Superior canal dehiscence (SCDS): the third-window syndrome

What superior canal dehiscence is

Superior canal dehiscence syndrome (SCDS) is a condition in which the thin bone that normally covers the superior (anterior) semicircular canal of the inner ear is absent or abnormally thinned, creating a small opening or gap in the bony labyrinth. This gap forms an abnormal third mobile window into the inner ear, which is why SCDS is sometimes called third-window syndrome.

The term third window refers to the normal two windows of the cochlea: the oval window, where the stapes footplate transmits sound vibrations into the inner ear, and the round window, which acts as a pressure release. When a third opening exists at the superior canal, sound energy and pressure transmitted into the cochlea can escape through this abnormal route rather than driving the basilar membrane efficiently. This disrupts both hearing and balance in characteristic ways.

The condition was first systematically described by the otologist Lloyd Minor and colleagues in 1998, following the identification of a pattern of symptoms and audiological findings that did not fit previously known inner-ear conditions.

How common it is

The true prevalence of superior canal dehiscence is not precisely established. Anatomical studies of temporal bone specimens have found evidence of thinning or true dehiscence in a notable proportion of subjects, with some estimates suggesting dehiscence in roughly 1 to 2 percent of temporal bones examined. However, the presence of an anatomical gap does not guarantee that symptoms will occur; clinical SCDS with significant functional impact appears to be less common than the anatomical finding alone.

SCDS can be present in one ear or both, and bilateral involvement creates more complex diagnostic and management challenges.

Symptoms

SCDS produces a distinctive but variable symptom profile. The most commonly reported features include:

Autophony

Autophony is the abnormal perception of one’s own internal body sounds at unusually high intensity. People with SCDS often describe hearing their own heartbeat (a pulsatile sound synchronized with the pulse), the sound of their own voice resonating loudly inside their head, the sound of their eyes moving, and footsteps transmitted through their skeleton to the labyrinth. This occurs because body-conducted vibrations that would normally dissipate into the skull can now stimulate the superior canal through the dehiscence.

Pulsatile tinnitus

Pulsatile tinnitus in SCDS is driven by transmission of vascular pulsations into the labyrinth through the third window. It is synchronous with the heartbeat and can be one-sided or most prominent in the affected ear. SCDS is one of several structural causes that should be investigated when pulsatile tinnitus is present, particularly if other SCDS features coexist.

Tullio phenomenon

The Tullio phenomenon is dizziness or vertigo triggered by exposure to loud sounds. In SCDS, a loud sound creates a pressure wave through the middle ear that, at the level of the dehiscence, deflects the membranous labyrinth of the superior canal and activates the semicircular canal’s motion-sensing hair cells. The resulting artificial sense of rotation causes dizziness or oscillating eye movement (nystagmus) that stops when the sound stops.

Pressure-induced vertigo

A similar mechanism can be triggered by pressure changes in the middle ear, such as those produced by coughing, straining, sneezing, or bearing down (Valsalva maneuver). These pressure changes, which transmit into the perilymph and deflect the superior canal membrane at the dehiscence, are called the Hennebert sign in clinical examination.

Low-frequency conductive hearing loss

Despite the problem originating entirely in the inner ear, SCDS often produces an audiometric pattern that looks like conductive hearing loss at low frequencies. Pure-tone audiometry may show an air-bone gap at low frequencies, meaning bone-conducted thresholds appear better than air-conducted thresholds. The difference from true conductive loss (such as middle-ear fluid or ossicular fixation) is that tympanometry is normal and stapedial reflexes are present. This characteristic pattern is one of the audiological clues that leads audiologists to consider SCDS.

Diagnosis

Diagnosis requires combining clinical symptoms, audiological findings, and imaging. High-resolution computed tomography (HRCT) of the temporal bones using thin sections (typically 0.5 mm or finer) in the plane of the superior canal is the primary imaging tool. The coronal and Poschl plane reconstructions are most useful for visualizing the superior canal roof.

A limitation of CT imaging is that partial-volume averaging effects can sometimes make a thin but intact bone cover appear absent. For this reason, the finding of a gap on CT must be correlated with clinical symptoms and physiological evidence.

Cervical vestibular evoked myogenic potentials (cVEMP) are a particularly useful physiological test for SCDS. In cVEMPs, a loud click stimulus is presented to the ear and the resulting reflex muscle response in the neck is recorded. In SCDS, the third window lowers the threshold at which the saccular hair cells respond to sound, producing cVEMP responses at lower intensities than normal and sometimes with higher amplitudes. Low cVEMP thresholds combined with the appropriate CT finding and symptoms form a strong diagnostic triad.

Ocular VEMPs (oVEMPs), which record muscle responses around the eye, can also show abnormalities in SCDS.

Management

For people with mild symptoms, conservative management focuses on avoiding triggers: straining, very loud environments, and activities that cause pressure changes. Many people with SCDS, particularly those with minimal functional impact, choose observation rather than intervention.

When symptoms are significantly disabling and conservative measures are insufficient, surgical repair of the dehiscence is an option. The two main approaches are canal resurfacing (patching the bony defect from above via a middle cranial fossa approach) and canal plugging (occluding the membranous superior canal so it no longer responds to pressure). Both procedures carry risks inherent in neurotological surgery, including potential impact on hearing and balance, and are performed by specialized neurotological surgeons.

The decision to pursue surgery is individualized and involves detailed discussion with a specialist about the specific risks, the severity of symptoms, and realistic expectations for improvement. Not all symptoms resolve after surgery, and the evidence base for SCDS surgery comes largely from case series rather than randomized controlled trials.

If symptoms persist or change, see an audiologist or physician.