Superior Semicircular Canal Dehiscence Syndrome

Diagnostic Evaluation

Patients with SCDS generally present with a primary complaint of dizziness; when evaluating a patient with this complaint, a thorough history is the most effective diagnostic tool. Vertigo symptoms related to SCDS are usually induced by loud sound or pressure changes and are brief in duration. Dizziness or oscillopsia induced by loud sound is present in 90% of patients with SCDS. Vestibular symptoms induced by pressure changes such as coughing or straining are present in 73% of patients, with 67% exhibiting both pressure- and sound-related symptoms. Chronic disequilibrium symptoms and cognitive impairment (“brain fog”) may also be attributed to SCDS. Among our patients, 43% endorse a complaint of disequilibrium.

In addition to dizziness, patients with SCDS may also present with a primary complaint of autophony, defined as the hyperperception of one’s own voice, breathing, or other internal sounds. Autophony is present in varying degrees in 60% of patients. Auditory symptoms are also a common feature of SCDS. Hyperacusis for bone-conducted sound is present in 52% of SCDS patients. The symptoms often include patients hearing their own pulse, eye movements, or the impact of the feet during walking. Patients with SCDS can occasionally hear, in the affected ear, a 512-Hz tuning fork placed against the foot or ankle. Pulsatile tinnitus is present in approximately one-third of patients seen at our institution.

Evoked eye movements in the plane of the superior canal are the hallmark of SCDS. The eyes should be examined under Frenzel lenses, infrared video goggles, or by some other means to eliminate the effect of visual fixation. Using an audiometer, pure tones at levels up to 110 dB nHL should be delivered in one ear at a time to cover the frequency range of 125 to 4000 Hz. Sound-evoked eye movements at one or more frequencies were noted in 82% of SCDS patients using such stimuli. Among our patient population, eye movements can also be induced with Valsalva maneuvers (34%) or pressure in the EAC (23%).

Depending on the type of stimulus, either excitation or inhibition of the superior canal may occur, as shown in Fig. 37.1 . Valsalva against pinched nostrils, pressure in the EAC (e.g., tragal compression), or sound will produce excitatory affects (ampullofugal deflection of cupula). Valsalva against a closed glottis, jugular venous compression, or negative external canal pressure will produce inhibitory secondary to ampullipetal cupula deflection. Pressure- or sound-evoked eye movements almost always occur in the plane of the superior canal, as shown in Fig. 37.2 . In the case of larger dehiscences, the eye movements may be shifted out of the superior canal plane. However, if the eye movements are not in this direction, the diagnosis of SCDS should be questioned, and alternative diagnoses of posterior canal dehiscence or horizontal canal fistula considered. Sound-evoked rotation of the head, which also tilts in the plane of the affected superior canal, occurred with tones in 14% of our patients with SCDS.

The audiogram ( Fig. 37.3 ) is an important part of the SCDS evaluation. A minority of patients have auditory symptoms in the absence of any vestibular signs or symptoms. ^{, , ,} CHL is often the greatest at lower frequencies, ^, and bone conduction thresholds are often less than 0 dB nHL (conductive hyperacusis). Historically, in patients with CHL and a normal-appearing ear, otosclerosis has been misdiagnosed. The key differences are (1) that conductive hyperacusis does not occur in otosclerosis, and (2) that the acoustic stapedial reflex, which is often normal in SCD, should be absent in an ear affected with otosclerosis.

Electrocochleography (ECOG) may also be used. Elevated summating potential (SP) to action potential (SP/AP) ratios greater than 0.4 were reported in all 21 patients with unilateral SCDS during the study period. In those with postoperative ECOG, the ratio normalized to less than 0.4 with an average decrease of 0.47 ± 0.36.

Cervical vestibular evoked myogenic potentials (cVEMPs) are inhibitory electromyographic (EMG) signals measured over the contracted sternocleidomastoid (SCM) muscle ipsilateral to the ear being stimulated by multiple loud clicks or tone bursts ( Fig. 37.4 ). It is thought that cVEMPs are activated through the stapes footplate to the saccule and vestibular nerve. In SCDS, abnormally low thresholds and enlarged peak-to-peak amplitudes are demonstrated. ^, The theory is that a dehiscent semicircular canal lowers the impedance of the vestibular system, resulting in a lower resistance for pressure and sound transmission. ^, Thus, cVEMP signals are enhanced with lower thresholds in patients with SCDS. For air-conducted 500-Hz tone bursts, for example, we have found that cVEMP thresholds were 80 to 95 dB SPL for 13 patients with SCDS (83.85 ± 1.40 dB SPL, mean ± SD), 20 to 30 dB lower than in healthy control subjects (110.25 ± 1.28 dB SPL). It has been argued that cVEMP is better than 90% sensitive and specific for SCD, whereas other series have found the sensitivity and specificity to be closer to 80%. The cVEMP is not measurable in all patients and is especially likely to be absent in patients who have had previous middle ear surgery. The cVEMP threshold may also be decreased in other conditions such as enlarged vestibular aqueduct syndrome.

Ocular VEMP (oVEMP) is also used for the diagnostic evaluation of suspected SCDS. An excitatory EMG response is obtained from the contralateral inferior oblique muscle, with the pathway thought to be a result of utricular activation. We have demonstrated that oVEMP results in response to air-conducted sound provide greater sensitivity and specificity than does cVEMP for diagnosing SCDS. In 29 patients with surgically confirmed SCDS, a peak-to-peak amplitude greater than 17.1 μV corresponded to 100% sensitivity and 98% specificity. The performance of oVEMP is also less time-consuming compared to cVEMP. oVEMPs may also be a good screening test for SCDS. In a prospective study, SCDS patients were more likely to have abnormal oVEMPs when compared to healthy controls.

