionhearing, or to enhance the vibrations of sound introducedvia bone conduction, causing an improvement in bone-conduction hearing (Fig. 1). Sensorineural hearing losshas also been observed in patients with SSCD (1), al-though currently this phenomenon is less well understoodthan conductive losses.

Given the intimate association between the superiorsemicircular canal and the cochlea both physiologicallyand anatomically, it is unsurprising that SSCD is asso-ciated with hearing loss. The treatment for SSCD, usu-ally performed for reasons of disabling vertigo, issurgical establishment of a barrier between the membra-nous labyrinth at the dehiscence and the overlying tem-poral lobe of the brain that eliminates the third mobilewindow created by the dehiscence. At our institution,this treatment is typically performed via a middlefossa craniotomy and superior semicircular canal plug-ging with resurfacing or resurfacing alone (Fig. 2) with

Address correspondence and reprint requests to Charles J. Limb, M.D.,Department of OtolaryngologyYHead and Neck Surgery, Johns HopkinsHospital, 601 N. Caroline Street, 6th Floor, Baltimore, MD 21287; E-mail:[email protected]

Otology & Neurotology27:969Y980 � 2006, Otology & Neurotology, Inc.

969


the assistance of intraoperative stereotactic image-guidednavigation (9). Other than our use of image-guided navi-gation, our technique is very similar to other methodsdescribed in the literature for both benign positionalvertigo and SSCD (10Y13). Because many cases ofSSCD with vertigo do not present with hearing loss, itis important to establish whether surgical treatment ofSSCD can cause new-onset hearing loss. Secondly, inpatients who do have preexisting (usually conductive)hearing loss, it is important to determine whether elim-ination of the dehiscence reverses this auditory deficit.Hence, this study seeks to determine audiologic charac-teristics in patients with surgically treated SSCD. Wehypothesized that SSCD is not associated with hearingloss in cases of primary repair, but that revision surgeryis associated with hearing loss.

METHODS

SettingThis study was performed at the Johns Hopkins Hospital in

Baltimore, MD, a tertiary referral center. This study was areview of existing clinical data with patient identifiers re-moved. It qualified for exemption from an institutional reviewboard protocol on the basis of United States Department ofHealth and Human Services criteria 45 CFR 46.101(b4). Theinstitutional review board made the determination that thestudy was exempt for a protocol.

Study DesignAll patients with surgically treated SSCD were identified. There

were no negative explorations in any of these cases. To satisfy in-clusion criteria for the study, only those subjects with documentedSSCD (by history, physical examination, auditory-vestibular

testing, high-resolution computed tomography, and intraoperativeconfirmation of dehiscence) were examined here. All subjectsunderwent extensive preoperative and postoperative testing of bothauditory and vestibular function. Only auditory test results arediscussed here. Pure-tone thresholds using both air conduction at250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz and bone con-duction at 500 Hz, 1 kHz, 2 kHz, and 4 kHz were obtained beforeand after surgery. Speech discrimination was also tested for allpatients. A retrospective case review was performed from thisselect group of patients.

Statistical AnalysisAll data were statistically analyzed using Stata (StataCorp

LP, College Station, TX) software and rendered using Sigma-Plot (SyStat Software, Inc., Point Richmond, CA). Two-tailedpaired t tests were used to assess statistical significance of databefore and after intervention.

RESULTS

Study PopulationOf 100 patients (60 men, 40 women) with SSCD evalu-

ated at our institution between 1995 and 2005, 29 subjectsunderwent surgical repair, which we offer for debilitatingvestibular symptoms (e.g., noise- or Valsalva-induced ver-tigo) or debilitating auditory symptoms (e.g., pulsatile tin-nitus, severe autophony). Of these 29 subjects, 15 were maleand 14 were female, with an age range of 27 to 64 years(median, 44 years). Of these 29 subjects, 19 had no history ofsurgical repair, whereas 11 subjects (including 1 subject that

FIG. 1. Third mobile window created by presence of SSCD.This mobile window (arrow), in addition to the oval and roundwindows, either dissipates incoming vibromechanical energy(leading to hearing loss by air conduction) or enhances transmis-sion of sound by bone (leading to low-frequency hyperacusis bybone conduction). Drawing by Brian Dunham, M.D.

