False-positive CT gas cisternogram.

Nonfilling of the normal internal auditory canal on computed tomographic (CT) gas cisternography was observed in seven (11%) of 62 cases in a retrospective study of three series of gas cisternograms. The meatal surface of the fluid-filled canal was convex and pointed, simulating a small acoustic neuroma. A meniscus effect at the gas-cerebrospinal fluid interface was considered the probable cause of nonfilling of the canal with gas. Shaking the patient's head briskly after injection of gas into the spinal fluid will facilitate filling of the internal auditory canal with gas. Bone erosion in the canal or meatus suggests the presence of acoustic neuroma. In the absence of conclusive findings of neuroma, persistent nonfilling of the canal is an indication for repeat cisternography with an alternate contrast medium.