11 Superior Semicircular Canal Dehiscence Syndrome Inner ear fistulas or areas of bony labyrinthine dehiscence can be associated with vertigo, especially when the affected ear is subjected to external pressure. This so-called “fistula sign” has classically been linked to oval and round window fistulas, cholesteatoma erosion of the horizontal semicircular canal, and syphilitic labyrinthitis. However, as noted by Baloh, any defect in the bony labyrinth can be the source of similar symptomatology: “Because of the rigid bony capsule, the vestibular part of the labyrinth is unaffected by sound or pressure changes in the middle ear and cerebrospinal fluid (CSF). However, a break in the bony capsule renders the vestibular labyrinth sensitive to sound and pressure changes.”1 In 1998, Minor and colleagues’ landmark description of a unique subset of patients in whom sound-and pressure-induced vertigo were found to be due to a dehiscence of the superior semicircular canal (sSCC) led to a new disease entity termed superior semicircular canal dehiscence syndrome (SSCD).2 Interestingly, some of the patients had undergone negative explorations for perilymphatic fistula in the past, but the presence of vertical nystagmus after sound (Tullio phenomenon) or pressure (Hennebert sign) led the group to suspect a defect at the level of the sSCC. This suspected defect was confirmed when computed tomography (CT) revealed a dehiscence of the bone overlying the sSCC in each case. Two of the patients with disabling vertigo experienced improvement in their symptoms after a middle cranial fossa surgical procedure, at which time the affected canals were plugged. The proposed mechanism by which a defect in the vestibular bony labyrinth renders the vestibular neuroepithelium sensitive to sound or pressure is based on the concept of a “third window” to the inner ear (in addition to the round and oval windows). In the normal setting, sound pressure transduction by the stapes results in only cochlear hair cell deflection due to the round window, which dissipates cochlear vibration by impedence matching. Because the semicircular canals do not have a membrane or release valve to dissipate vibration, their pressure remains constant and the neuroepithelium remains undisturbed. However, if there is a defect in the superior canal bone, the energy typically confined to the cochlea escapes along a path of least resistance toward the defect or “third window.” Displaced endolymphatic fluid within the sSCC activates the canal’s vestibular apparatus, leading to vertigo after sound or pressure changes.2,3,4 In the short time since Minor’s landmark paper, SSCD has become a validated disease entity. Superior semicircular canal dehiscence syndrome’s clinical presentation overlaps with many other otologic diseases, which has led it to be called the great otologic mimicker. In the modern otology practice, a thorough understanding of SSCD pathophysiology, variable presentations, diagnostic subtleties, and treatment options are critical to prevent patients from unnecessary tests, surgeries, or frustration with diagnostic ambiguity. This chapter discusses the evolution of SSCD, diagnostic pearls, new classification schemes, treatment options, and outcomes data. In 1929, an Italian biologist, Pietro Tullio, introduced the concept of a “third window” as the mechanism behind sound-induced vertigo, imbalance, and eye movements.5 Tullio created a fistula in the horizontal semicircular canals of pigeons and then exposed the birds to a loud sound, which led to quick deviation of their head away from the damaged ear. Tullio’s experiments with pigeons set the stage for the realization that openings in the bony labyrinth can render the semicircular canal sensitive to loud sounds; hence the term Tullio phenomenon was coined. Modern animal studies have further validated Tullio’s observations. Using a chinchilla model, Hirvonen et al measured neuronal firing rates in response to pressure from semicircular canal afferents before and after fenestration.6 Prior to fenestration of the sSCC, only one of nine superior canal afferents responded to pressure, whereas after fenestration, all such afferents were excited by pressure. Also, after fenestration, half of the otolithic and most of the horizontal canal afferents were still unaffected by pressure. These findings were reversed to normal when a rigid seal was applied to the fenestrated superior canal. In a second set of experiments, Carey et al performed an identical study using acoustic stimuli and similarly demonstrated that SSCD