FIG. 2. Surgical repair of SSCD via middle fossa craniotomy. A,Illustration showing intact membranous labyrinth with dehiscence ofoverlying bone. B, Use of canal plugging, using a combination offascia and bone chips, within the lumen of the superior semicircularcanal, followed by bony resurfacing using fascia and a bone graft.The membranous labyrinth is compressed by the fascial plug at thedehiscence site. C, Canal resurfacing procedure alone, with place-ment of fascia and bone graft over the membranous canal withoutplugging of the canal lumen (Figure modified from Minor (2)).

970 C. J. LIMB ET AL.

Otology & Neurotology, Vol. 27, No. 7, 2006


was revised) had a positive history of surgery (1 previousmiddle fossa repair of SSCD at an outside institution, 1previous middle fossa repair at our institution, 5 middleear explorations, 3 stapes procedures, and 1 tympanostomytube placement). Dehiscence was present bilaterally in 4cases, right-sided only in 9 cases, and left-sided only in16. The surgical method used for canal repair (pluggingwith resurfacing versus resurfacing alone) varied in accor-dance with modifications of surgical technique in responseto short-term outcomes. Initially, patients underwent canalplugging with resurfacing, followed by a period duringwhich resurfacing alone was attempted. Currently, we

have returned to plugging of the canal with resurfacing as themethod of choice because of a higher incidence of symptomrecurrence when we performed resurfacing alone (14). Ofthe 29 patients surgically treated, 18 patients underwentplugging and resurfacing, whereas 11 patients underwentresurfacing procedures alone.

Subjects Without Previous SurgeryIn patients without previous surgery, hearing remained

essentially stable after surgical repair via middle fossacraniotomy. Figures 3 and 4 show preoperative and post-operative mean audiograms on the side of dehiscence only.

FIG. 3. Audiogram showing mean preoperativehearing threshold responses for all subjects with-out previous surgery. Results for air conduction(open circles) and bone conduction (right-facingbracket) are shown. These audiograms repre-sent mean data for all subjects and hence in-clude error bars showing standard deviation.

FIG. 4. Audiogram showing mean postopera-tive hearing threshold responses for all subjectswithout previous surgery. Results for air conduc-tion (open circles) and bone conduction (right-facing bracket) are shown. These audiogramsrepresent mean data for all subjects and henceinclude error bars showing standard deviation.No major changes are seen before or after sur-gery in this group of patients.

971HEARING AFTER REPAIR OF SUPERIOR CANAL DEHISCENCE



Although subtle changes can be observed (e.g., bone-conduction thresholds at 4 kHz), these differences arecertainly small. Statistical analysis was performed usingtwo-tailed paired t tests at each frequency tested. Usingthis method, p values calculated at each frequency were asfollows for air-conduction thresholds: 250 Hz (p= 0.47); 500Hz (p= 0.91); 1,000 Hz (p= 0.72); 2,000 Hz (p= 0.42); 4,000Hz (p = 0.12); 8,000 Hz (p = 0.11). Bone-conduction thresh-old mean values were as follows: 500 Hz (p = 0.004); 1,000Hz (p = 1.0); 2,000 Hz (p = 0.92); 4,000 Hz (p = 0.65).

Insufficient data were obtained to allow paired t test calcula-tions at 250 Hz. The statistically significant difference be-tween preoperative and postoperative bone-conductionthresholds at 500 Hz are for a difference of 3.2 dB HL (pre-operative) and 6.54 dB HL (postoperative). Mean preopera-tive speech discrimination scores were 96.5% T 5.0%,whereas mean postoperative speech discrimination scoreswere 96.8% T 3.8% (paired t test p = 0.66). Of these 19patients, 13 patients underwent canal plugging with resurfa-cing, whereas 6 patients underwent canal resurfacing alone.

FIG. 5. Audiogram showing mean preoperativehearing threshold responses for six subjects withmiddle ear exploration or tympanostomy tube place-ment before SSCD repair. Results for air conduction(open circles) and bone conduction (right-facingbracket) are shown. These audiograms representmean data for all subjects and hence includeerror bars showing standard deviation.

