Superior canal dehiscence syndrome

Superior canal dehiscence syndrome (SCDS) is a rare medical condition of the inner ear, first described in 1998 by Dr. Lloyd Minor of Johns Hopkins University Hospital, 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 semicircular canal of the vestibular system. 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.

Diagnosis and treatment
The presence of dehiscence can be detected by a high definition (0.6mm or less) coronal CT scan of the temporal bone, currently the only reliable way to distinguish between superior canal dehiscence syndrome (SCDS) and other conditions of the inner ear such as Meniere's disease and perilymphatic fistula. This type of 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 Meniere's disease, causing the one illness to be confused with the other. There are reported cases of patients being affected by both Meniere's disease and SCDS simultaneously.

It should be noted that 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 bone can be repaired by surgical resurfacing of the affected bone or plugging of the superior semicircular canal.

Eponym
Increasingly, this disorder is becoming known, both inside and outside the medical profession, as Minor's syndrome, after its discoverer, Dr. Lloyd Minor. However, it is important that this disease is not confused with the paralysis and anaesthesia following a spinal injury, also known as Minor's syndrome after the Russian neurologist, Lazar Salomowitch Minor (1855-1942). In the latter case this term is now nearly obsolete.

Symptoms
Superior canal dehiscence (SCD) can affect both hearing and balance, to different extents in different patients.

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

Symptoms in detail

 * SCDS-related autophony differs greatly in quality and range from the more common form which results from an open, or patulous Eustachian tube through which sufferers of this disorder hear the sound of their own voice and breathing. In contrast, patients with SCDS-related autophony report hearing their own voice as a disturbingly loud and distorted "kazoo-like" 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. Some sufferers of this condition experience such a high level of hyperacusis that a tuning fork placed on the ankle will be heard in the affected ear. 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.


 * 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 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. The presence of Tullio may also mean that involuntary eye movements (nystagmus), sometimes rotational, are set off by sound, giving the sufferer the impression that the world is tipping, clockwise or anticlockwise, depending on the site of the dehiscence. Some patients report this tilt as being as much as 15°. 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 semicircular canal and up into the intracranial space where they become absorbed instead of being registered as sound in the hearing center, the cochlea. Due to the difference in resistance between the normal round window and the pathological dehiscence window this hearing loss is more serious 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 or as being like the chirrup of a cricket or grasshopper.


 * 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.