lowers the threshold for sound-evoked afferent stimulation of the superior canal.7 Taken together, these findings support the concept that SSCD creates selectively abnormal endolymphatic flow in the region of the sSCC when there is a dehiscence in that particular canal and provides further insight regarding the mechanisms behind key physical exam findings like the Tullio phenomenon and Hennebert sign. The Tullio phenomenon was first clinically relevant in patients with congenital syphilis, in which later temporal bone studies revealed gummatous osteomyelitis and fistulas of the labyrinth. Hennebert, who in 1911 described the finding of pressure-induced vestibular changes, had previously linked the syphilis patient population to inner ear dysfunction.8,9 The elicitation of pressure-induced nystagmus was later termed Hennebert sign and was linked to other otologic conditions, such as advanced Meniere’s disease and perilymphatic fistula.10,11 Despite overlap with other disease processes, the presence of concomitant Tullio phenomenon and Hennebert sign provided early insight into the possibility of a bony defect in the labyrinthine defect.12 The exact cause of SSCD remains unknown, but generally the theories are grouped into either congenital or acquired mechanisms. The congenital theory of SSCD argues that thin bone overlying the sSCC causes a persistent dehiscence or predisposes a patient to dehiscence later in life. The sSCC is the first semicircular canal to develop in utero but at birth it may still be covered with only a monolayer of periosteal bone. In most cases, a well-formed trilaminar bone does not cover the sSCC until 2 to 3 years of age.13 Even if bone eventually covers the sSCC, it is unclear if the overlying dura or superior petrosal sinus predisposes certain people to having a thin tegmen and eventual dehiscence later in life, when SSCD is most prevalent. In 2000, Carey et al reviewed temporal bones from 27 deceased infants and children all less than 4 years old whose bones had been donated to the Johns Hopkins Temporal Bone Collection. They found that, at birth, the average bone covering the sSCC measured 0.092 mm and the bone did not reach adult thickness until 32.4 months of age.13 On account of the rarity of pediatric temporal bone specimens, attempts have been made to characterize the incidence of congenital SSCD radiographically. Jackson et al in 2015 examined high-resolution temporal bone CT scans, including Pöschl reconstructed images, in 700 patients less than 18 years old. They found a dehiscent sSCC in 1.9% of bones and an additional 15.6% with a thin covering. The prevalence of a thin or dehiscent sSCC was highest among patients less than 12 months old, which supports Carey’s observation that bone overlying the sSCC grows postnatally.13,14 Additionally, in 2011 Nadgir et al reviewed high-resolution temporal bone CT scans from 304 patients with ages ranging from 7 months to 89 years. Their results showed a 93% increase in SSCD prevalence from pediatric to adult populations. They also noted a trend toward tegmen thinning with age. Their findings correlate with SSCD’s being diagnosed more commonly in middle-aged or older age groups. Of the 46 patients they studied who were less than 20 years old, only one had SSCD. Their conclusion was that congenital SSCD does exist, but more often is an acquired condition.15 Despite its being less common than the acquired form, the congenital variant of SSCD is important to recognize for pediatric patients hindered by auditory or vestibular symptoms.16 Carey et al’s temporal bone survey also examined 1,000 adult temporal bones from 596 adults. In control specimens without dehiscence, they found an average bone thickness of 0.96 ± 0.61 mm between the sSCC and the middle fossa dura, and 1.79 ± 1.2 mm between the sSCC and the superior petrosal sinus. Five of the specimens (0.5%) demonstrated dehiscence, and an additional 14 specimens (1.4%) were markedly thinned (≤ 0.1 mm).13 The first radiographic correlate to Carey’s histologic analysis was performed in 2003 and used coronal reconstructions of temporal bone CT imaging in 442 temporal bones of patients 7 to 87 years old (mean age = 45 years). This radiographic survey identified 39 bones (9%) with a dehiscent sSCC.17 The overestimation is largely due to the resolution limitations of conventional multislice CT temporal bone imaging, which typically has a slice thickness of 1.0 mm, leading to a resolution limit of 0.324 mm. Additionally, the coronal view is not in the plane of the sSCC, which may or may not