ENLARGEMENT OF THE COCHLEAR AQUEDUCT - FACT OR FICTION

Enlargement of the cochlear aqueduct (CA) is often mentioned in the ot ologic literature, usually in its purported association with sensory h earing loss, stapes gusher, and transotic cerebrospinal fluid leak. In CT scans of 100 ears, the diameter of the CA medial aperture was foun d to be highly variable, ranging from 0 to 11 mm, with a mean of 4.5 m m. In contrast, the otic capsule segment was very narrow in every case . It could be visualized in only 56% of cases, none of which exceeded 2 mm in diameter. Several published reports of supposed CA enlargement presented images of a dilated medial aperture that was well within th e range of normal variability according to the present study. In a tho rough review of the literature on radiology of the CA, we were unable to find a single published image that convincingly demonstrated enlarg ement of the otic capsule portion. As radiographic CA enlargement has not been convincingly reported to date, it appears to be an exceedingl y rare or perhaps even nonexistent malformation. It is important to re cognize than even a radiographically normal CA may be hyperpatent. It is theoretically possible for increased fluid flow to result from eith er deficiencies in intraluminal membrane baffles or subtle canal enlar gement beneath the resolution limits of CT scanning. However, as fluid flow through a tube is regulated by its narrowest point, it is extrem ely improbable that stapes gusher, transotic CSF leak, and vigorous pe rilymphatic fistula are generated by the CA when CT scans show any por tion of it to be very narrow. A substantial body of evidence points to a deficient partition between the internal auditory canal and inner e ar as causative in such cases. We propose that the criteria for the di agnosis of CA enlargement on high-resolution CT scan be a diameter exc eeding 2 mm throughout its course from the posterior fossa to the vest ibule.