FIG. 6. Audiogram showing mean postopera-tive hearing threshold responses for six subjectswith middle ear exploration or tympanostomytube placement before SSCD repair. Results forair conduction (open circles) and bone conduc-tion (right-facing bracket) are shown. Theseaudiograms represent mean data for all subjectsand hence include error bars showing standarddeviation. No significant changes are seen beforeor after surgery in this group of patients.




Subjects With Previous Surgery: MiddleEar Exploration and Pressure Equalization

Tube PlacementFive patients underwent middle ear exploration, typi-

cally for a planned stapes replacement procedure, thatwas subsequently aborted when otosclerosis could notbe confirmed intraoperatively. One patient underwentmyringotomy and tympanostomy tube placement. Be-cause each of these variables could have affected initialhearing before middle fossa canal repair, they were con-sidered separately. Given that no oval window or inner

ear manipulation was performed in these cases, however,we hypothesized that there would be no significant dif-ference between preoperative and postoperative audio-logic results in this patient group and that they wouldshow similar audiologic characteristics with patientsthat had no previous surgical history. Figures 5 and 6show preoperative and postoperative mean audiogramsof this patient group. There were no quantitative dif-ferences in audiologic threshold levels by air or bone con-duction or speech discrimination scores (mean, 98% T 2.3%preoperatively versus 99% T 2.1% postoperatively) between

FIG. 7. Audiogram showing mean preoperativehearing threshold responses for three subjectswith previous stapes repair procedures usingair conduction (open circles) and bone conduc-tion (right-facing bracket). Error bars indicatestandard deviation.

FIG. 8. Audiogram showing mean postopera-tive hearing threshold responses for three sub-jects with previous stapes repair proceduresusing air conduction (open circles) and boneconduction (right-facing bracket). Error bars in-dicate standard deviation. Average data reveala significant worsening of auditory function afterSSCD repair in these patients, which can beattributed to one patient in particular, whosedata are shown in Figure 9.




these patients and those without previous surgery, nor wasthere any significant effect of the middle fossa canal repairon hearing levels after surgery by paired t test (p 9 0.1). Inthis group of six patients, three underwent canal pluggingwith resurfacing and three underwent resurfacing alone.

Subjects With Previous Surgery: Stapes ProceduresThree patients had undergone previous stapes proce-

dures that were ineffective in treating conductive hearing

loss. These patients therefore had surgical factors thatcould have affected both initial pre-SSCD repair hearinglevels and post-SSCD repair hearing levels, given the closeassociation between the oval window and the superiorsemicircular canal. Therefore, these patients were alsoconsidered separately from those without previous sur-gery. Audiograms for preoperative and postoperativeresults are shown below in Figures 7 and 8. These graphsreveal mean levels of hearing both preoperatively and

FIG. 10. Audiogram showing mean hearing threshold responses for two subjects with previous attempts at canal repair via middle fossacraniotomy, using air conduction (open circles) and bone conduction (right-facing bracket). Preoperative (dashed lines) and postoperative (solidlines) audiologic data are shown.Error bars indicate standard deviation. Because bone-conduction data were not routinely obtained in cases ofnormal hearing in the early part of this series, the preoperative bone-conduction line represents results in one patient only. This group of patientshad significant loss of hearing after SSCD repair, despite preservation of intraoperative auditory evoked brainstem potentials.

FIG. 9. Preoperative and postopera-tive hearing threshold results in onepatientwhohad threepreviousattemptsat stapes mobilization and who devel-oped a significant mixed hearing lossafter SSCD repair, with nontestablespeech discrimination in the operativeear. Preoperative (dashed lines) andpostoperative (solid lines) audiologicdata are shown. Appropriate maskingwas used for all measurements.




postoperatively that are poorer than that in patients withoutprevious stapes surgery, with extremely wide variances thatmade statistical analysis in this small subject group difficult.Speech discrimination scores also showed a similar decline(mean, 98.7% T 2.3% preoperatively versus 65.3% T 56.6%postoperatively). Much of the variability in this group wasattributed to one patient that had undergone three previousattempts at stapes repair (Fig. 9). Intraoperative monitoringof auditory brainstem responses showed a loss of the

evoked waveform during surgery, and this patient had asignificant loss of sensorineural function after middle fossacanal repair with nontestable speech discrimination. Thispatient’s audiogram shows a very large postoperativeABG, which may be exaggerated by the persistence oflow-frequency conductive hyperacusis; all thresholdswere measured with appropriate masking levels, reduc-ing the likelihood of a shadow curve effect. In contrast,the other two patients (each of whom had undergone a

FIG. 11. Audiogram showing normalization of low-frequency conductive hyperacusis and closure of ABG in one patient after middlefossa repair of SSCD.

