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Superior Semicircular Canal Dehiscence
CHAPTER 12
Superior Semicircular Canal Dehiscence GERARD J. GIANOLI, MD, FACS • JAMES S. SOILEAU, MD
INTRODUCTION Superior semicircular canal dehiscence (SSCD) can be defined as an anatomic anomaly found (occasionally incidentally) on high-resolution temporal bone computed tomography (CT) scan or at surgical exploration (Fig. 12.1) or as a syndrome, a constellation of symptoms, that frequently accompanies this anatomic deviation. An anatomic dehiscence of bone overlying the superior semicircular canal (SSC) at its interface with the middle cranial fossa dura has been proposed to be a developmental abnormality.1 Bone overlying the middle ear cavity and mastoid air cell system (tegmen tympani and tegmen mastoideum) separating the ear from the middle cranial fossa (a.k.a. the floor of the middle cranial fossa) thickens progressively during the first 3 years of life, such that CT findings of SSCD in a 1-year-old patient may “disappear” by the time the patient is 3 or 4 years old.2 At this age, the incidence of an anatomic finding of SSCD is believed to be relatively stable, rendering 1%–2% of the general population at risk for developing SSCD syndrome.3 The calvarium initially thickens in early childhood, but then, later in life, the thickness of the calvarium and the area of the middle fossa floor in question, very slowly thins. This has also been proposed as a possible cause for development of SSCD in patients who had very thin bone covering the SSC earlier in life.4 Additional factors that may make an individual susceptible to SSCD are erosive processes such as arachnoid granulations, cholesteatoma, other neoplastic processes, and trauma/fractures. More recently, the CDH23 gene (associated with Usher Syndrome and nonsyndromic hearing loss) has been found to be a genetic risk factor for the development of SSCD.5 SSCD has been called the great otologic mimicker because of the myriad of clinical presentations that are similar or identical to other major otologic disorders, such as patulous Eustachian tube, otosclerosis, Meniere’s disease, benign paroxysmal positional vertigo (BPPV), perilymph fistula, sudden sensorineural hearing loss, and acute vestibular neuritis.6 This variety
of presentations gives rise to a disorder that can go undiagnosed or misdiagnosed for quite some time. Consider the SSCD patient who presents initially with conductive hearing loss without vestibular symptoms and is diagnosed with otosclerosis. Unless the patient undergoes some investigation beyond basic audiometry, the patient can go for quite some time labeled as having “otosclerosis” before a different diagnosis is considered. Even after the patient develops vestibular symptoms, many clinicians may preserve the otosclerosis label while adding other diagnoses, such as BPPV or “otosclerosis with Meniere’s disease.” Clinicians must maintain a level of open-mindedness and not “pigeonhole” patients into a single diagnosis when dealing with patients with symptoms of SSCD. To make the condition even more interesting, a patient with the absence of bone overlying the SSC may be completely asymptomatic. It has been recognized since the initial description of the problem in 1998 that a “second event” is suspected to be the root cause of the onset of the symptoms of SSCD. The leading suspected cause is head trauma or conditions that cause increased intracranial pressure. This is the presumed reason that SSCD symptoms are rarely seen in children.
INCIDENCE AND ETIOLOGY The prevalence of SSCD has been found to be much higher in a series of analyzed CT scans than on temporal bone histology. Carey et al. identified a complete absence of bone over the superior canal histologically in 0.5% of 1000 vertically sectioned adult temporal bones.3 There was an additional 1.4% with very thin (<0.1 mm) bone covering the SSC. Added together, the histologic prevalence of thin or dehiscent SSCs approached 2%. This study also reported that 50% of patients with SSCD had bilateral involvement. Carey and colleagues3 also analyzed 36 infant temporal bones and concluded that the thickness of the bone overlying the superior canal was consistently thin. The thickness of the bone covering the SSC gradually increased with age, reaching adult thickness by 3 years of age. 143
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Arcuate eminence
TLD
MFR
FIG. 12.1 Intraoperative, middle fossa craniotomy view of the middle fossa floor demonstrating superior
semicircular canal dehiscence (SSCD) (multiple arrows). MFR, middle fossa retractor; TLD, temporal lobe dura.
Roberto et al.7 used tetracycline staining to investigate the deposition of bone in a dog model at 10, 25, and 50 days of age. This study demonstrated progressive deposition of endochondral and endosteal bone over the SSC postnatally. The bone deposition decreased with age. These findings are in agreement with observations in the study by Carey et al.3 In a related study, Hirvonen and colleagues8 reported a CT temporal bone study of bone thickness over the SSC in patients with SSCD and those without SSCD. Among those with SSCD, the contralateral SSC bone was thinner (or dehiscent), compared with those patients without SSCD. This finding of thin bone overlying the SSC bilaterally supports the notion of SSCD as a developmental anomaly related to bony deposition in early life. Several observations point to SSCD as a developmental anomaly requiring a “second event” to produce symptoms: 1. Studies demonstrating development of bone over the SSC occurring later (postnatally) than other parts of the inner ear. 2. Clinical observations of asymptomatic but anatomic SSCD noted during intraopera tive exploration of the middle cranial fossa for encephalocele repair. 3. Symptoms of SSCD rarely present in the pediatric population. Thus, a second event may be required in addition to the congenital anomaly of thin or absent superior
canal bone to produce clinical symptoms. Roughly half of patients with SSCD report an event they attribute to symptom onset. This “second event” is typically noted to be head trauma, a Valsalva-type episode, barotrauma, or some other type of intracranial pressurealtering event.
CLINICAL PRESENTATION SSCD was first reported by Minor et al.1 in eight patients who exhibited symptoms of short-lived episodic vertigo in response to certain sounds or activities that would cause transient increases in intracranial or middle ear pressure (e.g., Valsalva, coughing, sneezing, nose-blowing, auto-insufflation). These activities produce torsional nystagmus, which directly implicates SSC stimulation. Activities causing increased middle ear pressure (e.g., loud sound, positive pressure in the ear canal, autoinsufflation) induce nystagmus with the slow phase upward and the superior pole of the eye directed away from the affected ear. Activities causing transient elevation in intracranial pressure (e.g., Valsalva against a closed glottis, jugular venous compression) or negative pressure in the ear canal result in the slow phase of nystagmus directed downward and the superior pole of the eye directed toward the affected ear. The clinical findings of Tullio’s phenomenon and pressure-induced nystagmus associated with SSCD has been termed “Minor’s syndrome.” Although the vertigo caused by SSCD is most characteristically
CHAPTER 12 Superior Semicircular Canal Dehiscence reported as short-lived, other descriptions of vestibular symptoms have been reported as well, including more prolonged vertigo spells, chronic disequilibrium, and drop attacks. Since the first identification of SSCD as a cause for Minor’s syndrome, other symptoms and clinical presentations have been identified.9 Although the presentation of patients with SSCD is highly variable, the most recognizable presentation will include Tullio’s phenomenon, pressure-induced vertigo with transient increases in intracranial or middle ear pressure, and autophony. While these symptoms are characteristic of SSCD, they are certainly not present in all SSCD patients, and their absence cannot be used as a means to exclude the diagnosis of SSCD. The more nonspecific symptoms of vestibulopathy such as head movement–induced disequilibrium is frequently described by patients but not particularly helpful in confirming the diagnosis of SSCD. Aural pressure and aural fullness are commonly reported, as are complaints of hearing loss, distorted hearing, pulsatile tinnitus, aural fullness, and hyperacute hearing. There are also vague cognitive and neurobehavioral symptoms frequently reported by SSCD patients that are not specific to SSCD but should be included in the discussion during patient education. These symptoms include depression, “brain fog,” short-term memory problems, and difficulty with concentration. These symptoms often improve after surgical repair of SSCD.10 Patients with SSCD may have been given other diagnoses prior to presentation. The biggest indicator of a misdiagnosis is nonresponse to treatment. This should always prompt the clinician to reassess the prior diagnosis.
