lunedì 7 settembre 2009
Superior semicircular canal dehiscence
Figure 1: On the left, there is dehiscence of the superior semicircular canal.
Figure 2: Oblique view through the plane of the superior semicircular canal shows bone dehiscence on the left.
Diagnosis: Superior semicircular canal dehiscence
Superior semicircular canal dehiscence refers to absence of the bony roof over the superior semicircular canal. Dehiscence of the lateral and posterior canals may also occur, but is much less common. Although this may be asymptomatic, it can result in Tullio phenomenon as in this case, conductive hearing loss, or chronic disequlibrium. When symptomatic, it is often referred to as superior semicircular canal dehiscence syndrome (SCDS). Thinning of the bone over the canal is thought to predispose patients to this entity, which has been demonstrated in 2% of persons at autopsy.
Normally, the semicircular canals are a closed hydraulic system. With dehiscence of the semicircular canal, a "third window" is created. When this occurs, movement of the oval window and stapes in response to sound can result in slight movement of the covering of this third window and subsequent unphysiologic motion of endolymph in the semicircular canal.
Imaging is critical in the diagnosis of semicircular canal dehiscence, and it is only with the advancements in thin section MDCT and multiplanar reconstructions that have made effective evaluation possible. Non contrast temporal bone CT is the examination of choice. Oblique coronal reconstructions are often the most useful. MR imaging cannot be used to diagnose semicircular canal dehiscence but may be of benefit in ruling out other pathology.
Semicircular canal dehiscence is fortunately a treatable form of vestibular disturbance. A conservative approach with earplug use and avoidance of provoking stimuli may be appropriate in certain circumstances. Surgical correction varies from middle fossa craniotomy and covering of the defect with bone wax, cement, or fascia to less invasive approaches involving reinforcement of the oval and round windows to decrease their movement. Techniques are still evolving.