For the diagnosis of SCD to be considered, imaging of the temporal bone using CT must show the absence of bone over the superior canal. If the superior canal appears surrounded with bone on CT, the diagnosis of SCDS is effectively excluded; however, the appearance of a dehiscence on CT does not rule out thin bone covering the SC below the resolution of the scanner. Thus, CT is a highly sensitive test for SCD but it is not specific.

Optimal imaging uses high-resolution CT (HRCT) in the plane of the superior canal. ^, Unfortunately, the term “high-resolution” has been applied to a wide variety of CT scanning parameters that continue to change as technology is updated. In a review of temporal bone CT scans performed in the general population, 9% of scans had apparent SCD, with one observer reporting up to 12%. Many of these are likely false dehiscences caused by the limits of resolving thin bone. In scans with a thickness of greater than 1 mm, thin structures are subjected to partial volume artifacts. Furthermore, with bone structures less than 0.1 mm thick, volume artifacts can give the impression that bone is absent, leading to a higher perceived rate of dehiscence.

A properly performed scan should have a resolution near 0.2 mm. This requires attention to a number of parameters, the most important of which is slice thickness. Collimation of the x-ray beam to 0.5 mm allows the data to be represented by nearly isotropic voxels, so that the images can be reformatted in any plane without distortion.

Helical CT scanning, in which the table moves along the z-axis while the gantry rotates and scans, may lead to some loss of resolution. The “step, scan, and repeat” mode is preferred. The field of view used to reconstruct the images of the inner ear should be of the smallest size possible, so that the labyrinth is displayed to maximal resolution over the fixed size of the image matrix (usually 512 × 512 pixels). Image filters should be set for bone edge detection, as those filters producing less “noisy” images are likely to filter out a thin layer of bone that might remain over the canal. The images should be reconstructed in the plane of the superior canal as well as orthogonal to it so that any dehiscence can be demonstrated definitively ( Fig. 37.5 ). Parallel (Pöschl position) and perpendicular (Stenver) reformatted planes can allow for more accurate assessment. In a study of 850 patients (1700 temporal bones), the prevalence of any semicircular canal dehiscence decreased from 7% to 2.5% when the use of HRCT was combined with a semicircular canal evaluation that confirmed dehiscence in two perpendicular planes. However, even optimized scans are not without the risks of false-positive findings, so the diagnosis of SCD must never be based on a CT scan alone. A finding of SCD on CT should be considered in the context of the findings on physical examination, cVEMP or oVEMP, audiogram, and the patient’s symptoms before concluding that the patient has SCDS.

Differential Diagnosis

The CHL component of SCDS can appear similar to otosclerosis because both occur in adulthood in ears that appear normal on physical examination. The audiograms differ in that SCDS patients often have conductive hyperacusis (see Fig. 37.3 ), and if there is no previous history of middle ear surgery the acoustic reflex is often intact. Otosclerosis is not associated with decreased cVEMP or oVEMP thresholds, vertigo symptoms, or CT findings of SCD.

Ménière disease (MD) is characterized by the quartet of low-frequency hearing loss, vertigo, aural fullness, and tinnitus. Although the hearing loss in MD is classically sensorineural hearing loss, CHL has also been described. The attacks of vertigo associated with MD are usually severe and last for hours with normal periods between attacks. The dizziness associated with SCDS can be chronic but there are often shorter periods of vertigo associated with exposure to noise or pressure changes.

Autophony is often the predominant symptom in patients with a patulous eustachian tube, but it can also be the most disturbing symptom in SCDS. One distinguishing feature between the two conditions is that patients with a patulous eustachian tube typically have autophony for their own breath sounds whereas patients with SCDS usually do not. A history of vertigo symptoms and hyperacusis of bone-conducted sound are not typical of a patulous eustachian tube. The audiogram, VEMP testing, and CT will typically differentiate a patulous eustachian tube from SCDS.

A perilymphatic fistula, along with fenestrations of the other semicircular canals, is often considered in the differential diagnosis of SCDS. A perilymphatic fistula is a leak of perilymph within the vestibular labyrinth and generally is used to describe a fistula involving the round or oval window. The leak creates an abnormal compliance that allows fluid to move and stimulate the vestibular end-organs in response to sound or pressure changes. The diagnosis of a perilymphatic fistula should be considered in the context of a recent stapes surgery, temporal bone fracture, or barotrauma injury. In these cases, acute vertigo is usually accompanied by a sensorineural hearing loss. A fistula in the horizontal canal can be acquired in cases of cholesteatoma or prior mastoidectomy. Spontaneous perilymphatic fistula is a controversial diagnosis and should be considered as a diagnosis of exclusion.

One of the most common causes of spontaneous (nonpositional) vertigo is migraine-associated vertigo and should be considered in the differential with SCDS. The incidence of migraine is 17.6% of females and 5.7% of males, and approximately 25% of patients with migraine report some vertigo. Thus, migraine is much more common than is SCDS, and inevitably we have found some patients with radiographically apparent SCD whose symptoms were nonspecific and better explained by migraine. Particularly challenging are those patients who have specific symptoms of both SCDS and migraine. For example, it may be difficult to determine if their sound sensitivity is due to one more than the other. Their chronic disequilibrium may be related to migraine, or it may be due to the constant transmission of intracranial pressure pulsations through the dehiscence to the labyrinth. Moreover, the physiological disturbances of the labyrinth caused by SCDS could serve as triggers to exacerbate migraine in susceptible individuals. However, the neurotologist must also consider that the failure to recognize and treat coexistent migraine can lead to disappointing results in SCDS surgery.