PHYSICAL EXAMINATION Routine head and neck examinations are typically normal in the SSCD patient. Microscopic otoscopy is normal as well, unless there has been prior surgical intervention. The vestibular component of the physical examination should include evaluation with infrared video goggles. This is typically unremarkable but may reveal spontaneous nystagmus and head thrust or head-shake abnormalities if there has been any vestibular loss. In some extreme cases, one can identify spontaneous torsional nystagmus that is synchronous with the pulse. Tuning fork testing can also be helpful. Patients may “hear” the tuning fork in the affected (SSCD) ear when
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the tuning fork is placed on the ankle or some other remote bone location. Pneumatic otoscopy can be an effective screening tool for SSCD. Symptoms of vertigo, or a feeling that the world shifts or moves, will often be reported during pneumatic otoscopy. Infrared video-oculography (VOG) cameras can be used to observe and/or record the resulting pressure-induced nystagmus. If VOG cameras are not available, a second examiner, an assistant or family member can also watch for pressure-induced nystagmus. The direction of pressureinduced nystagmus is dependent on the location of dehiscence and whether positive or negative pressure is applied.
TESTING Much of the published literature on SSCD discusses CT imaging and vestibular evoked myogenic potential (VEMP), an electrical potential measured from the sternocleidomastoid muscle elicited by a sound stimulus to the ipsilateral ear. Relatively high-intensity sound stimulates the hair cells in the ampulla of the saccule, which in turn send afferent impulses along the inferior vestibular nerve to vestibular nuclei in the brainstem. The vestibular nuclei send efferent projections along the vestibulospinal (vestibulocollic) tract to the sternocleidomastoid muscle to facilitate head stabilization. Athough the CT scan is imperative (the “gold standard”), and VEMP testing is often helpful, we feel this limited testing is inadequate for patients presenting with symptoms of SSCD. Because SSCD is a disorder that can mimic many other otologic disorders, can cause a number of secondary pathologies, and may require invasive surgery to resolve, we feel a full audiovestibular test battery is warranted. CT scan sections should be performed at the submillimeter level, preferably 0.24 mm thickness, but no thicker than 0.6 mm. The thinner section scan gives a more accurate portrayal of the defect. Thicker scans can be prone to both false-positive and false-negative findings.11 Both coronal and Pöschl views will demonstrate the dehiscence, but the Pöschl view is a parasagittal view perpendicular to the petrous ridge/long axis of the temporal bone displaying the entirety of the SSC (Fig. 12.2). Because of this, Poschl views tend to only identify large dehiscences. Slices perpendicular to the SSC, such as coronal or Stenvers views, are more sensitive in detecting a dehiscence. MRI should be performed to evaluate for concomitant intracranial abnormalities. One of the more frequent findings in SSCD patients is Chiari
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Tegmen defect
Sub. semicircular Canal dehiscence at superior petrosal sinus
Malleus head
FIG. 12.2 Left temporal bone computed tomography (CT) scan in a Pöschl view demonstrating malleus
impinged by overlying temporal lobe encephalocele with CSF leak and superior semicircular canal dehiscence (SSCD) at the superior petrosal sinus.
malformation.12 Additionally, MRI findings suggestive of elevated intracranial pressure, such as empty sella, vertical tortuosity of the optic nerves, prominent arachnoid spaces around the optic nerves, flattening of the globe, slit-like ventricles, venous sinus abnormalities, and Chiari/cerebellar ectopia should also be sought. Audiometric testing may be normal, may show some degree of sensorineural hearing loss in the affected ear, or more characteristically, may demonstrate low-frequency conductive loss, often with supranormal bone conduction thresholds. To distinguish the patient with low-frequency conductive loss who has SSCD from a patient with otosclerosis or a patient with other middle ear pathology, impedance testing is critical. The patient with SSCD typically has normal tympanograms and intact acoustic reflexes, whereas patients with otosclerosis will show absent acoustic reflexes.13 VEMP testing may show reduced threshold responses compared with laboratory norms or the contralateral ear. VEMP testing may show an asymmetric result in patients with unilateral SSCD or may be completely
normal. Amplitude of the VEMP has not been found to be a reliable measure for SSCD. In patients with reduced vestibular function, the VEMP response may be absent.6 Electrocochleography is frequently abnormal in SSCD patients and often normalizes after successful surgery to plug the dehiscent canal.14 A full vestibular evaluation should be performed on any patient who is to undergo SSCD surgery. Any identified vestibular abnormality is helpful in (1) the decision to proceed with surgery and (2) outcome expectations. Patients with SSCD have varied vestibular test profiles including severe unilateral vestibular hypofunction and occasionally severe bilateral vestibular hypofunction. Such patients will continue to experience symptoms from these vestibular deficits despite successful surgical treatment of their SSCD. Additionally, BPPV is a frequent secondary pathology that may need treatment in addition to treatment of SSCD. Video head impulse testing (vHIT) has been introduced to clinical practice recently (see Chapter 8). However, because clinical experience with this testing methodology has not yet been widespread and
CHAPTER 12 Superior Semicircular Canal Dehiscence has only been available for a limited time, we recommend caution when using this as a means to determine “normal” semicircular canal function. Recent studies have shown poor correlation of vHIT with caloric irrigation studies15; therefore, we cannot recommend using vHIT in place of caloric irrigation. However, in the context of SSCD, vHIT may prove to be invaluable in determining superior canal function preoperatively and postoperatively. In patients who have undergone SSCD occlusion, vHIT may be helpful in determining whether the posterior semicircular canal has actually been occluded and therefore hypofunctioning.