impair diagnostic accuracy.18,19 Since that 2003 study, attention to ultra-high-resolution CT scans with a slice thickness of 0.5 mm or less and orientation in the plane of the sSCC (Pöschl or Stenvers view) has led to improved diagnostic accuracy. Another radiographic survey in 2011 also overestimated the radiographic dehiscence, at 3% of the 164 temporal bones assessed, although only 0.6% had clinical manifestations consistent with SSCD.20 The diagnosis of SSCD on imaging alone should be avoided, because even modern techniques can overestimate the size of the defect or falsely detect a dehiscence.21 Regardless of the true SSCD incidence in the adult population, the vast majority of cases are acquired defects. Some patients may be predisposed to developing SSCD if the bone overlying their sSCC is inherently thin. Other patients may have a dehiscence associated with a traumatic event, such as a car accident, postpartum strain, or barotrauma.21,22,23 There is growing recognition that obesity can cause thinning of the lateral skull base, which may lead to temporal bone encephaloceles, cerebrospinal fluid leaks, SSCD, or all of the above.24,25,26 The presumed mechanism of SSCD symptoms is that a dehiscent segment of the superior canal produces a third window, so that sound- or pressureevoked changes (either via external stimuli or internal CSF pressure) induce cupular deviation secondary to endolymphatic fluid displacement. Fluid displacement then results in either ampullofugal or ampullopetal displacement of the superior canal cupula and thus excitatory or inhibitory stimuli to the superior vestibular nerve, with resulting upbeating or downbeating torsional nystagmus and vertigo, respectively.6,7,27 A clinical example of this pathophysiology would be a patient with a left dehiscent sSCC hearing a loud noise in his left ear. The acoustic energy enters the inner ear via the stapes and oval window. Rather than being confined to the cochlea, a portion of the transmitted energy escapes into the vestibule and then to the ampullated end of the sSCC before being released at the third window. In this example, the energy would push the sSCC cupula away from the vestibule (ampullofugal displacement), thus exciting the left sSCC and causing the brain to think the patient’s head is rotating down and to the left. As a result of this excitation, the eyes will rotate upward and to the right (from the patient’s perspective, this is a clockwise, torsional, slow-phase nystagmus). The nystagmus-defining fast phase would then be down and to the left (counterclockwise from the patient’s perspective). The nystagmus can be reversed if the initiating force comes from an intracranial source, like a Valsalva maneuver, which would result in ampullopetal sSCC cupular displacement and inhibition of the affected canal (Fig. 11.1).27 The mechanism of the previously mentioned pathway has been studied in both human subjects and animal models. Hirvonen et al’s elegant chinchilla study from 2001 is described previously in the introduction. In 2004, Rowsowski et al also used both human subjects with known SSCD and the chinchilla model to investigate the effects of SSCD on inner ear fluid mechanics and how it affects hearing. Acoustic symptomatology of SSCD includes reduced (hypersensitive) thresholds for bone-conducted stimuli, increased thresholds for air-conducted sounds at low frequencies (< 2 kHz), with a resultant air–bone gap of as much as 30 to 60 dB in the low-frequency range. Rowsowski et al’s first experiment used laser-Doppler vibrometry (LDV) to measure the magnitude of tympanic membrane change in response to acoustic stimulation in patients with and without SSCD. They showed that, in the low-frequency range, the LDV magnitudes in four out of five SSCD ears was 0.9 standard deviation larger than mean normal magnitude. This finding suggests a decrease in load on the tympanic membrane, likely due to SSCD-induced decrease in cochlear impedance and shunting of the acoustic energy away from the cochlea and toward the dehiscence. In another experiment, bone-conduction-evoked cochlear potentials were studied in the chinchilla model. After induction of SSCD, cochlear potential increased by a factor of 3 over the 200–4,000 Hz range, and this effect was reversed when the dehiscence was plugged. The data also demonstrated that altering the inner ear impedance made the cochlea become more sensitive to bone-conducted sound, particularly at low frequencies.28 Fig. 11.1 Superior semicircular canal activation. Head rotation down and 45° to the left