About Superior Semicircular Canal Dehiscence
Superior Semicircular Canal Dehiscence (SSCD) is a rare medical condition of the inner ear leading to vestibular and auditory symptoms. The symptoms arise because of the absence or thinning of the bony part of the labyrinth that is overlying the superior semicircular canal.
Patients with SSCD can experience vertigo (know more) and oscillopsia evoked by loud noises and also by activities that alter the intracranial (or middle ear) pressure, such as straining, sneezing or coughing. Auditory manifestations of the syndrome include autophony (meaning, amplification of one’s own voice), hypersensitivity to noises, and a distinct conductive hearing loss revealed on audiometry. Some patients have either vestibular or auditory consequences while some may have both.
A striking sign of SSCD, a sensation of oscillopsia, is described by patients as the swaying of the eyes or horizon in up and down direction or experiencing vertigo on an upward plane.
Some patients, respond well when they know the cause of their symptoms and withdraw from the triggers that induce SSCD symptoms; such as avoiding loud noises. Those patients who do not show any improvement to trigger avoidance may have to undergo surgery. If the symptoms are severe, the avoidance of triggers will not be of much help. Symptoms like constant disequilibrium, autophony, extreme sound intolerance and pulsatile oscillopsia, may cause extreme uneasiness and do not get controlled by avoiding the triggers.
Signs & symptoms
The symptoms seen in auditory functions in SSCD are hearing loss, unsteadiness oscillopsia and extreme sensitivity to sound. There can be unusual symptoms too. Many times, we have seen that the patients exhibit conductive hearing loss for low-frequency sounds that are very close to the characteristic pattern in otosclerosis. Patients with SSCD may also complain of symptoms such as hearing their eyes move, hearing their voice more loudly (autophony) or having a distorted sensation of sound in the affected ear during activities such as running. Bone conducted sounds amplify the effects of the dehiscence. On the other hand, sounds that are transmitted by air to the cochlea are reduced by the dehiscence.
Vestibular evaluation should be done which may show nystagmus on VNG during Valsalva maneuver and vibration testing, reduced thresholds for VEMP and conductive hearing loss on audiometry.
Vestibular Evoked Myogenic Potential (VEMP)
Loud noises provoke short-latency relaxation potential in the ipsilateral sternocleidomastoid muscle. The VEMP response of the patients will be below the normal threshold. The amplitude of the VEMP waveform will be larger for equal stimuli intensities in the affected ear, than in an ear without dehiscence. The VEMP examination plays a significant role in the evaluation of patients with suspected SSCD.
A high-resolution CT scan of temporal bones proves extremely helpful in the diagnosis of Superior Semicircular Canal Dehiscence. The scan displays the breach in the bone that covers the superior canal. Even then, extreme care and caution are required while studying the scanned images, so that the thin layer of uninjured bone overlying the superior canal is not overlooked.
For surgical repair of SSCD, the middle cranial fossa approach is widely used. In this surgery, the surgeon closes the superior canal with soft fibrous tissue and bone patty holds the fascia in place. This surgical correction provides long-term relief from both vestibular and auditory symptoms of the patients Although, the functionality of the operated canal may diminish, the other semicircular canals work normally.
The decline of functional efficiency in the superior canal has littleeffecton the overall balance of the patient.