Audiologists commonly see patients with an unusual sensitivity to loud sounds and/or dizziness. There could be a number of reasons for this condition, including psychological issues, sensorineural hearing loss (e.g., recruitment)—or a disorder termed superior canal dehiscence (SCD).
SCD is a relatively uncommon disorder first described by Lloyd Minor in 1998. It is characterized by a small opening or thinning in the temporal bone that encases the superior semicircular canal and can lead to bothersome auditory and vestibular manifestations (see Figure 1 [PDF]). Although the etiology of SCD is currently unknown, two primary causes of SCD are reported to be congenital malformation or physical trauma to the temporal bone.
The audiologist is in a unique position to identify patients with suspected SCD because the auditory symptoms of this disorder include decreased sensitivity to air conduction stimuli and increased sensitivity to bone conduction stimuli. The vestibular system symptoms associated with SCD include disequilibrium or nausea when the patient encounters everyday sounds. For example, it is not uncommon for patients with SCD to experience dizziness when listening to a phone conversation with the impaired ear. An audiologist with an awareness of the auditory and vestibular symptoms experienced by a patient with SCD and the tests that can help confirm this disorder can play a valuable role in differential diagnosis.
The fluid-filled membranous labyrinth of the inner ear is almost completely encased by the bone of the otic capsule. A normal inner ear has two primary openings or windows in the otic capsule that facilitate the transmission of sound from the middle ear space into the inner ear (i.e., oval and round windows). In some cases a pathological third window can occur in the bony roof overlying the superior semicircular canal (SSC; see Figure 1 [PDF]). Patients with this clinical entity—SCD—often present with vestibular and auditory symptoms when the impaired ear is subjected to intracranial pressure changes or loud sounds (Tullio's phenomenon; Minor et al., 1998). These patients often report tinnitus that pulsates, dizziness when subjected to loud sounds, and autophony (a condition in which self-generated sounds such as breathing are unusually loud to the individual), making a hearing test the logical first step in the differential diagnosis.
The presence of a third mobile window in the otic capsule allows air-conducted acoustic energy to be shunted away from the cochlea, resulting in increased thresholds for air-conducted stimuli (Merchant & Rosowski, 2008). Conversely, the third window does not reduce bone-conducted thresholds and, in some cases, can even lower thresholds. SCD appears to have little effect on middle-ear compliance or acoustic reflexes, presenting the clinician with an unexplained air/bone gap on the audiogram. This pattern of findings illustrates the importance of always including comprehensive immitance testing in the routine audiometric examination when SCD is suspected.
In our experience, some patients with SCD are symptomatic, describing Tullio's phenomenon, autophony, and a visual disturbance in which objects move in the field of vision. However, some patients who have SCD have no symptoms. These individuals' diagnoses of SCD are confirmed with the use of imaging, yet they have no symptoms or complaints that would suggest the presence of a third window.
Recently, data have been presented that suggest the size of the dehiscence may relate to patient complaints and the size of the air-bone gap (ABG). Using computerized tomography (CT) to confirm the SCD in their study participants, Yuen and colleagues (2009) showed that 100% of patients with dehiscence larger than 3 mm had an ABG. Furthermore, the size of the ABG was positively correlated with the size of the dehiscence (R2=0.828 using a quadratic fit). Pframmatter and colleagues (2010) reported that patients with larger superior canal dehiscences presented with significantly more vestibular and cochlear symptoms than those individuals with smaller dehiscences. The “gold standard” for confirmation of dehiscence is high-resolution (0.6 mm or less) coronal computed tomography (CT) of the temporal bone. However, CT is an expensive screening test for SCD. The findings above illustrate how the audiologist is in an excellent position to collaborate with the physician to help determine if a patient should undergo additional testing to confirm or deny the presence of SCD.
Modifying Hearing Tests
When an SCD is suspected there are several ways to modify the routine hearing test to begin the process of determining the presence of a third window. First, special attention should be paid to determining the lowest possible bone conduction threshold. Bone conduction thresholds obtained from ears with SCD are often less than 0 dB nHL (Brantberg et al., 2001; Minor et al., 2001; Mikulec et al., 2004). Second, a Weber tuning fork test (512 Hz) usually will localize to the affected side. Third, the audiologist can place the audiometer bone-vibrator on the lateral round bony prominence on the side of an affected patient's ankle and the patient may hear it in the affected ear.
If SCD is suspected following the hearing test and case history, a vestibular-evoked myogenic potential (VEMP) test may be warranted. The VEMP is an evoked potential test of otolith function (i.e., saccule or utricle) that consists of an initial positivity occurring at approximately 13 ms (P13) followed by a negativity at 23 ms (N23). The VEMP has been shown to be highly sensitive to the presence of SCD (Brantberg et al., 1999). Specifically, patients with confirmed SCD demonstrate abnormally low VEMP thresholds. In these patients, VEMPs recorded from the affected side are measurable at a lower sound pressure level than would normally be expected. Additionally, VEMP recordings from SCD ears typically demonstrate larger P13 and N23 amplitudes than ears without SCD (Brantberg et al., 1999).
At first it may seem inconsistent that air conduction thresholds would be inordinately poor when VEMP thresholds would be incredibly good. However, it is believed that the presence of a dehiscence increases the motion of the endolymphatic fluid in the saccule, resulting in transporation of stimuli to the nervous system at lower than normal intensity levels. Occasionally, symptoms are severe enough to be disabling and surgery may be considered. Surgical correction typically consists of either resurfacing the dehiscence or ablating the canal.
A multidisciplinary approach incorporating audiology and otolaryngology is optimal in the diagnosis and management of patients with SCD. When each professional pays close attention to the characteristic signs and symptoms presented, a clinical profile can be generated and the appropriate tests ordered. The diagnosis of SCD should be established based on a case history, audiometry, and VEMP responses. The diagnosis is confirmed with the CT scan. If SCD is suspected, the audiologist can adapt the audiometric examination and initiate the process of correctly
identifying those patients with a dehiscence.