PATHOPHYSIOLOGY The most commonly espoused theory for the pathophysiology of SSCD is the “third window” phenomenon. This theory posits that the flexible nature of the SSCD allows for possible egress of endolymph from the SSC resulting in abnormal stimulation of the cupula. Additionally, low-frequency sound energy transmitted through the inner ear is allowed to dissipate through this bony defect, resulting in the auditory conductive component noted in some SSCD patients. Merchant and Rosowski16 proposed that SSCD could be classified among a number of anomalies that produce a third mobile window on the scala vestibuli side of the cochlea. Included among these are lateral or posterior canal dehiscence, enlarged vestibular aqueduct, dehiscence of the internal auditory canal, carotid dehiscence (into the cochlea), diffuse dehiscence (such as in Paget’s disease), and other congenital abnormalities of the inner ear. The hearing loss in these patients with a pathologic third mobile window is characterized by poor air conduction thresholds and normal or supranormal bone conduction thresholds. However, the third window theory does not completely explain all of the findings of SSCD. Among these are the existence of asymptomatic patients, the presentation of patients with only auditory and no vestibular findings or vice versa, Ménière’s-type vertigo spells, and the absence of symptoms in pediatric patients who have definite dehiscence on imaging. It has been proposed that a second event such as trauma or a major pressurealtering event (Valsalva-type maneuver; barotrauma) causes a disruption of very thin bone over the superior canal, thus creating a symptomatic SSCD. However, this only explains adult onset of symptoms and not the other findings, nor does it explain a true dehiscence in an asymptomatic patient. Gianoli and Soileau17 proposed the theory that alteration of intracranial pressure
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may result in increased compliance at the round and oval windows and, if pressure changes were extreme, potential disruption of the windows resulting in a frank middle ear perilymph fistula. This theory could explain the above exceptions to the third mobile window theory and also explain why round window reinforcement has been noted to resolve SSCD symptoms (at least temporarily) in many patients. They further proposed a grading system for SSCD: Stage 1: Asymptomatic SSCD—anatomic dehiscence with no symptoms. Stage 2: Minor’s syndrome—Tullio’s phenomenon and Valsalva-induced vertigo, correlating with increased compliance of the cochlear windows. Stage 3: Meniere’s syndrome—vertigo and hearing loss mimicking Meniere’s disease, correlating with a frank oval or round window perilymph fistula. Stage 4: End stage—profound hearing loss and/or vestibular areflexia, as a result of repeated damage from stage 3. It should be noted that BPPV is a frequent comorbidity at any of these stages.
OTHER DEHISCENCES Among patients who present with symptoms and testing consistent with SSCD, there are some who do not have SSCD. These patients may be found to have dehiscence of labyrinthine bone in other areas. Among these are posterior semicircular canal dehiscence at the posterior fossa dura or at the jugular bulb, horizontal canal dehiscence (usually due to erosive processes such as cholesteatoma), cochlear dehiscence at the labyrinthine segment of the facial nerve, cochlear dehiscence at the carotid artery, and horizontal canal dehiscence near the second genu of the facial nerve. Each of these has minor variations from the typical presentation of SSCD, but the most common unifying symptom seems to be pressure-induced dizziness/vertigo from either internal or external sources. These dehiscences are more commonly found among SSCD patients and may be clinically identical in their presentation. Finally, patients may present with a syndrome identical to SSCD but have no radiographically visible dehiscence. The collection of patients presenting with this syndrome has been named “otic capsule dehiscence syndrome.”18
Near Dehiscence Many clinicians have noted patients with SSCD symptoms and no bony defect of the SSC or any other place
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in the labyrinthine bone. Some of these patients will have extreme thinning of SSC bone to the point where the bone itself is flexible enough to transmit pressure to the inner ear. These so-called near dehiscences share many features with definite SSCD but typically do not have reduced thresholds on VEMP testing. These patients also have a generally favorable outcome with SSCD surgery.19
TEGMEN DEHISCENCE AND OSSICULAR HEAD IMPINGEMENT Among anomalies frequently seen with SSCD are multiple dehiscences of the tegmen tympani and tegmen mastoideum.20 Usually these are of no significance unless there has been dural herniation through the dehiscence, resulting in an encephalocele and possible CSF leak (Fig. 12.2). An encephalocele with prolapse onto the ossicular heads in the epitympanum can cause a conductive hearing loss and autophony, which can accompany SSCD. Similarly, a large tegmen tympani dehiscence may allow impingement of the ossicular heads without a prolapsing encephalocele. If SSCD repair does not include repair of this type defect, symptoms of autophony and conductive hearing loss will persist.
TREATMENT: NONSURGICAL Treatment options for SSCD have emphasized surgical intervention, but it must be kept in mind that there are considerations for nonsurgical treatment. In patients with no symptoms, surgery should be deferred. Some patients may have hearing loss as their only symptom. For these patients we advise against surgery. Surgery is most successful for patients with vestibular symptoms and for the symptom of autophony, but hearing is seldom improved with surgery. Restricted activity or vestibular rehabilitation can be very successful nonsurgical options for patients who have minimal symptoms. Lastly, for patients who have concomitant elevated intracranial pressure, measures directed at lowering CSF pressure can be immensely rewarding.
WINDOW REINFORCEMENT Among the more minimally invasive procedures is window reinforcement surgery.21 The technique varies among surgeons but is basically perilymph fistula repair surgery of the round window. We advocate reinforcement of both the round and the oval windows if this option is chosen. Outcomes for this procedure are initially quite good, but there is a fairly high recurrence
rate within 1 year after surgery. In general, the larger the dehiscence, the less likely this procedure seems to be adequate for long-term control of symptoms. This procedure, like the others, is most successful in controlling vestibular symptoms. It also seems to be relatively unsuccessful for alleviation of autophony. The main advantage of this approach is its minimally invasive nature and low risk for serious complications, while its main disadvantage is the low rate of long-term success. Although we still offer this minimal approach to patients, we are careful to counsel about the lower success rate. However, we will also typically include window reinforcement concomitantly with resurfacing or occlusion of the SSCD.
RESURFACING In general, SSC resurfacing involves placing some material over the dehiscence to prevent transmission of pressure from the intracranial cavity to the inner ear.9 Resurfacing techniques also vary quite significantly from surgeon to surgeon. Among the materials used for resurfacing are bone, bone chips, hydroxyapatite bone cement, silastic sheeting, cartilage, and glass ceramic implants. The goal of resurfacing is a rigid repair between the SSCD and the temporal lobe dura. Surgical outcomes for resurfacing are highly successful for controlling vestibular symptoms and autophony. Resurfacing to completely cover the dehiscent area is relatively easy with smaller dehiscences but more challenging with larger dehiscences that extend to the edge of the petrous ridge and into the posterior fossa. The biggest problem with resurfacing is inadequate coverage of the defect, which can be the result of slippage of the resurfacing material, incorrect placement, or inadequate curing of the hydroxyapatite cement. However, when successfully performed, this technique gives excellent results while preserving SSC patency and function.
OCCLUSION/PLUGGING The concept of occlusion of the SSC stems from our collective experience with posterior semicircular canal occlusion for intractable BPPV. The goal is to occlude the SSC to prevent motion of the ampulla. By definition, the function of the SSC is removed. This is not significant for most patients, but in some situations the loss of SSC function can have significant untoward consequences. In particular, surgical occlusion of bilateral SSCD will result in bilateral loss of SSC function and can be problematic for some patients. Older patients, patients at risk for poor central compensation, and patients who
CHAPTER 12 Superior Semicircular Canal Dehiscence have already lost significant vestibular function should be counseled carefully regarding plugging. Lastly, in very large defects, occlusion runs the risk of plugging the crus commune, which would effectively result in the loss of both superior and posterior semicircular canal function.
