DIAGNOSTIC EVALUATION

With any patient presenting with complaints of dizziness, a good history is the most effective diagnostic tool. Patients with SCDS usually present with a primary complaint of either autophony or dizziness. There are many possible causes of dizziness, and patient evaluation should focus on narrowing the diagnosis. 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 are present in 90% of SCDS patients.⁸ Vestibular symptoms induced by pressure changes such as coughing or straining are present in 73% of patients, with 67% exhibiting pressure-related and sound-related symptoms.⁸ Chronic dysequilibrium symptoms and cognitive impairment (“brain fog”) may also be attributed to SCDS.

Auditory symptoms are also a common feature of SCDS. Hyperacusis for bone-conducted sound⁹ is present in 52% of SCDS patients.⁸ Symptoms often include hearing one’s pulse, eye movements, or the impact of the feet during walking. More uncommon but quite dramatic conductive hyperacusis symptoms include being able to hear others’ bowel sounds when sitting in the same church pew or the motor of a faraway vehicle while sitting on a park bench. Autophony or the patient’s own voice sounding disturbing to them is present in varying degree in 60% of patients.⁸ Patients with SCDS occasionally can hear in the affected ear a 512 Hz tuning fork placed against the foot or ankle.¹⁰

Evoked eye movements in the plane of the superior canal are the hallmark of SCDS¹¹; these are present in 60% of our patients. The eyes should be examined under Frenzel lenses, with 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 normal hearing level (nHL) should be delivered in one ear at a time covering 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.⁸ Eye movements can also be induced with Valsalva maneuvers (75%) or pressure in the external auditory canal (45%). Depending on the type of stimulus, either excitation or inhibition of the superior canal may occur as shown in Figure 42-1. Pressure-evoked or sound-evoked eye movements almost always occur in the plane of the superior canal as shown in Figure 42-2. In the case of larger dehiscences, eye movements may be shifted out of the superior canal plane¹²; however, if eye movements are not in this direction, the diagnosis of SCDS should be questioned, and alternative diagnoses of posterior canal dehiscence¹³ or horizontal canal fistula¹⁴ must be 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 SCDS patients.

FIGURE 42-1 Route of excitatory and inhibitory pressure changes causing stimulation of superior canal ampulla in superior canal dehiscence syndrome. Superior canal excitation is caused by ampullofugal displacement of the cupula typically by positive external auditory canal pressure, nasal Valsalva maneuver, or sound. Superior canal inhibition is caused by ampullopetal displacement of the cupula from negative external auditory canal pressure or glottic Valsalva maneuver, which transiently increases intracranial pressure.

FIGURE 42-2 Direction of slow phase of eye movements with superior canal excitation. Eye movement occurs in the plane of the superior canal regardless of direction of gaze. There are vertical and torsional components when the patient is looking directly ahead (center gaze). The torsional and vertical components can be separated by having the patient look to the right or left during stimulation.

The audiogram (Fig. 42-3) is an important part of the SCDS evaluation, and a few patients have auditory symptoms in the absence of any vestibular signs or symptoms.^8,10,15,16 Conductive hearing loss is often largest at lower frequencies,^15–17 and bone conduction thresholds are often less than 0 dB nHL (conductive hyperacusis). Because of the conductive hearing loss and normal appearance of the ear, some patients with primarily auditory symptoms have historically been misdiagnosed as having otosclerosis.¹⁰ 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.

FIGURE 42-3 Typical audiogram in a patient with right-sided superior canal dehiscence syndrome. Circles represent air conduction, and brackets represent bone conduction. There is a negative bone conduction threshold at 250 Hz and 500 Hz, and the air-bone gap is largest at low frequencies.

Vestibular evoked myogenic potential (VEMP) thresholds are usually decreased in patients with SCDS. These potentials are most commonly measured from the sternocleidomastoid muscles using averaged electromyography in response to multiple loud clicks or tone bursts delivered to the ear (Fig. 42-4). The reflex is thought to be activated by sound transmitted through the stapes footplate to the saccule and inferior vestibular nerve.¹⁸ Decreased VEMP thresholds are indicative of SCDS. For air-conducted 500 Hz tone bursts, we have found that cervical VEMP thresholds were 80 to 95 dB sound pressure level (SPL) for 13 patients with SCDS (83.85 ± 1.40 dB SPL, mean ± SD), 20 to 30 dB lower than in normal control subjects (110.25 ± 1.28 dB SPL).¹⁹ It has been argued that VEMP is better than 90% sensitive and specific for SCD,²⁰ whereas other series have found the sensitivity and specificity closer to 80%.²¹ The VEMP is not measurable in all patients and is especially likely to be absent in patients who have had previous middle ear surgery. The VEMP threshold may also be decreased in other conditions, such as enlarged vestibular aqueduct syndrome.²²

FIGURE 42-4 Typical vestibular evoked myogenic potential (VEMP) results in a patient with right-sided superior canal dehiscence syndrome and an intact left side. VEMP is initially measured with clicks at 95 dB nHL, and stimulus amplitude is decreased until response is no longer measurable. In the left ear, the patient has VEMP response at 95 dB, but not with lower amplitude stimuli. In the right ear, the amplitude of VEMP is much larger at 95 dB, and the response continues to be detectable at amplitudes of 60 dB. In this example, VEMP threshold is 95 dB nHL on the left and 60 dB nHL on the right.

