Abstract

Superior canal dehiscence syndrome (SCDS) is a set of hearing and balance symptoms, related to a rare medical condition of the inner ear, known as "superior canal dehiscence". The symptoms are caused by a thinning or complete absence of the part of the temporal bone overlying the superior semicircular canal of the vestibular system. There is evidence that this rare defect, or susceptibility, is congenital. There are also numerous cases of symptoms arising after physical trauma to the head. It was first described in 1998 by Lloyd B. Minor of Johns Hopkins University in Baltimore.

Symptoms

Superior canal (SCD) can affect both hearing and balance to different extents in different people.

Symptoms of SCDS include:

- Autophony – person's own speech or other self-generated noises (e.g. heartbeat, eye movements, creaking joints, chewing) are heard unusually loudly in the affected ear

- Dizziness/ vertigo/ chronic disequilibrium caused by the dysfunction of the superior semicircular canal

- Tullio phenomenon – sound-induced vertigo, disequilibrium or dizziness, nystagmus and oscillopsia

- Pulse-synchronous oscillopsia

- Hyperacusis – the over-sensitivity to sound

- Low-frequency conductive hearing loss

- A feeling of fullness in the affected ear

- Pulsatile tinnitus

- Brain fog

- Fatigue

- Headache/migraine

- Tinnitus – high pitched ringing in the ear

Causes

According to current research, in approximately 2.5% of the general population the bones of the head develop to only 60–70% of their normal thickness in the months following birth. This genetic predisposition may explain why the section of temporal bone separating the superior semicircular canal from the cranial cavity, normally 0.8 mm thick, shows a thickness of only 0.5 mm, making it more fragile and susceptible to damage through physical head trauma or from slow erosion. An explanation for this erosion of the bone has not yet been found.

Diagnosis

The presence of dehiscence can be detected by a high definition (0.6 mm or less) coronal CT scan of the temporal bone, currently the most reliable way to distinguish between superior canal dehiscence syndrome (SCDS) and other conditions of the inner ear involving similar symptoms such as Ménière's disease and perilymphatic fistula. Other diagnostic tools include the vestibular evoked myogenic potential or VEMP test, videonystagmography (VNG), electrocochleography (ECOG) and the rotational chair test. An accurate diagnosis is of great significance as unnecessary exploratory middle ear surgery may thus be avoided. Several of the symptoms typical to SCDS (e.g. vertigo and Tullio) may also be present singly or as part of Ménière's disease, sometimes causing the one illness to be confused with the other. There are reported cases of patients being affected by both Ménière's disease and SCDS concurrently.

As SCDS is a very rare and still a relatively unknown condition, obtaining an accurate diagnosis of this distressing (and even disabling) disease may take some time as many health care professionals are not yet aware of its existence.

Treatment

Once diagnosed, the gap in the temporal bone can be repaired by surgical resurfacing of the affected bone or plugging of the superior semicircular canal. These techniques are performed by accessing the site of the dehiscence either via a middle fossa craniotomy or via a canal drilled through the transmastoid bone behind the affected ear. Bone cement has been the material most often used, in spite of its tendency to slippage and resorption, and a consequent high failure rate; recently, soft tissue grafts have been substituted.

Eponym

Occasionally this disorder has been referred to as Minor's syndrome, after its discoverer, Lloyd B. Minor. However, that eponym has also been given to an unrelated condition, the paralysis and anaesthesia following a spinal injury, which is named after the Russian neurologist, Lazar Salomowitch Minor (1855–1942). In the latter case this term is now nearly obsolete.