FIG. 12. Audiogram showing partial closure of large ABG (55 dB at 500 Hz) in one patient after middle fossa repair of SSCD.




single stapes procedure) had stable or improved hearingafter SSCD repair, with speech discrimination scores of96% and 100%, respectively. Although the number ofcases here is small, it suggests that a history of multipleprevious stapes procedures is a risk factor for sensorineuralhearing loss after SSCD repair. This patient also underwentcanal plugging with resurfacing, although of the two otherprevious stapes procedure patients, canal pluggingwith resurfacing was used in one patient without ad-verse effects.

Patients With Previous Surgery: MiddleFossa Repair

Revision middle fossa craniotomy patients that hadundergone attempted canal repair with subsequent failure(usually through slippage or resorption of the bone graft)were analyzed separately. Two patients were identified.Different methods of repair (plugging with resurfacingversus resurfacing only) were used in each case. Despitethe preservation in both cases of intact auditory brainstemevoked potentials intraoperatively, significant loss in

FIG. 13. Audiogram showing closure of small ABGs (largest gap of 15 dB) in one patient after middle fossa repair of SSCD.

FIG. 14. Audiogram showing partial closure of small-moderate ABGs (largest gap of 30 dB) after middle fossa repair of SSCD.




sensorineural auditory function was observed in each case(Fig. 10), with a decrease in mean pure-tone average by airconduction (5.8 dB T 4.9 dB preoperatively versus 61.7 T12.5 dB postoperatively) and bone conduction (j6.7 dB T5.8 dB preoperatively versus 48.3 T 12.1 dB postopera-tively), as well as speech discrimination (preoperativemean score 92% versus postoperative mean score 22%).Again, the number of such cases is small because sur-gical repair for SSCD typically does not require revision;but these cases strongly suggest that previous middlefossa craniotomy and manipulation of the open canalsuperior are major risk factors for postoperative senso-rineural hearing loss. Furthermore, these limited casessuggest that intraoperative auditory brainstem responsemonitoring may not be effective in identifying intra-labyrinthine trauma that might lead to delayed sensori-neural hearing loss.

Closure of Air-Bone GapA total of five patients demonstrated at least partial

closure of their ABGs after surgical repair of SSCD.The preoperative and postoperative audiograms forthese patients are included in Figures 11Y14 below,whereas the fifth patient is discussed separately as anunusual case below. One patient (Fig. 11) demonstratednormalization of low-frequency conductive hyperacusis,with both improvement in air conduction and decrease inbone conduction after surgical repair. This group of fivepatients also included one patient with previous stapessurgery (Fig. 12), and two other patients who had rela-tively small ABGs (Figs. 13, 14). For all audiograms, thedashed lines indicate preoperative hearing thresholds,whereas solid lines indicate postoperative thresholds; cir-cles refer to air conduction, whereas brackets refer tobone conduction.

FIG. 16. Preoperative audiologic thresholds in both ears in response to air and bone conduction, with bilateral conductive hearing loss inthis patient with bilateral SSCD. Characteristic Carhart notches can be seen at 2 kHz bilaterally.

FIG. 15. Temporal bone computed tomo-graphic scans reformatted in the plane of thesuperior semicircular canal, showing bilateraldehiscence in one patient (arrows). The rightside was the more symptomatic side and had thelarger dehiscence by imaging. Bone-conductiontesting showed bilateral 2-kHz notch depres-sions similar to Carhart notches.




