Superior Canal Dehiscence
Superior canal dehiscence (SCD), one form of perilymphatic fistula, is a clinical condition that results in a constellation of auditory and vestibular symptoms, such as sound- and/or pressure-induced vertigo and nystagmus, hearing loss, and/or autophony (hearing one’s own body sounds at an abnormally high sound level). First described by Minor, Solomon, Zinreich, & Zee (1998), SCD occurs when the temporal bone overlying the superior semicircular canal is unusually thin or absent.
Under normal conditions, sound pressure by air conduction enters the fluid-filled inner ear via the oval window and exits via the round window. In superior canal dehiscence, the deficit of bone overlying the semicircular canal results in a third mobile window effect, leading to areas of increased compliance and allowing for the movement of endolymph within the canal in the presence of sound and/or pressure stimuli. These stimuli increase the pressure gradient within the membranous labyrinth resulting from the change in compliance at the dehiscence site, with resulting deflection of the superior canal cupula and a subsequent inhibitory or excitatory response from the vestibular nervous system.
Incidence and Prevalence
Prevalence refers to the estimated population of people who are living with SCD in a given time period. It is estimated that the prevalence of dehiscence of the bone overlying the superior canal is 0.5%–0.6%. An additional 1%–2% of individuals have been found to have an extremely thin (<0.1mm) layer of bone overlying the canal, which is also associated with vestibular or auditory symptoms. It should be noted, however, that not all persons with complete or partial dehiscence experience symptoms. The proportion that does is not well established (Carey, Minor & Nager, 2000; Crovettoa, et al., 2010).
Signs and Symptoms
In SCD, vestibular symptoms are the most common and also the most debilitating. However, patients can also present with auditory symptoms, with or without vestibular symptoms. The following is a list of signs and symptoms associated with SCD. Note that not all patients will experience each symptom.
Common vestibular symptoms include
- chronic unsteadiness (other symptoms are typically episodic)
- Tullio phenomenon: vertigo and nystagmus induced by loud sounds
- Hennebert sign: vertigo and nystagmus induced by pressure
- oscillopsia: a visual disturbance in which objects in the visual field appear to oscillate, provoked in this case by loud sounds and/or pressure to the external auditory canal
Evoked eye movements that are seen in patients with SCD will typically be in the vertical-torsional plane either (a) torsioning away with an upward movement (this is for the vestibular ocular reflex [VOR]—slow component response—not the beat) from the affected ear in the case of positive pressure changes and resulting excitatory deflection of the cupula or (b) torsioning toward with downward movement the affected ear in the case of negative pressure changes and resulting inhibitory deflection of the cupula (Hagr, 2010). For example, right SCD will result in rightward torsion with an upbeat in response to an excitatory stimulus (sound, valsalva with closed nostrils, and positive pressure in the middle ear) or leftward torsion with a downbeat in response to an inhibitory stimulus (valsalva against closed glottis and negative pressure in the middle ear). Conversely, left SCD will result in leftward torsion with an upbeat in response to excitatory stimulus or a rightward torsion with a downbeat response due to an inhibitory stimulus (Minor, Solomon, Zinreich, & Zee, 1998).
Common auditory symptoms include
- pulsatile tinnitus
- hearing loss— most commonly low-frequency conductive or mixed (not of middle ear origin; tympanograms are normal and acoustic reflex thresholds are preserved)
- conductive hyperacusis (increased sensitivity to bone-conducted sounds)
- autophony (perception of one’s own body sounds at unusually high levels, such as hearing eye movements or pulse or, most commonly, one’s own voice)
- aural fullness
Several theories have been proposed regarding the cause of SCD, including
- embryological—a postnatal failure of bone formation over the superior semicircular canal due to a malpositioned primitive otocyst
- bone thinning associated with aging and/or vascular malformations
- head trauma, typically to the temporal bone
- surgical complication following surgical removal of the bone over the superior canal, usually in an attempt to “blue line” the labyrinth
- increased intracranial pressure resulting from activities such as straining, sneezing or coughing in already thinned bone
Roles and Responsibilities
Audiologists play a critical role in the identification of an open clinically active SCD. See ASHA's Scope of Practice in Audiology.
