Superior Semicircular Canal Dehiscence Syndrome

Abstract

This article discusses superior semicircular canal dehiscence syndrome. This syndrome has been identified in 1998 by Minor etal. This condition is caused due to dehiscence of bone overlying the superior semicircular canal. This causes vertigo, oscillopsia, dysequilibrium due to exposure to sound. Ultra high resolution CT scans help in identification of this condition. This dehiscence has been confirmed by surgical exploration of middle cranial fossa. Introduction: Superior semicircular dehiscence syndrome is a reasonably recently identified condition. It was Minor etal 1 during 1998 who documented first this condition. This condition causes sound / pressure induced vertigo, autophony, conductive hearing loss and hyperacusis in some patients. Sound induced vertigo also known as Tullio phenomenon 2 classically seen in this condition can also be caused by perilymphatic fistula, congenital syphilis (due to excessive laxity of annular ligament). Clinical features of Superior semicircular canal dehiscence syndrome: 1. Sound / pressure induced vertigo (Tullio phenomenon) 3 2. Nystagmus is vertical and rotatory in nature. This is in line with the involved superior semicircular canal. This may vary in patients with large dehiscence over superior canal due to hyperfunction. 3. Vertigo during coughing, sneezing and valsalva manoeuvre. This is due to increased middle ear pressure. Pathophysiology: Under normal circumstances the round and oval windows are the only two openings seen in the hydraulic system of inner ear. Inner ear corresponds to a hydarulically closed system. There is no substantial movement of inner ear fluid in the semicircular canal even when stapes vibrates in reponse to sound. When a third window is present in this system in the form of labyrinthine fistula then membranous labyrinth over the fistulous area bulges out causing movement of endolymph. Otolaryngology online This affects the hydraulic purity of the system causing the brain to interpret this movement as movement of the body. This causes a conflict with input received from visual and proprioceptive receptors leading on to a sense of reeling by the patient. Presence of third window in the labyrinth is the cause for positive Tullio phenomenon. According to Minor lack of bony covering over superior semicircular canal could make it to function like a third window in addition to round and oval windows.