eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > Inner Ear

Superior Canal Dehiscence

Author: Wayne T Shaia, MD, Consulting Staff, Department of Otology/Neurotology, Medical College of Virginia
Coauthor(s): Jack M Kartush, MD, Clinical Professor, Department of Otolaryngology, Wayne State University; Director of Otology, Neurotology, and Skull Base Surgery, Providence Hospital; President, Michigan Ear Institute; President, American NeuroMonitoring
Contributor Information and Disclosures

Updated: Sep 2, 2009

Introduction

Background

Superior canal dehiscence syndrome (SCDS) is a newly described condition in which vestibular symptoms are elicited by sound or pressure secondary to a dehiscent superior semicircular canal. More than 70 years have passed since Tullio and Hennebert described their findings of sound-induced and pressure-induced vestibular activation.

Since then, the Tullio phenomenon, wherein vestibular symptoms are induced by loud sounds, has been associated with syphilis, perilymphatic fistula, congenital deafness, Ménière disease, head trauma, Lyme disease, cholesteatomas with labyrinthine fistula, and fenestration operations. The Hennebert sign of vestibular symptoms due to changes in external auditory canal pressure is frequently found in conjunction with the Tullio phenomenon (as in perilymphatic fistula, syphilis, Ménière disease).

In 2000, Minor found a series of patients with positive Tullio and Hennebert signs.¹ He was the first to relate these positive findings directly to an anatomical defect of the superior semicircular canal that was detected with high-resolution computed tomography (HRCT; see Image 1). Minor theorized that, when the bone over the canal becomes thin or dehiscent, it acts as an additional window for the vestibular system, allowing pressure and noise changes to induce vestibular activity. SCDS, as it has become known, is now recognized and treated by otolaryngologists and neurootologists throughout the world.

Pathophysiology

The cochleovestibular system has 2 functional windows. The oval window, which houses the footplate of the stapes, allows sound to enter the inner ear (vestibule) and to be carried via hydroacoustic waves through the perilymph. This allows the mechanical wave to be transduced into neural activity, and, thus, sound is perceived.

The function of the round window is more controversial. It is thought to have several roles. Its first role is thought to involve the release of sound and mechanical energy from the scala tympani. Another proposed role is its participation in the secretion and absorption of substances in the inner ear. The round window may also play a role as a defense mechanism of the inner ear.

These 2 windows of the inner ear work together to regulate hearing and balance. When a dehiscence in the superior semicircular canal is created, a third-window effect is thought to take place. As a result, endolymph within the labyrinthine system continues to move in relation to sound or pressure, which causes an activation of the vestibular system. The intracranial pressure transmission to the round window may also result in increased compliance of the inner ear from stretching of the round window membrane. This pressure transmission may also result in a frank round window (or oval window) fistula.

Frequency

United States

The true incidence of persons with symptomatic SCDS is currently unknown. One study of 1000 cadaveric temporal bones revealed that a dehiscence of bone that overlies the superior canal was present in approximately 0.5% of temporal bone specimens. In an additional 1.4% of the specimens, the bone was markedly thin (≤ 0.1 mm) compared with the normal bone.

Race

SCDS has no racial bias.

Sex

SCDS appears to affect males and females equally.

Age

In 2000, Minor reported that, in his original series of 17 patients, the median age at diagnosis was 40 years (range, 27-70 y).¹

Clinical

History

Patients with superior canal dehiscence syndrome (SCDS) usually present with symptoms of sound- or pressure-induced dizziness. Chronic imbalance is another symptom of SCDS. The patient's symptoms may be re-created when sound or pressure is presented to the affected ear. Patients often control these symptoms with strict avoidance of noisy environments. In addition, some patients may hear a swishing noise when they move their eyes in a certain direction. This gaze-evoked tinnitus can be found in almost 25% of patients with SCDS. Hyperacusis is defined as an unusual sensitivity to normal everyday sounds. Although not specific, this condition is found in a high percentage of patients with SCDS.

Physical

After a general head and neck examination is performed, a detailed neurootological examination should be performed in all patients with vertigo. Other, more common, causes of vertigo and imbalance must be eliminated before SCDS can be diagnosed.