Appropriate roles for audiologists include, but are not limited to
- being familiar with symptoms and clinical signs for SCD
- screening for SCD using pure-tone testing, acoustic immittance testing, and vestibular evoked myogenic potentials (VEMP)
- referring patients to ENT if testing reveals positive results
- remaining informed of the research in the area of SCD and advancing the knowledge base surrounding this topic.
As indicated in the Code of Ethics (ASHA, 2010), audiologists who serve this population should be specifically educated and appropriately trained to do so.
Symptoms of SCD occasionally overlap with other auditory and vestibular disorders, such as otosclerosis, Meniere’s disease, benign paroxysmal positional vertigo (BPPV), and other forms of perilymphatic fistula (Mechant, Rosowski & McKenna, 2007; Minor, 2000; Smullen, Andrist & Gianoli, 1999) Differential diagnosis of SCD requires careful correlation of case history information, symptomology, comprehensive audiological and vestibular test batteries, and confirmation with medical imaging (high-resolution high resolution computerized tomography [CT] scan of the temporal bone).
A thorough hearing and balance history should be obtained from the patient. An awareness of the unique auditory and vestibular red flags reported in cases SCD can play a valuable role in guiding the assessment and in differential diagnosis. Below are key questions the audiologist can ask the patients in cases of suspected SCD.
- Do certain events/stimuli precipitate symptoms? If so, what are they?
- Noise triggers: Patient complaints may be associated with loud music, being in a noisy environment such as a sports event, sounds on the telephone such as the dial tone or a busy signal, and/or sounds made by the patient such as singing at certain pitches.
- Pressure triggers: Patient complaints may be associated with pushing on the outer aspect of the ear, blowing through the nose while pinching the nostrils, and/or straining while lifting heavy objects.
- Are you unusually sensitive to internal body sounds, such as the sound of your own voice, heartbeat, heel strike, etc.?
- Do you have chronic imbalance?
Audiologists typically perform a battery of audiologic evaluations. Below is a list of evaluations that are typically included in this battery and the results that might lead to an SCD diagnosis.
Purpose: Identify and classify existence, type, and degree of hearing loss
Procedure: Standard behavioral pure-tone audiometry via air and bone conduction
Results: No one pattern of hearing loss defines SCD; rather a variety of abnormalities may appear on the audiogram, including
- air-bone gaps of 10 dB or greater at lower frequencies (Minor, 2005),
- better-than-normal absolute bone conduction (lower than 0 dB HL) (Banerjee, Whyte & Atlas, 2005; Hagr, 2010),
- mild-to-moderate hearing loss in the affected ear.
Purpose: Rule out middle ear dysfunction in patients with conductive hearing loss
Procedure: Standard tympanometry with a 226 Hz probe tone and acoustic reflex threshold testing at 500, 1000, and 2000 Hz
Results: Tympanometric peak pressure and compliance are normal; acoustic reflexes are normal, although pressure changes may evoke vestibular symptoms (Carey, Minor & Nager, 2000; Minor, 2005). Acoustic reflexes are often present with air-bone gaps.
Transient Evoked Otoacoustic Emissions (TEOAE)
Purpose: Assess the integrity of the cochlea noninvasively using moving time window analysis (MTWA), which enables determination of the TEOAE duration and more accurately analyzes cochlear amplifier nonlinearities (Thabet, 2011)
Procedure: Standard wideband TEOAE protocol followed by off-line MTWA
Results: Duration of TEOAE is significantly lower than normal (Mechant, Rosowski & McKenna, 2007) (normal duration is ≥8 ms) (Pytel & Büki, 1995).