TRANSMASTOID VERSUS MIDDLE FOSSA VERSUS COMBINED APPROACH The surgical approach for repair/resurfacing or occlusion of SSCD can be accomplished via middle fossa craniotomy, transmastoid approach, or combined middle fossa/transmastoid approach. The advantage of the middle fossa approach is the visibility of the defect. The middle fossa approach gives the best exposure of the SSCD defect, but the nature of SSCD anatomy is such that there can be multiple middle fossa defects. In some patients, tegmen defects can camouflage the SSCD. Because of this issue, some surgeons have advocated the use of navigational systems to ensure the correct defect is addressed. This point cannot be stressed enough because we have had occasion to do revision surgery on patients who had the wrong defect plugged or repaired with obvious consequences. Conversely, the middle fossa approach allows the repair of multiple tegmen defects, as long as the correct defect is also repaired! The other disadvantage of the middle fossa craniotomy is the need for a larger craniotomy defect and more extensive brain retraction than the other approaches with subsequent inpatient hospital admission. The transmastoid approach has the advantage of minimal brain retraction, which converts the surgery into an outpatient procedure. The major disadvantage of the transmastoid approach is poor visibility of the defect. Although an experienced surgeon should have no trouble identifying the SSC in the mastoid, actual visualization of the dehiscence is usually difficult or impossible; therefore, the occlusion or repair is often done blindly, without actually seeing the defect. Fortunately, most of the time this is not necessary but may be problematic depending on the individual anatomy. Another disadvantage of the transmastoid approach relates to the “normal” anatomy of most SSCD patients. We find that most SSCD patients have a very low tegmen and, consequently, there is not much room for dissection superior to the level of the horizontal semicircular canal compared with non-SSCD patients, making dissection somewhat challenging in the area of the SSC. The combined transmastoid/middle fossa craniotomy approach has the advantage of both approaches, and consequently, this is the one we advocate and utilize in the vast majority of surgery for SSCD. The
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combined approach allows for minimal brain retraction while allowing for excellent visibility of the defect and localization of the defect, obviating the need for a navigational system. The minimal brain retraction also allows this to be an outpatient procedure. The main disadvantage of the combined approach is the need for mastoid obliteration at the conclusion of the procedure to prevent future encephalocele formation.
OUTCOMES Two large reviews comparing various surgical treatments for SSCD have been recently published.22,23 The authors note no significant differences in outcomes for surgical treatment of SSCD, with the exception of window reinforcement surgery, with the difference attributed to limited patient numbers presented in the literature. It is noted in the reviews that individual surgical techniques vary quite considerably from one surgeon to another, that the number of reported surgeries is relatively low, and that there is little consistency between reports with regard to outcome measures, thus making the reviewers’ job of comparison quite difficult. With that disclaimer, conclusions from these reviews are that surgical outcomes generally provide excellent improvement rates in vestibular symptoms and autophony. In our experience, resolution or significant improvement in vestibular symptoms occurs in over 95% of patients. Autophony is improved in more than 90% of patients. Hearing improvement is seldom seen; therefore, we do not advocate surgery if hearing loss is the sole symptom. Similarly, we do not advocate the surgery for the sole symptom of tinnitus. Tinnitus can improve, worsen, or remain unchanged postoperatively. However, one observation we have made is that tinnitus may continue to improve long after the 1-year postoperative period.
COMPLICATIONS Complications reported following SSCD surgery include early failure with recurrence of symptoms, late failure, sensorineural hearing loss, vestibular loss, postoperative BPPV, tinnitus, infection, facial paralysis, and other complications seen with major ear surgery. Severe hearing loss is seen in approximately 4% of patients following surgical techniques directly addressing the SSCD (transmastoid and middle fossa approaches), whereas hearing loss following window reinforcement surgery does not seem to occur. There have been anecdotal reports of higher risk of hearing loss in revision surgery and patients who have undergone prior stapes surgery.
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Early failure of repair/occlusion is fairly uncommon. Late failures have been reported, although it is too soon to estimate whether this is common. In our experience with over 400 SSCD surgeries since 1998, delayed failure for resurfacing is quite low and is probably in the range of 1% or so. Delayed failures seem to be somewhat more common with occlusion techniques. Some patients who have initially excellent results with occlusion procedures will have recurrence of SSCD symptoms, albeit milder than their preoperative symptoms. These patients do quite well with revision surgery. We have postulated that this may be secondary to retraction of the soft tissue plug, allowing movement in the SSC, a.k.a. “loose plug syndrome.” This has led some surgeons who had advocated occlusion procedures to adopt an approach of occlusion and resurfacing concomitantly. Other complications typically seen with major ear surgery are, of course, expected but do not seem to be any more common than other otologic procedures of similar complexity.
CONCLUSIONS Diagnosis of SSCD requires symptoms consistent with the pathology, physiologic testing consistent with SSCD, and high-resolution CT confirming the presence of SSCD. Comprehensive evaluation should be performed to rule out concomitant pathologies or other plausible causes for the patient’s symptoms. Choosing among the various treatment options for SSCD and individualizing the care for each patient can significantly improve the quality of these patients’ lives. The range of treatment options includes nonsurgical strategies geared toward reducing pressure-altering episodes to the ear, or vestibular rehabilitation which may be the best options for those patients who are averse to surgery or patients for whom surgery/ anesthesia represents greater risk. Minimally invasive approaches with window reinforcement procedures can be effective for patients whose main symptoms are vestibular in nature, albeit at a lower success rate and higher recurrence rate than more invasive surgical procedures. For those patients with the most severe vestibular symptoms and incapacitating autophony, more direct surgical alternatives are preferred: occlusion or resurfacing of the SSCD, through a transmastoid, middle fossa, or a combined approach. The ultimate goal of SSCD treatment is symptom alleviation/control; SSCD is not a dangerous condition; and each patient, in consultation with the specialist who presents the various treatment options, must decide how debilitating the symptoms are and whether the risks of treatment are outweighed by the benefit of alleviation of symptoms.
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CHAPTER 12 Superior Semicircular Canal Dehiscence 17. Gianoli G. Superior semicircular canal dehiscence repair. In: Babu S, ed. Practical Otology for the Otolaryngologist. San Diego: Plural Publishing; 2013:287–296. 18. Wackym PA, Wood SJ, Siker DA, Carter DM. Otic capsule dehiscence syndrome: superior semicircular canal dehiscence syndrome with no radiographically visible dehiscence. Ear Nose Throat J. 2015;94(8):E8–E24. 19. Ward BK, Wenzel A, Ritzl EK, et al. Near-dehiscence: clinical findings in patients with thin bone over the superior semicircular canal. Otol Neurotol. 2013;34(8):1421–1428. 20. Gianoli GJ. Deficiency of the superior semicircular canal. Curr Opin Otolaryngol Head Neck Surg. 2001;9:336–341.