activates the left superior semicircular canal (sSCC). Excitatory interneurons from the left vestibular nuclei synapse on the oculomotor nucleus (CN III) and trochlear nucleus (CN IV), which excite the ipsilateral superior oblique (SO) and superior rectus (SR) as well as the contralateral inferior oblique (IO) and SR muscles. The slow phase component is upward/clockwise (from patient’s perspective) eye deviation. The corrective nystagmus beats downward and counterclockwise (from patient’s perspective). In the case of SSCD, acoustic energy and positive middle ear pressure excite the sSCC, whereas Valsalva maneuver and negative middle ear pressure inhibit the sSCC. hSCC, horizontal semicircular canal; LR, lateral rectus; MR, medial rectus; pSCC, posterior semicircular canal. Used with permission from Semaan MT, Wick CC, Megerian CA. Vestibular physiology. In: Pensak ML, Choo DI, eds. Clinical Otology. 4th ed. New York, NY: Thieme; 2015:41. Patients with SSCD can present with vestibular symptoms, auditory symptoms, or both. The mean age at diagnosis is 43 years old.29,30 The syndrome is rare in pediatrics, but reported examples do exist.16,31 Up to a third of patients with SSCD will have bilateral defects.13,29 Most patients will have had symptoms for many years and some even report prior middle ear explorations for perilymphatic fistula or conductive hearing loss. Patients may have had extensive work-ups for Meniere’s disease, tests for spirocheterelated inner ear conditions with negative Lyme, Venereal Disease Research Laboratory (VDRL) testing, and fluorescent treponemal antibody (FTA)-ABS testing. Most patients have no history of otologic disease, but 25% of Minor’s original group of patients had an antecedent history of head trauma.2 Vestibular signs and symptoms associated with SSCD include sound- and pressure-induced vertigo and chronic disequilibrium. Sensitivity to sound or pressure can be grouped into four categories: eye movement evoked by external pressure on the ear canal (Hennebert sign), eye movement evoked by internal pressure (Valsalva maneuver, cough, sneeze), eye movement evoked by sound (Tullio phenomenon), or sound-induced head tilt in the plane of the affected canal. Minor’s review of 60 patients with vestibular symptoms from SSCD noted a prevalence of 45%, 75%, 82%, and 20%, respectively, for each group. Overall, vestibular symptoms appear to be more commonly triggered by loud noise (90%) than by pressure (73%), but some patients will experience symptoms from both (67%).30 Chronic disequilibrium is also a common, and often debilitating, complaint affecting up to 76% of patients in one series.29 Disequilibrium and gait disturbances may worsen when the patient is exposed to loud sounds. Patients with bilateral SSCD may experience oscillopsia.32 Still, a small subset of patients may experience no vestibular symptoms.30 In general, the vestibular characteristics can be perplexing for the patient to describe. Some articulate patients, like the one described in Minor’s original report, have provided colorful descriptions of their SSCD experience, such as the environment’s appearing to “move like on a clock face” whenever he whistled or hummed a specific tune.2 Auditory symptoms are also variable and include: hyperacusis, autophony, aural fullness, hearing loss, and pulsatile tinnitus. As previously described, SSCD causes increased sensitivity to bone-conducted sounds, and bone-conduction thresholds on audiometry can be less than 0 dB normal hearing level (NHL). This suprathreshold bone conduction creates an air–bone gap, particularly at low frequencies, and manifests as a conductive hearing loss.28,33,34 The conductive hearing loss can mimic otosclerosis, a key difference being preservation of the stapedial reflex in SSCD but not in otosclerosis. It is less common for patients to experience auditory symptoms without vestibular complaints (7.7%).30,34,35 Autophony is the increased awareness of hearing one’s voice or bodily movements. Patients with SSCD often hear their own voice and may also describe hearing their eyes move. This differs from the autophony associated with a patulous eustachian tube, which is typically amplified by respiration and correlates with direct visualization of tympanic membrane movement.36 Other auditory symptoms, such as conductive hyperacusis, defined as hearing or feeling the pulse in the affected ear, occur in 39% of SSCD patients. A smaller subset will experience gaze-evoked tinnitus, presumably due to abnormal neuronal sprouting between the cochlear and vestibular