Superior canal dehiscence syndrome (SCDS)

Superior canal dehiscence syndrome (SCDS)

Superior canal dehiscence syndrome (SCDS) is a rare condition of the inner ear, first described in 1998 by Dr.Lloyd Minor of Johns Hopkins University, Baltimore, USA, leading to hearing and balance disorders in those affected. The symptoms are caused by a thinning or complete absence of the part of the temporal bone overlying the superior semi circular canal. This may result from slow erosion of the bone or physical trauma to the skull and there is evidence that the defect or susceptibility is congenital.
Symptoms

Autophony – Patients with SCDS-related autophony report hearing their own voice as a disturbingly loud and distorted sound deep inside the head as if relayed through “a cracked loudspeaker.” Additionally they may hear the creaking and cracking of joints, the sound of their footsteps when walking or running, their heartbeat and the sound of chewing and other digestive noises. A distinctive feature of this condition almost exclusively associated with SCDS is hearing the sound of the eyeballs moving in their sockets (e.g. when reading in a quiet room) “like sandpaper on wood”.
Tullio phenomenon, another of the more identifiable symptoms leading to a positive SCD diagnosis is sound-induced loss of balance. Patients showing this symptom may experience vertigo, a feeling of motion sickness or even actual nausea, triggered by normal everyday sounds. Although this is often associated with loud noises, volume is not necessarily a factor. Patients describe a wide range of sounds that affect balance: a telephone ringing; a knock at the door; music; the sound of children playing and even the patient’s own voice are typical examples of sounds that can cause a loss of balance when this condition is present, although there are countless others. For such persons, a visit to the concert hall or to a noisy playground may seem like being at the epicenter of an earthquake. A change of pressure within the middle ear (for example when flying or nose-blowing) may equally set off a bout of disequilibrium or nystagmus.
Low-frequency conductive hearing loss is present in many patients with SCDS and is explained by the dehiscence acting as a “third window.” Vibrations entering the ear canal and middle ear are then abnormally diverted through the superior semi circular canal and up into the intracranial space instead of being registered as sound in the hearing center, the cochlea. This hearing loss being greater in the lower frequencies and may initially be mistaken for otosclerosis.

Pulsatile tinnitus is yet another of the typical symptoms of SCDS and is caused by the gap in the dehiscent bone allowing the normal pulse-related pressure changes within the cranial cavity to enter the inner ear abnormally. This pressure change thus becomes audible and an existing tinnitus will be perceived as containing a pulse-synchronized “wave” or “blip” which patients describe as a “swooshing” sound.
Brain fog and fatigue are both common SCDS symptoms and are caused by the brain having to spend an unusual amount of its energy on the simple act of keeping the body in a state of equilibrium when it is constantly receiving confusing signals from the dysfunctional semicircular canal.
Headache and migraine are also often mentioned by patients showing other symptoms of SCDS. A direct causal link has yet to be proven.
Causes
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. In SCDS the section of temporal bone separating the superior semicircular canal from thecranial 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.

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 Meniere’s disease and Perilymph fistula. Other diagnostic tools include the VMPtest, CENG (Computerized ElectroNystagmography) or 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.

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.