Purpose: Measure the electrical responses of the cochlea and eighth nerve in response to acoustic stimulation
Procedure: Standard clinical ECOG protocol (transtympanic or extratympanic)
Results: Summating potential (SP) is elevated relative to action potential (AP) ratio (SP/AP > 0.4) (Adams, et al., 2011; Arts, Adams, Telian, El-Kashlan & Kileny, 2009).
Vestibular Evoked Myogenic Potential (VEMP)
Purpose: Assess vestibular function— specifically the ipsilateral sacculo-collic reflex (cervical VEMP) and the crossed utriculo-ocular reflex (ocular VEMP)—by air conducted auditory stimuli
Procedure: Cervical VEMP (responses measured at the sternocleidomastoid muscle) or ocular VEMP (responses measured at the inferior oblique muscle of the eye)
Results: Thresholds are lower than normal and amplitudes greater than normal for both cervical and ocular VEMPs (Mudduwa, Kara,Whelan & Banerjee, 2010; Welgampola, Myrie, Minor & Carey, 2008).
Purpose: Assess peripheral vestibular function in order to rule out BPPV
Procedure: Begins with patient in sitting position with head turned 45° right or left; moves rapidly to supine head hanging;. after nystagmus subsides, patient returns to sitting position; procedure repeated to determine fatigability of response
Results: Results are negative for BBPV (positive response would be transient, fatiguable, torsional–vertical nystagmus with delayed onset).
Valsalva Maneuver Against Closed Nostrils or Closed Glottis
Purpose: Observe if pressure changes induce nystagmus and/or vertigo
Procedure: Requires patient to take a deep breath, hold the nose, and blow forcefully against pinched nostrils OR take a deep breath and try to blow forcefully against closed glottis
Results: Patient may experience subjective vertigo/disequilibrium and/or nystagmus (downbeating and torsional beat in the direction of the affected ear for closed nostrils and upbeating and torsional beat away from the affected ear for closed glottis) (Carey, Minor & Nager, 2000; Pfammatter, et al., 2010).
Purpose: Assess peripheral vestibular function
Procedure: Standard clinical caloric irrigation protocol using air or water
Results: Patient with a large dehiscence may show reduced vestibular function on the affected side (Cremer, Minor & Carey, 2000).
Additional Vestibular Tests
Videonystagmography (VNG), frenzel goggles, tragal compressions, and pneumatic otoscopy may all cause eye movements in the plane of the superior canal as well as concurrent dizziness.
If SCD is suspected from case history and audiologic/vestibular test battery, the patient should be referred to an otolaryngologist to confirm the diagnosis. SCD is confirmed by CT scan of the temporal bone is used to confirm the presence of a dehiscence, but audiological studies are needed to confirm that the dehiscence is active as it can be closed by dura.
All patients exhibiting positive clinical findings for SCD should be referred to an otolaryngologist. A multidisciplinary approach incorporating audiology and otolaryngology is optimal in the management of patients with SCD. The following treatment options may be considered.
For less severe cases, counseling on avoidance of precipitating stimuli is sufficient to prevent the symptoms from becoming debilitating (Minor, 2000).
Reserved for patients with severely debilitating symptoms, intervention consists of surgical repair of the superior canal via resurfacing or plugging (middle cranial fossa or transmastoid approach) (Brantberg, et al., 2001).
In cases of persistent hearing loss, with or without surgical intervention, hearing aids may be helpful, based on the degree of loss and perceived handicap. However, consideration must be given to the Tullio effect.
Superior Canal Dehiscence Audiologists Play Vital Role in Differential Diagnosis
Dehiscence of Bone Overlying the Superior Semicircular Canal as a Cause of an Air-Bone Gap on Audiometry
Clinical Assessment of Otolith Function
Signs and Symptoms of Central Vestibular Disorders
Evaluating Patients With Dizziness and Unsteadiness: A Team Approach
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