Unusual Case: Bilateral SSCD With Carhart NotchesOne patient with definite bilateral SSCD dehiscence

(Fig. 15) presented with the symptoms of bilateral con-ductive hearing loss (right 9 left), bilateral sound-inducedvertigo and nystagmus, right external auditory canal pres-sure-induced nystagmus, and bilaterally reduced vestibu-lar evoked myogenic potentials (80 dB right, 75 dB left).Audiologic testing in this individual before canal repairrevealed bilateral (right 9 left) conductive hearing losseswith a distinctive notch at 2 kHz reminiscent of the classicCarhart notch observed in otosclerosis (15Y18) (Fig. 16).This patient underwent surgical repair of the right, moresymptomatic side via middle fossa craniotomy and canalplugging with resurfacing; despite postoperative findingsof vestibular hypofunction in all three semicircular canals,the audiogram showed improvement in hearing. In particu-lar, his postoperative air-conduction thresholds at 2 kHzwere better than his preoperative bone levels, demonstratingoverclosure of the ABG and confirming the artifactual na-ture of the depressed notch at 2 kHz (Fig. 17).

DISCUSSION

Our results show that hearing preservation after surgicalrepair of SSCD is highly feasible and that in cases withoutprevious surgical treatment, there is no association be-tween SSCD repair and sensorineural hearing loss. In ourseries of 29 patients, hearing loss occurred in patients thatunderwent previous surgical procedures on either the co-chlea (stapes procedures) or the labyrinth (previous semi-circular canal repair). Procedures that were performedwithout manipulating the inner ear, such as middle earexploration or tympanostomy tube placement, were not

associated with hearing loss. In the three patients that suf-fered hearing loss after surgical repair, one had undergonethree previous stapes procedures, and two had undergonemiddle fossa craniotomy with canal repair procedures. Al-though numerous possible factors may have contributed tothe loss of hearing in these cases (i.e., hearing loss couldhave occurred regardless of previous surgical history), it isdifficult to ignore the relevance of previous surgical fac-tors, particularly given the fact that each of our patientswith hearing loss belonged to the group of patients withprevious surgery.

Our results showed overall stability of bone-conductionthresholds before and after canal repair in patients withoutprevious surgery. These results are somewhat perplexingin light of the fact that major symptom control wasachieved in all cases. This barrier should therefore havesignificantly reduced or eliminated altogether any conduc-tive hyperacusis, as well as an ABG. It is possible, how-ever, that the presence of a dehiscence introduces smallartifacts in our measurement of sensorineural function bybone conduction that would cause an effective decrease inhearing measurements (akin to that observed in otoscle-rosis). Five patients demonstrated improvement in air-conduction thresholds and at least partial closure of theABG, including the one patient described here with bilat-eral Carhart notches who showed both significant hearingimpairment preoperatively and improvement postopera-tively. Although animal studies have confirmed that theconductive hearing loss created by a dehiscence should bereversible (8), further study of this phenomenon in humanpatients is required.

Of the 19 subjects without previous surgery, 6 pa-tients had a preoperative conductive hearing loss. Thisnumber does not include patients that had normal air

FIG. 17. Preoperative and postoperative au-diologic thresholds after middle fossa repair ofthe right side only in the same individual withpreoperative Carhart notches. The postopera-tive air conduction (solid line) at 2 kHz is betterthan the preoperative bone conduction at2 kHz (dashed line), demonstrating ABG over-closure and confirming the artifactual natureof the preoperative notch.




conduction with low-frequency conductive hyperacusis.Of the six patients with conductive hearing loss, fourdemonstrated at least partial closure of their ABG aftersurgery (Figs. 11Y14). Of the two remaining patients, onehad 5- to 25-dB increases in both air and bone conduc-tion, whereas the other had a 15-dB increase in air-conduction thresholds (at 2, 4, and 8 kHz) and 15- to 25-dBincrease in bone-conduction thresholds (at all frequen-cies tested), implying a degree of sensorineural hearingloss in the high frequencies but also partial closure of theABG in the low frequencies (which may represent someimprovement in low-frequency conductive hyperacusis).One patient developed a temporary conductive hearingloss after surgery because of a middle ear effusion thathas since resolved (with normalization of hearing) sincethe initial submission of this manuscript. The improve-ments in conductive hearing in the four subjects werepartially offset by the hearing losses in these two sub-jects but were also relatively minor in the context of thewhole group, so that paired t tests at each frequencyshow no statistically significant changes before andafter surgery in this group of patients who had conduc-tive hearing loss due to superior canal dehiscence.