For the diagnosis of SCD to be considered, imaging of the temporal bone with computed tomography (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 excluded; however, the appearance of a dehiscence on CT does not rule out thin bone covering the superior canal below the resolution of the scanner. CT is a highly sensitive test for SCD, but it is not specific.²¹ Optimal imaging uses high-resolution CT formatted in the plane of the superior canal.^21,23 The term high-resolution has been applied to a wide variety of CT scanning parameters, which continue to change as technology is updated. In a review of temporal bone CT scans done in the general population, 9% of scans had apparent SCD with one observer identifying 12%.²⁴ Many of these are likely false dehiscences caused by the limits of resolving thin bone because the incidence of SCD in a survey of temporal bones was only 0.7%.⁷

A properly done CT scan should have a resolution near 0.2 mm. Attaining this resolution requires attention to numerous parameters. The most important of these 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 because filters producing less “noisy” images are likely to filter out a thin layer of bone that might remain over the canal.

Images should be reconstructed in the plane of the superior canal and orthogonal to it so that any dehiscence can be definitively shown (Fig. 42-5). Even optimized scans are not without the risks of false-positive findings, however, 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 findings on physical examination, VEMP, and audiogram, and the patient’s symptoms before concluding that the patient has SCDS.

FIGURE 42-5 CT showing superior canal dehiscence. A, CT image is reformatted in plane of superior canal. Area of dehiscence between the superior canal and middle fossa is present. B, Orthogonal reconstructions are performed at 3 degree intervals for 180 degrees around superior canal. These planes of reconstruction are shown as white lines. C, Orthogonal reconstruction shows superior canal dehiscence. Region of reconstruction is shown in small view in lower left.

DIFFERENTIAL DIAGNOSIS

SCDS should be considered in the differential diagnosis of other disorders. When conductive hearing loss is present in a setting without trauma and with a normal otoscopic examination, SCDS should be considered along with otosclerosis. Autophony raises the possibility of a patulous eustachian tube, but SCDS can produce a similar sensation. Episodic vertigo evoked by intracranial or middle ear pressure changes could indicate a perilymphatic fistula, but SCDS should strongly be considered as an alternative diagnosis. We have seen several patients who have undergone previous surgical explorations for such presumed otologic disorders, only later to be found to have SCDS.

The conductive hearing loss component of SCDS can appear similar to otosclerosis because both occur in adulthood in ears that appear normal on physical examination.¹⁰ The audiograms differ in that SCDS patients often have conductive hyperacusis (see Fig. 42-3), and if there is no previous history of middle ear surgery, the acoustic reflex is often intact. Otosclerosis is not associated with decreased VEMP thresholds, vertigo symptoms, or CT findings of SCD.

Meniere’s disease is characterized by the triad of low-frequency hearing loss, vertigo, aural fullness, and tinnitus.²⁵ Although the hearing loss in Meniere’s disease is classically sensorineural, conductive hearing loss has also been described.²⁶ The attacks of vertigo associated with Meniere’s disease usually are severe and last hours with normal periods between attacks. The dizziness associated with SCDS can be chronic, but there are also often short periods of vertigo associated with exposure to noise or pressure changes; a careful history usually differentiates these two conditions.

Autophony is often the predominant symptom in patients with a patulous eustachian tube,²⁷ but it can also be the most disturbing symptom in SCDS. One distinguishing feature between the two conditions is that patients with patulous eustachian tube typically have autophony for their own breath sounds, whereas patients with SCDS usually do not.²⁷ Also, a history of vertigo symptoms and hyperacusis of bone-conducted sound are not typical of a patulous eustachian tube. The audiogram, VEMP, and CT typically differentiate a patulous eustachian tube and SCDS.

A perilymphatic fistula is a leak of perilymph somewhere in the vestibular labyrinth, and the leak creates an abnormal compliance that allows fluid to move and stimulate the vestibular end organs in response to sound or pressure changes. Perilymph fistula along with fenestrations of other semicircular canals are often considered in the differential diagnosis of SCDS.²⁸ The term perilymphatic fistula is usually used to describe a fistula into the middle ear through the round or oval window. The perilymphatic fistula diagnosis is most clear in the presence of 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.²⁹ Spontaneous perilymphatic fistula is a controversial diagnosis, which if considered at all should be considered only after all other possible causes are excluded.³⁰

Just as it is possible for patients with SCDS to have prior incorrect diagnoses, patients with other disorders are occasionally diagnosed with SCDS. In recent years, the SCDS diagnosis has become well publicized with the disorder featured on nationally televised news programs and Internet forums. Radiologists are also aware of the diagnosis, and 12% of temporal bone CT scans of the general population may be read as showing SCD,²⁴ even though the true prevalence of the disorder is likely less than 1%.⁷

The most common cause of spontaneous (nonpositional) vertigo is migraine-associated vertigo.³¹ The incidence of migraine is 17.6% of women and 5.7% of men,³² and approximately 25% of migraine patients report some vertigo.³³