• Gait test: Determine whether the patient staggers or is off-balance with gait. Typically, patients with SCDS demonstrate a normal gait pattern.
• Oculomotor examination: All patients must be assessed for an intranuclear ophthalmoplegia and other signs of multiple sclerosis. In addition, gaze-dependent nystagmus must be eliminated as a cause of the imbalance. Nystagmus of peripheral (ie, labyrinthine) origin is usually unidirectional. Nystagmus of central origin (ie, brainstem) is usually bidirectional. Patients with SCDS do not typically demonstrate nystagmus upon routine examination. Nystagmus can be induced with loud sounds or with a pressure fistula test.
• Romberg test: This test is used to evaluate peripheral sensation, dorsal column function, and midline cerebellar function. The results of this test are usually abnormal in patients with a central pathologic condition. Patients with SCDS demonstrate normal Romberg test results.
• Fukuda test: Patients who undergo this test are asked to step in place for 20-30 seconds with their eyes closed. Rotation of the patient may indicate a unilateral loss of vestibular function. The results of this nonspecific test are typically normal in patients with SCDS.
• Dix-Hallpike maneuver: The Dix-Hallpike maneuver is performed by laying a patient back suddenly with the patient's head turned to one side. The test results are considered abnormal (or positive) if the examiner sees geotropic or ageotropic rotatory nystagmus that typically lasts less than 60 seconds. An abnormal or positive Dix-Hallpike examination result is most likely due to benign paroxysmal positional vertigo (see the eMedicine article Benign Paroxysmal Positional Vertigo). Patients with SCDS do not typically have positive Dix-Hallpike test results.
• Head-shake test: The patient wears Frenzel lenses, and the examiner shakes the patient's head at approximately 1 Hz in the horizontal plane for 20 seconds. After the shaking is stopped, the eyes are observed for nystagmus. This test can reveal latent nystagmus and indicate which labyrinth is malfunctioning. In this test, the fast phase of nystagmus is directed toward the normal (or better-performing) labyrinth. The results of this nonspecific test may be abnormal in patients with SCDS.
• Head-thrust test: The patient is asked to gaze steadily at a target in the room while the examiner briskly moves the patient's head from side to side. If the patient's eyes remain fixed on the target, the test result is normal. When the eyes make a compensatory movement after the head is stopped to reacquire the target (a refixation saccade), the test results are abnormal. This test can indicate if the output of one or both labyrinths is depressed. This is usually not a typical finding in SCDS.
• Visual dynamic acuity test: Before and during vigorous vertical shaking, followed by horizontal head shaking, the patient is asked to read the smallest visible line on the Snellen eye chart. A normal result is the ability to maintain acuity within 2 lines of the acuity at rest. An abnormal visual acuity test suggests bilateral vestibulopathy, which is most commonly observed in ototoxicity. The dynamic visual acuity test results are normal in patients with SCDS.
• Fistula test: The fistula test is designed to elicit symptoms and signs of an abnormal connection between the labyrinth and the surrounding structures. This is usually performed while the patient wears Frenzel lenses. Pressure can be applied to the patient's ear by pushing the tragus over the ear canal or with the use of a Bruening otoscope. If vertiginous symptoms are elicited or if nystagmus is seen the patient has positive fistula test results. In SCDS, the superior canal can be thought of as a fistula with connection to the middle cranial fossa. With SCDS, the direction of the nystagmus, as a result of pressure applied to the ear, results in vertical-torsional nystagmus with slow waves directed away from the labyrinth suspected of being dehiscent. Other tests, such as the pinched nostril test, can demonstrate similar findings, whereas a Valsalva maneuver produces nystagmus in the opposite direction.
• Eye movements evoked by sound are found in most patients with SCDS. These sound-induced eye movements are typically found at frequencies of 500-2000 Hz, with an intensity of 100-110 dB. The eye movements are typically vertical and torsional, away from the side of the stimulus.
• A Barany noise box can also be used to help elicit the noise-induced vertigo (Tullio phenomenon). This commercially available box simply makes a loud (100 dB) noise. When the box is slowly moved towards the patient's symptomatic ear, the vertiginous symptoms may be re-created.

Causes

An embryological etiology of SCDS has been proposed; this theory involves a postnatal failure of bone formation over the superior semicircular canal. Tsunoda and Terasaki, with the use of a computer simulation model, determined that the cause of bony dehiscence of the superior semicircular canal was due to a malpositioned primitive otocyst.² When this otocyst lies too close to the developing brain, the migratory patterns of the loose reticular cells are altered. These mesenchymal cells are thought to be necessary for completion of the bony development of the labyrinthine structures. This region may be left with incomplete or thin bony development over the superior semicircular canal.

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