The relationship between previous surgery in the case ofstapes procedures and hearing loss after SSCD repair islikely to be based on the loss of integrity in such cases atboth the dehiscence (the third mobile window) and the ovalwindow. Although the oval window is normally mobile,previous stapedectomy with removal of the footplate oreven multiple stapedotomy procedures are likely to in-crease the mobility at the oval window. Given the factthat stapes procedures alone are associated with a smallbut definite risk of sensorineural hearing loss due to innerear injury, one might speculate that stapes footplate removalcreates a situation that maximizes the risk of perilymphoutflow when excess pressure is exerted on the system,such as when the lumen of the superior semicircularcanal is plugged. The sudden outflow of perilymph, proba-bly through or around the oval window graft, could causethe severe sensorineural hearing loss that we observed inthese patients. Alternatively, stapes surgery may violatethe blood-labyrinthine barrier, which subsequently predis-poses the inner ear to sensorineural injury after furthermanipulation. Although the numbers of these cases aresmall (and only one of three patients with previous stapesprocedures had hearing loss), the relationship between pre-vious stapes surgery and SSCD repair deserves furtherinvestigation to improve the counsel provided to patientsin this clinical context. Furthermore, because otosclerosisis often suspected and stapes surgery attempted when adiagnosis of SSCD is missed (19), this patient group islikely to persist. If in fact, as suggested by one of ourpatients reported here, Carhart notch (defined here as anartifactual decrease in bone conduction at 2 kHz in thesetting of conductive hearing loss) can be observed incases of SSCD, the proper identification of SSCD inpatients with conductive hearing loss becomes evenmore difficult. It should be noted that Carhart notch isnot exclusively a finding limited to otosclerosis. Most

importantly, this category of patients emphasizes the im-portance of obtaining acoustic reflex measurements or ves-tibular evoked myogenic potentials in all patients with anABG, so that the proper cause of the conductive hearingloss can be identified before surgical attempts at repair.

In the case of previous middle fossa surgery, the relation-ship between hearing loss and SSCD repair is likely to bebased on postsurgical scarring and adhesions that develop atthe site of the repair. We have observed slippage of the bonegraft during release of the temporal lobe and resorption aftersurgery (3), which would expose the dehiscent canal (andmembranous canal) to the now surgically manipulated dura.It is also likely that adhesions develop between the dura andmembranous canal, and that subsequent manipulation of thedura can cause significant trauma to the delicate membra-nous canal, with possible rupture and outflow of perilymph.Again, in both patients that underwent revision craniotomy,we had subsequent postoperative sensorineural hearing loss.It might also be postulated that there is an implicit tendencyduring revision cases to be more aggressive during canalplugging, in light of the fact that revisions are usually per-formed in cases where symptoms are refractory to treatment,and that definitive control of symptoms is sought at the timeof revision. If so, more aggressive plugging could causesubsequent injury to the cochlea, with resultant hearingloss. Although our numbers here do not show a clear asso-ciation between method of repair and hearing loss, this isvery likely to be because of the small number of cases in-volved. Clinically, it is our suspicion that canal pluggingachieves superior control over symptoms (also supportedby Mikulec et al. (13)), but that it also carries a slightlyincreased risk of sensorineural hearing loss.

Most patients that elect to receive surgical treatment forSSCD choose so because of vestibular symptoms or audi-tory symptoms related to conductive hyperacusis (e.g., hear-ing one’s own heartbeat, autophony, or eye movements), notconductive hearing loss. It is unusual for a patient to seeksurgical repair of SSCD for conductive hearing loss alone.Furthermore, the exact mechanisms behind sensorineuralhearing loss and SSCD remain unclear. Given the findingsshown in this study and the vagaries associated with hearingin SSCD, it is recommended that patients receiving revisiontreatment for SSCD be advised of the possibility of hearingloss after treatment. In primary cases of SSCD repair, thereis no statistically increased risk of hearing loss. The in-creased identification of cases of SSCD and treatmentwith canal repair will yield important data regarding theauditory consequences of SSCD and will continue to shedlight on this interesting condition.

Acknowledgments: The authors thank Peter Elfert andKarin Placidet for their assistance in providing superior canaldehiscenceYformatted temporal bone images, and Gautam Bhatiafor assistance in obtaining information in the medical records.

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