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21. Silverstein H, Kartush JM, Parnes LS, et al. Round window reinforcement for superior semicircular canal dehiscence: a retrospective multi-center case series. Am J Otolaryngol. 2014;35(3):286–293. 22. Gioacchini F, Alicandri-CIufelli M, Kaleci S, Scarpa A, Cassandro E, Re M. Outcomes and complications in superior semicircular canal dehiscence surgery: a systematic review. Laryngoscope. 2016;126:1218–1224. 23. Ziylan F, Kinaci A, Beynon A, Kunst H. A comparison of surgical treatments for superior semicircular canal dehiscence: a systematic review. Otol Neurotol. 2017;38: 1–10.
Superior Semicircular Canal Dehiscence GERARD J. GIANOLI, MD, FACS • JAMES S. SOILEAU, MD
INTRODUCTION Superior semicircular canal dehiscence (SSCD) can be defined as an anatomic anomaly found (occasionally incidentally) on high-resolution temporal bone computed tomography (CT) scan or at surgical exploration (Fig. 12.1) or as a syndrome, a constellation of symptoms, that frequently accompanies this anatomic deviation. An anatomic dehiscence of bone overlying the superior semicircular canal (SSC) at its interface with the middle cranial fossa dura has been proposed to be a developmental abnormality.1 Bone overlying the middle ear cavity and mastoid air cell system (tegmen tympani and tegmen mastoideum) separating the ear from the middle cranial fossa (a.k.a. the floor of the middle cranial fossa) thickens progressively during the first 3 years of life, such that CT findings of SSCD in a 1-year-old patient may “disappear” by the time the patient is 3 or 4 years old.2 At this age, the incidence of an anatomic finding of SSCD is believed to be relatively stable, rendering 1%–2% of the general population at risk for developing SSCD syndrome.3 The calvarium initially thickens in early childhood, but then, later in life, the thickness of the calvarium and the area of the middle fossa floor in question, very slowly thins. This has also been proposed as a possible cause for development of SSCD in patients who had very thin bone covering the SSC earlier in life.4 Additional factors that may make an individual susceptible to SSCD are erosive processes such as arachnoid granulations, cholesteatoma, other neoplastic processes, and trauma/fractures. More recently, the CDH23 gene (associated with Usher Syndrome and nonsyndromic hearing loss) has been found to be a genetic risk factor for the development of SSCD.5 SSCD has been called the great otologic mimicker because of the myriad of clinical presentations that are similar or identical to other major otologic disorders, such as patulous Eustachian tube, otosclerosis, Meniere’s disease, benign paroxysmal positional vertigo (BPPV), perilymph fistula, sudden sensorineural hearing loss, and acute vestibular neuritis.6 This variety
of presentations gives rise to a disorder that can go undiagnosed or misdiagnosed for quite some time. Consider the SSCD patient who presents initially with conductive hearing loss without vestibular symptoms and is diagnosed with otosclerosis. Unless the patient undergoes some investigation beyond basic audiometry, the patient can go for quite some time labeled as having “otosclerosis” before a different diagnosis is considered. Even after the patient develops vestibular symptoms, many clinicians may preserve the otosclerosis label while adding other diagnoses, such as BPPV or “otosclerosis with Meniere’s disease.” Clinicians must maintain a level of open-mindedness and not “pigeonhole” patients into a single diagnosis when dealing with patients with symptoms of SSCD. To make the condition even more interesting, a patient with the absence of bone overlying the SSC may be completely asymptomatic. It has been recognized since the initial description of the problem in 1998 that a “second event” is suspected to be the root cause of the onset of the symptoms of SSCD. The leading suspected cause is head trauma or conditions that cause increased intracranial pressure. This is the presumed reason that SSCD symptoms are rarely seen in children.
INCIDENCE AND ETIOLOGY The prevalence of SSCD has been found to be much higher in a series of analyzed CT scans than on temporal bone histology. Carey et al. identified a complete absence of bone over the superior canal histologically in 0.5% of 1000 vertically sectioned adult temporal bones.3 There was an additional 1.4% with very thin (<0.1 mm) bone covering the SSC. Added together, the histologic prevalence of thin or dehiscent SSCs approached 2%. This study also reported that 50% of patients with SSCD had bilateral involvement. Carey and colleagues3 also analyzed 36 infant temporal bones and concluded that the thickness of the bone overlying the superior canal was consistently thin. The thickness of the bone covering the SSC gradually increased with age, reaching adult thickness by 3 years of age. 143
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Arcuate eminence
TLD
MFR
FIG. 12.1 Intraoperative, middle fossa craniotomy view of the middle fossa floor demonstrating superior
semicircular canal dehiscence (SSCD) (multiple arrows). MFR, middle fossa retractor; TLD, temporal lobe dura.
Roberto et al.7 used tetracycline staining to investigate the deposition of bone in a dog model at 10, 25, and 50 days of age. This study demonstrated progressive deposition of endochondral and endosteal bone over the SSC postnatally. The bone deposition decreased with age. These findings are in agreement with observations in the study by Carey et al.3 In a related study, Hirvonen and colleagues8 reported a CT temporal bone study of bone thickness over the SSC in patients with SSCD and those without SSCD. Among those with SSCD, the contralateral SSC bone was thinner (or dehiscent), compared with those patients without SSCD. This finding of thin bone overlying the SSC bilaterally supports the notion of SSCD as a developmental anomaly related to bony deposition in early life. Several observations point to SSCD as a developmental anomaly requiring a “second event” to produce symptoms: 1. Studies demonstrating development of bone over the SSC occurring later (postnatally) than other parts of the inner ear. 2. Clinical observations of asymptomatic but anatomic SSCD noted during intraopera tive exploration of the middle cranial fossa for encephalocele repair. 3. Symptoms of SSCD rarely present in the pediatric population. Thus, a second event may be required in addition to the congenital anomaly of thin or absent superior
canal bone to produce clinical symptoms. Roughly half of patients with SSCD report an event they attribute to symptom onset. This “second event” is typically noted to be head trauma, a Valsalva-type episode, barotrauma, or some other type of intracranial pressurealtering event.
CLINICAL PRESENTATION SSCD was first reported by Minor et al.1 in eight patients who exhibited symptoms of short-lived episodic vertigo in response to certain sounds or activities that would cause transient increases in intracranial or middle ear pressure (e.g., Valsalva, coughing, sneezing, nose-blowing, auto-insufflation). These activities produce torsional nystagmus, which directly implicates SSC stimulation. Activities causing increased middle ear pressure (e.g., loud sound, positive pressure in the ear canal, autoinsufflation) induce nystagmus with the slow phase upward and the superior pole of the eye directed away from the affected ear. Activities causing transient elevation in intracranial pressure (e.g., Valsalva against a closed glottis, jugular venous compression) or negative pressure in the ear canal result in the slow phase of nystagmus directed downward and the superior pole of the eye directed toward the affected ear. The clinical findings of Tullio’s phenomenon and pressure-induced nystagmus associated with SSCD has been termed “Minor’s syndrome.” Although the vertigo caused by SSCD is most characteristically
CHAPTER 12 Superior Semicircular Canal Dehiscence reported as short-lived, other descriptions of vestibular symptoms have been reported as well, including more prolonged vertigo spells, chronic disequilibrium, and drop attacks. Since the first identification of SSCD as a cause for Minor’s syndrome, other symptoms and clinical presentations have been identified.9 Although the presentation of patients with SSCD is highly variable, the most recognizable presentation will include Tullio’s phenomenon, pressure-induced vertigo with transient increases in intracranial or middle ear pressure, and autophony. While these symptoms are characteristic of SSCD, they are certainly not present in all SSCD patients, and their absence cannot be used as a means to exclude the diagnosis of SSCD. The more nonspecific symptoms of vestibulopathy such as head movement–induced disequilibrium is frequently described by patients but not particularly helpful in confirming the diagnosis of SSCD. Aural pressure and aural fullness are commonly reported, as are complaints of hearing loss, distorted hearing, pulsatile tinnitus, aural fullness, and hyperacute hearing. There are also vague cognitive and neurobehavioral symptoms frequently reported by SSCD patients that are not specific to SSCD but should be included in the discussion during patient education. These symptoms include depression, “brain fog,” short-term memory problems, and difficulty with concentration. These symptoms often improve after surgical repair of SSCD.10 Patients with SSCD may have been given other diagnoses prior to presentation. The biggest indicator of a misdiagnosis is nonresponse to treatment. This should always prompt the clinician to reassess the prior diagnosis.
