Superior canal dehiscence
What is SCDS?
(SCDS) is a rare medical condition of the inner ear 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 (the bone containing the internal parts of the ear) overlying the superior semicircular canal of the vestibular system (our body’s center of balance). The semicircular canals are components of the inner ear part of our vestibular system that control the body’s ability to perceive rotational motion (e.g. when we spin about ourselves or rolling down a hill for instance). Erosion of the superior semicircular canal 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 (occurs during our development as an embryo or fetus).
Superior canal dehiscence (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 in a “kazoo like” fashion or as if coming from a “cracked loudspeaker”. Additionally one may hear the creaking and cracking of joints, the sound of footsteps when walking or running, the heartbeat and the sound of chewing and other digestive noises. The bizarre phenomenon of being able to hear the sound of the eyeballs moving in their sockets (e.g. when reading in a quiet room) “like sandpaper on wood” is one of the more distinctive features of this condition and is almost exclusively associated with SCDS.
- Dizziness/ vertigo/ chronic lack of balance caused by the dysfunction of the superior semicircular canal.
- Tullio phenomenon – sound-induced vertigo, disequilibrium or dizziness, nystagmus. Patients showing this symptom may experience a loss of equilibrium, 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: the ‘rattle’ of a plastic bag; a cashier tossing coins into the register; 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.
- Pulse-synchronous oscillopsia – a visual disturbance in which objects in the visual field appear to oscillate. The severity of the effect may range from a mild blurring to rapid and periodic jumping.
- Hyperacusis – the over-sensitivity to sound or perception that sounds are louder and possibly causing discomfort than they should normally be perceived.
- Low-frequency conductive hearing loss
- A feeling of fullness in the affected ear
- Pulsatile tinnitus – a pulse-synchronized “wave” or “blip” which patients describe as a “swooshing” sound or as being like the chirrup of a cricket or grasshopper.
- Brain fog – A subjective sensation of mental clouding described as feeling “foggy” 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.
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 remainder of the skull, 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.
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.
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 neurosurgical type of approach (middle fossa craniotomy) or via a canal drilled in the bone behind the affected ear (a transmastoid approach). Bone cement and more recently soft tissue grafts have been the materials most often used for the plugging of the superior semicircular canal.