nuclei induced by SSCD.37 One report describes a patient who experienced bradycardia and hypotension evoked by sound and ear pressure, which were believed to be related to saccular stimulation. The otolithic receptors are known to have a role in the vestibulosympathetic reflex and play a role in cardiovascular regulation. The fact that the autonomic symptoms in this particular patient improved after plugging of the affected superior canal further supports the concept that, in rare circumstances, cardiac autonomic symptomatology in response to sound or pressure can herald the presence of SSCD.29 Patients with SSCD will often demonstrate a conductive hearing loss on physical examination, with the Weber test at 512 Hz lateralizing to the affected ear.1,2 Brantberg et al, in a series of eight patients, demonstrated that all had Weber test lateralization to the affected side, but stapedial reflex testing was always normal, thus differentiating the audiometric scenario from otosclerosis.38 Neurotologic examination will often reveal vertical-torsional movement of the eyes during sound presentation. Offending sound frequencies can range from 250 to 3,000 Hz and are effective in causing nystagmus, usually between 100 and 110 dB. In some cases, only one tone (440 Hz, for example) will elicit symptoms, whereas in most other cases a range of frequencies are equally effective in producing symptoms.2 Vertical-torsional eye movement induced by loud sounds was seen in 89% of the 28-patient Hopkins group, whereas 82% of the group had such eye findings during a Valsalva maneuver, and only 54% demonstrated these findings with pneumatic otoscopy.37 In some rare circumstances, jugular venous compression with pressure to the upper aspect of the neck near the jugular foramen will elicit symptoms and nystagmus, presumably via increased intracranial pressure. Most patients demonstrate the absence of nystagmus after horizontal or vertical head shaking, and head thrust testing typically reveals symmetrical vestibulo-ocular reflexes.2 Spontaneous nystagmus is usually not present in SSCD. However, in the rare circumstance that it occurs, it is quite debilitating. Spontaneous pulsesynchronous vertical nystagmus was described in a patient with bilateral SSCD who also presented with complaints of oscillopsia. The spontaneous nystagmus can also assume a vertical-rotatory appearance; both of these scenarios are likely due to large-enough defects in the sSCC to allow pulse-initiated intracranial pressure variations to activate the sSCC.39 Most ears affected by SSCD will have an audiogram with at least a 10 to 20 dB low-frequency (250 to 1,000 Hz) conductive hearing loss, with normal speech discrimination score, stapedial reflex, and tympanogram (Fig. 11.2).2,30 The fact that many SSCD patients have decreased (hypersensitive) thresholds to bone-conducted sound helps explain why normal body sounds (heartbeat, eye movement, voice) can become bothersome.1 Patients with a characteristic low-frequency air–bone gap with an intact acoustic reflex and a lateralizing Weber without evidence of tympanic membrane or ossicular chain abnormality should undergo further testing for SSCD. Prior to the discovery of SSCD, patients with SSCD-related conductive hearing loss were often diagnosed with otosclerosis and incorrectly underwent stapedectomy or middle ear exploration.34,40 The conductive hearing loss associated with SSCD has been described as an “inner ear conductive hearing loss.”33,34 Recent studies have attempted to characterize the length and location of the dehiscence and how it relates to audiometric testing. Pisano et al used intracochlear sound pressure measurements to show that, for low-frequency sound (< 600 Hz), more energy was shunted from the cochlea as the defect size increased, thus producing the characteristic low-frequency air– bone gap. Interestingly, small (pinhole) defects created a more pronounced loss at frequencies greater than 1,000 Hz.41 The literature does have conflicting reports on how defect size and location relate to cochleovestibular symptoms.34,35,42,43 The discrepancies in these studies may relate to small sample sizes and varied methods of measuring the dehiscence length. Additionally, the relationship between defect size and hearing sensitivity may be more complex than a monotonic relationship.28,41,43,44
Introduction
Historical Background
Etiology and Pathophysiology
Symptoms and Clinical Presentation
Diagnostic Evaluation
Physical Examination Signs
Audiologic Testing
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