PHYSICAL EXAMINATION Routine head and neck examinations are typically normal in the SSCD patient. Microscopic otoscopy is normal as well, unless there has been prior surgical intervention. The vestibular component of the physical examination should include evaluation with infrared video goggles. This is typically unremarkable but may reveal spontaneous nystagmus and head thrust or head-shake abnormalities if there has been any vestibular loss. In some extreme cases, one can identify spontaneous torsional nystagmus that is synchronous with the pulse. Tuning fork testing can also be helpful. Patients may “hear” the tuning fork in the affected (SSCD) ear when
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the tuning fork is placed on the ankle or some other remote bone location. Pneumatic otoscopy can be an effective screening tool for SSCD. Symptoms of vertigo, or a feeling that the world shifts or moves, will often be reported during pneumatic otoscopy. Infrared video-oculography (VOG) cameras can be used to observe and/or record the resulting pressure-induced nystagmus. If VOG cameras are not available, a second examiner, an assistant or family member can also watch for pressure-induced nystagmus. The direction of pressureinduced nystagmus is dependent on the location of dehiscence and whether positive or negative pressure is applied.
TESTING Much of the published literature on SSCD discusses CT imaging and vestibular evoked myogenic potential (VEMP), an electrical potential measured from the sternocleidomastoid muscle elicited by a sound stimulus to the ipsilateral ear. Relatively high-intensity sound stimulates the hair cells in the ampulla of the saccule, which in turn send afferent impulses along the inferior vestibular nerve to vestibular nuclei in the brainstem. The vestibular nuclei send efferent projections along the vestibulospinal (vestibulocollic) tract to the sternocleidomastoid muscle to facilitate head stabilization. Athough the CT scan is imperative (the “gold standard”), and VEMP testing is often helpful, we feel this limited testing is inadequate for patients presenting with symptoms of SSCD. Because SSCD is a disorder that can mimic many other otologic disorders, can cause a number of secondary pathologies, and may require invasive surgery to resolve, we feel a full audiovestibular test battery is warranted. CT scan sections should be performed at the submillimeter level, preferably 0.24 mm thickness, but no thicker than 0.6 mm. The thinner section scan gives a more accurate portrayal of the defect. Thicker scans can be prone to both false-positive and false-negative findings.11 Both coronal and Pöschl views will demonstrate the dehiscence, but the Pöschl view is a parasagittal view perpendicular to the petrous ridge/long axis of the temporal bone displaying the entirety of the SSC (Fig. 12.2). Because of this, Poschl views tend to only identify large dehiscences. Slices perpendicular to the SSC, such as coronal or Stenvers views, are more sensitive in detecting a dehiscence. MRI should be performed to evaluate for concomitant intracranial abnormalities. One of the more frequent findings in SSCD patients is Chiari
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Tegmen defect
Sub. semicircular Canal dehiscence at superior petrosal sinus
Malleus head
FIG. 12.2 Left temporal bone computed tomography (CT) scan in a Pöschl view demonstrating malleus
impinged by overlying temporal lobe encephalocele with CSF leak and superior semicircular canal dehiscence (SSCD) at the superior petrosal sinus.
malformation.12 Additionally, MRI findings suggestive of elevated intracranial pressure, such as empty sella, vertical tortuosity of the optic nerves, prominent arachnoid spaces around the optic nerves, flattening of the globe, slit-like ventricles, venous sinus abnormalities, and Chiari/cerebellar ectopia should also be sought. Audiometric testing may be normal, may show some degree of sensorineural hearing loss in the affected ear, or more characteristically, may demonstrate low-frequency conductive loss, often with supranormal bone conduction thresholds. To distinguish the patient with low-frequency conductive loss who has SSCD from a patient with otosclerosis or a patient with other middle ear pathology, impedance testing is critical. The patient with SSCD typically has normal tympanograms and intact acoustic reflexes, whereas patients with otosclerosis will show absent acoustic reflexes.13 VEMP testing may show reduced threshold responses compared with laboratory norms or the contralateral ear. VEMP testing may show an asymmetric result in patients with unilateral SSCD or may be completely
normal. Amplitude of the VEMP has not been found to be a reliable measure for SSCD. In patients with reduced vestibular function, the VEMP response may be absent.6 Electrocochleography is frequently abnormal in SSCD patients and often normalizes after successful surgery to plug the dehiscent canal.14 A full vestibular evaluation should be performed on any patient who is to undergo SSCD surgery. Any identified vestibular abnormality is helpful in (1) the decision to proceed with surgery and (2) outcome expectations. Patients with SSCD have varied vestibular test profiles including severe unilateral vestibular hypofunction and occasionally severe bilateral vestibular hypofunction. Such patients will continue to experience symptoms from these vestibular deficits despite successful surgical treatment of their SSCD. Additionally, BPPV is a frequent secondary pathology that may need treatment in addition to treatment of SSCD. Video head impulse testing (vHIT) has been introduced to clinical practice recently (see Chapter 8). However, because clinical experience with this testing methodology has not yet been widespread and
CHAPTER 12 Superior Semicircular Canal Dehiscence has only been available for a limited time, we recommend caution when using this as a means to determine “normal” semicircular canal function. Recent studies have shown poor correlation of vHIT with caloric irrigation studies15; therefore, we cannot recommend using vHIT in place of caloric irrigation. However, in the context of SSCD, vHIT may prove to be invaluable in determining superior canal function preoperatively and postoperatively. In patients who have undergone SSCD occlusion, vHIT may be helpful in determining whether the posterior semicircular canal has actually been occluded and therefore hypofunctioning.
PATHOPHYSIOLOGY The most commonly espoused theory for the pathophysiology of SSCD is the “third window” phenomenon. This theory posits that the flexible nature of the SSCD allows for possible egress of endolymph from the SSC resulting in abnormal stimulation of the cupula. Additionally, low-frequency sound energy transmitted through the inner ear is allowed to dissipate through this bony defect, resulting in the auditory conductive component noted in some SSCD patients. Merchant and Rosowski16 proposed that SSCD could be classified among a number of anomalies that produce a third mobile window on the scala vestibuli side of the cochlea. Included among these are lateral or posterior canal dehiscence, enlarged vestibular aqueduct, dehiscence of the internal auditory canal, carotid dehiscence (into the cochlea), diffuse dehiscence (such as in Paget’s disease), and other congenital abnormalities of the inner ear. The hearing loss in these patients with a pathologic third mobile window is characterized by poor air conduction thresholds and normal or supranormal bone conduction thresholds. However, the third window theory does not completely explain all of the findings of SSCD. Among these are the existence of asymptomatic patients, the presentation of patients with only auditory and no vestibular findings or vice versa, Ménière’s-type vertigo spells, and the absence of symptoms in pediatric patients who have definite dehiscence on imaging. It has been proposed that a second event such as trauma or a major pressurealtering event (Valsalva-type maneuver; barotrauma) causes a disruption of very thin bone over the superior canal, thus creating a symptomatic SSCD. However, this only explains adult onset of symptoms and not the other findings, nor does it explain a true dehiscence in an asymptomatic patient. Gianoli and Soileau17 proposed the theory that alteration of intracranial pressure
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may result in increased compliance at the round and oval windows and, if pressure changes were extreme, potential disruption of the windows resulting in a frank middle ear perilymph fistula. This theory could explain the above exceptions to the third mobile window theory and also explain why round window reinforcement has been noted to resolve SSCD symptoms (at least temporarily) in many patients. They further proposed a grading system for SSCD: Stage 1: Asymptomatic SSCD—anatomic dehiscence with no symptoms. Stage 2: Minor’s syndrome—Tullio’s phenomenon and Valsalva-induced vertigo, correlating with increased compliance of the cochlear windows. Stage 3: Meniere’s syndrome—vertigo and hearing loss mimicking Meniere’s disease, correlating with a frank oval or round window perilymph fistula. Stage 4: End stage—profound hearing loss and/or vestibular areflexia, as a result of repeated damage from stage 3. It should be noted that BPPV is a frequent comorbidity at any of these stages.
OTHER DEHISCENCES Among patients who present with symptoms and testing consistent with SSCD, there are some who do not have SSCD. These patients may be found to have dehiscence of labyrinthine bone in other areas. Among these are posterior semicircular canal dehiscence at the posterior fossa dura or at the jugular bulb, horizontal canal dehiscence (usually due to erosive processes such as cholesteatoma), cochlear dehiscence at the labyrinthine segment of the facial nerve, cochlear dehiscence at the carotid artery, and horizontal canal dehiscence near the second genu of the facial nerve. Each of these has minor variations from the typical presentation of SSCD, but the most common unifying symptom seems to be pressure-induced dizziness/vertigo from either internal or external sources. These dehiscences are more commonly found among SSCD patients and may be clinically identical in their presentation. Finally, patients may present with a syndrome identical to SSCD but have no radiographically visible dehiscence. The collection of patients presenting with this syndrome has been named “otic capsule dehiscence syndrome.”18
Near Dehiscence Many clinicians have noted patients with SSCD symptoms and no bony defect of the SSC or any other place
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in the labyrinthine bone. Some of these patients will have extreme thinning of SSC bone to the point where the bone itself is flexible enough to transmit pressure to the inner ear. These so-called near dehiscences share many features with definite SSCD but typically do not have reduced thresholds on VEMP testing. These patients also have a generally favorable outcome with SSCD surgery.19
TEGMEN DEHISCENCE AND OSSICULAR HEAD IMPINGEMENT Among anomalies frequently seen with SSCD are multiple dehiscences of the tegmen tympani and tegmen mastoideum.20 Usually these are of no significance unless there has been dural herniation through the dehiscence, resulting in an encephalocele and possible CSF leak (Fig. 12.2). An encephalocele with prolapse onto the ossicular heads in the epitympanum can cause a conductive hearing loss and autophony, which can accompany SSCD. Similarly, a large tegmen tympani dehiscence may allow impingement of the ossicular heads without a prolapsing encephalocele. If SSCD repair does not include repair of this type defect, symptoms of autophony and conductive hearing loss will persist.
TREATMENT: NONSURGICAL Treatment options for SSCD have emphasized surgical intervention, but it must be kept in mind that there are considerations for nonsurgical treatment. In patients with no symptoms, surgery should be deferred. Some patients may have hearing loss as their only symptom. For these patients we advise against surgery. Surgery is most successful for patients with vestibular symptoms and for the symptom of autophony, but hearing is seldom improved with surgery. Restricted activity or vestibular rehabilitation can be very successful nonsurgical options for patients who have minimal symptoms. Lastly, for patients who have concomitant elevated intracranial pressure, measures directed at lowering CSF pressure can be immensely rewarding.
WINDOW REINFORCEMENT Among the more minimally invasive procedures is window reinforcement surgery.21 The technique varies among surgeons but is basically perilymph fistula repair surgery of the round window. We advocate reinforcement of both the round and the oval windows if this option is chosen. Outcomes for this procedure are initially quite good, but there is a fairly high recurrence
rate within 1 year after surgery. In general, the larger the dehiscence, the less likely this procedure seems to be adequate for long-term control of symptoms. This procedure, like the others, is most successful in controlling vestibular symptoms. It also seems to be relatively unsuccessful for alleviation of autophony. The main advantage of this approach is its minimally invasive nature and low risk for serious complications, while its main disadvantage is the low rate of long-term success. Although we still offer this minimal approach to patients, we are careful to counsel about the lower success rate. However, we will also typically include window reinforcement concomitantly with resurfacing or occlusion of the SSCD.
RESURFACING In general, SSC resurfacing involves placing some material over the dehiscence to prevent transmission of pressure from the intracranial cavity to the inner ear.9 Resurfacing techniques also vary quite significantly from surgeon to surgeon. Among the materials used for resurfacing are bone, bone chips, hydroxyapatite bone cement, silastic sheeting, cartilage, and glass ceramic implants. The goal of resurfacing is a rigid repair between the SSCD and the temporal lobe dura. Surgical outcomes for resurfacing are highly successful for controlling vestibular symptoms and autophony. Resurfacing to completely cover the dehiscent area is relatively easy with smaller dehiscences but more challenging with larger dehiscences that extend to the edge of the petrous ridge and into the posterior fossa. The biggest problem with resurfacing is inadequate coverage of the defect, which can be the result of slippage of the resurfacing material, incorrect placement, or inadequate curing of the hydroxyapatite cement. However, when successfully performed, this technique gives excellent results while preserving SSC patency and function.
OCCLUSION/PLUGGING The concept of occlusion of the SSC stems from our collective experience with posterior semicircular canal occlusion for intractable BPPV. The goal is to occlude the SSC to prevent motion of the ampulla. By definition, the function of the SSC is removed. This is not significant for most patients, but in some situations the loss of SSC function can have significant untoward consequences. In particular, surgical occlusion of bilateral SSCD will result in bilateral loss of SSC function and can be problematic for some patients. Older patients, patients at risk for poor central compensation, and patients who
CHAPTER 12 Superior Semicircular Canal Dehiscence have already lost significant vestibular function should be counseled carefully regarding plugging. Lastly, in very large defects, occlusion runs the risk of plugging the crus commune, which would effectively result in the loss of both superior and posterior semicircular canal function.
TRANSMASTOID VERSUS MIDDLE FOSSA VERSUS COMBINED APPROACH The surgical approach for repair/resurfacing or occlusion of SSCD can be accomplished via middle fossa craniotomy, transmastoid approach, or combined middle fossa/transmastoid approach. The advantage of the middle fossa approach is the visibility of the defect. The middle fossa approach gives the best exposure of the SSCD defect, but the nature of SSCD anatomy is such that there can be multiple middle fossa defects. In some patients, tegmen defects can camouflage the SSCD. Because of this issue, some surgeons have advocated the use of navigational systems to ensure the correct defect is addressed. This point cannot be stressed enough because we have had occasion to do revision surgery on patients who had the wrong defect plugged or repaired with obvious consequences. Conversely, the middle fossa approach allows the repair of multiple tegmen defects, as long as the correct defect is also repaired! The other disadvantage of the middle fossa craniotomy is the need for a larger craniotomy defect and more extensive brain retraction than the other approaches with subsequent inpatient hospital admission. The transmastoid approach has the advantage of minimal brain retraction, which converts the surgery into an outpatient procedure. The major disadvantage of the transmastoid approach is poor visibility of the defect. Although an experienced surgeon should have no trouble identifying the SSC in the mastoid, actual visualization of the dehiscence is usually difficult or impossible; therefore, the occlusion or repair is often done blindly, without actually seeing the defect. Fortunately, most of the time this is not necessary but may be problematic depending on the individual anatomy. Another disadvantage of the transmastoid approach relates to the “normal” anatomy of most SSCD patients. We find that most SSCD patients have a very low tegmen and, consequently, there is not much room for dissection superior to the level of the horizontal semicircular canal compared with non-SSCD patients, making dissection somewhat challenging in the area of the SSC. The combined transmastoid/middle fossa craniotomy approach has the advantage of both approaches, and consequently, this is the one we advocate and utilize in the vast majority of surgery for SSCD. The
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combined approach allows for minimal brain retraction while allowing for excellent visibility of the defect and localization of the defect, obviating the need for a navigational system. The minimal brain retraction also allows this to be an outpatient procedure. The main disadvantage of the combined approach is the need for mastoid obliteration at the conclusion of the procedure to prevent future encephalocele formation.
OUTCOMES Two large reviews comparing various surgical treatments for SSCD have been recently published.22,23 The authors note no significant differences in outcomes for surgical treatment of SSCD, with the exception of window reinforcement surgery, with the difference attributed to limited patient numbers presented in the literature. It is noted in the reviews that individual surgical techniques vary quite considerably from one surgeon to another, that the number of reported surgeries is relatively low, and that there is little consistency between reports with regard to outcome measures, thus making the reviewers’ job of comparison quite difficult. With that disclaimer, conclusions from these reviews are that surgical outcomes generally provide excellent improvement rates in vestibular symptoms and autophony. In our experience, resolution or significant improvement in vestibular symptoms occurs in over 95% of patients. Autophony is improved in more than 90% of patients. Hearing improvement is seldom seen; therefore, we do not advocate surgery if hearing loss is the sole symptom. Similarly, we do not advocate the surgery for the sole symptom of tinnitus. Tinnitus can improve, worsen, or remain unchanged postoperatively. However, one observation we have made is that tinnitus may continue to improve long after the 1-year postoperative period.
COMPLICATIONS Complications reported following SSCD surgery include early failure with recurrence of symptoms, late failure, sensorineural hearing loss, vestibular loss, postoperative BPPV, tinnitus, infection, facial paralysis, and other complications seen with major ear surgery. Severe hearing loss is seen in approximately 4% of patients following surgical techniques directly addressing the SSCD (transmastoid and middle fossa approaches), whereas hearing loss following window reinforcement surgery does not seem to occur. There have been anecdotal reports of higher risk of hearing loss in revision surgery and patients who have undergone prior stapes surgery.
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Early failure of repair/occlusion is fairly uncommon. Late failures have been reported, although it is too soon to estimate whether this is common. In our experience with over 400 SSCD surgeries since 1998, delayed failure for resurfacing is quite low and is probably in the range of 1% or so. Delayed failures seem to be somewhat more common with occlusion techniques. Some patients who have initially excellent results with occlusion procedures will have recurrence of SSCD symptoms, albeit milder than their preoperative symptoms. These patients do quite well with revision surgery. We have postulated that this may be secondary to retraction of the soft tissue plug, allowing movement in the SSC, a.k.a. “loose plug syndrome.” This has led some surgeons who had advocated occlusion procedures to adopt an approach of occlusion and resurfacing concomitantly. Other complications typically seen with major ear surgery are, of course, expected but do not seem to be any more common than other otologic procedures of similar complexity.
CONCLUSIONS Diagnosis of SSCD requires symptoms consistent with the pathology, physiologic testing consistent with SSCD, and high-resolution CT confirming the presence of SSCD. Comprehensive evaluation should be performed to rule out concomitant pathologies or other plausible causes for the patient’s symptoms. Choosing among the various treatment options for SSCD and individualizing the care for each patient can significantly improve the quality of these patients’ lives. The range of treatment options includes nonsurgical strategies geared toward reducing pressure-altering episodes to the ear, or vestibular rehabilitation which may be the best options for those patients who are averse to surgery or patients for whom surgery/ anesthesia represents greater risk. Minimally invasive approaches with window reinforcement procedures can be effective for patients whose main symptoms are vestibular in nature, albeit at a lower success rate and higher recurrence rate than more invasive surgical procedures. For those patients with the most severe vestibular symptoms and incapacitating autophony, more direct surgical alternatives are preferred: occlusion or resurfacing of the SSCD, through a transmastoid, middle fossa, or a combined approach. The ultimate goal of SSCD treatment is symptom alleviation/control; SSCD is not a dangerous condition; and each patient, in consultation with the specialist who presents the various treatment options, must decide how debilitating the symptoms are and whether the risks of treatment are outweighed by the benefit of alleviation of symptoms.
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CHAPTER 12 Superior Semicircular Canal Dehiscence 17. Gianoli G. Superior semicircular canal dehiscence repair. In: Babu S, ed. Practical Otology for the Otolaryngologist. San Diego: Plural Publishing; 2013:287–296. 18. Wackym PA, Wood SJ, Siker DA, Carter DM. Otic capsule dehiscence syndrome: superior semicircular canal dehiscence syndrome with no radiographically visible dehiscence. Ear Nose Throat J. 2015;94(8):E8–E24. 19. Ward BK, Wenzel A, Ritzl EK, et al. Near-dehiscence: clinical findings in patients with thin bone over the superior semicircular canal. Otol Neurotol. 2013;34(8):1421–1428. 20. Gianoli GJ. Deficiency of the superior semicircular canal. Curr Opin Otolaryngol Head Neck Surg. 2001;9:336–341.
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