[Dehiscence of the superior semicircular canal: approach and CT scan classifications]

The syndrome of dehiscence of the superior semicircular canal (DCSS) is primarily associated with vertigo and/or hearing loss. The dehiscence may be completely asymptomatic and represent an incidental finding on radiological investigation.

To demonstrate the advantages of a volume rendered CT study of the petrous temporal bone of patients with hearing loss, and to demonstrate the effectiveness of its systematic application in the protocols of examination. To propose a radiological classification of DCSS with a therapeutic application.

The examination technique which was performed in incremental mode (axial and frontal sections) and in "volume rendered" mode, on a high resolution apparatus is described. The authors studied 154 scans of the petrous temporal bone obtained by this technique. They correlated the cases of DCSS with the indications for the radiological examination. Each 3d CT scan was studied and the type of fistula described. The authors propose a classification of fistulae into three types, depending on 3d CT scan appearance.

Out of 154 CT scans of the petrous temporal bone (77 patients), 13 cases of DCSS were discovered. DCSS was bilateral in 4 cases. The primary indication for investigation was the assessment of conductive or mixed hearing loss. The "volumetric" technique was compared with standard imaging techniques and/or reconstructed images in the superior canal plane. The correlation was perfect in all the cases. The description of the fistulae allowed a classification into 3 types: Type I (symmetrical fistula, 8 cases); Type II (asymmetrical fistula, 3 cases) corresponding to the canal dome; Type III (2 cases) involving the foot of the canal.

The increased frequency of DCSS in this series (prevalence of 17% against 0.5% in post mortem studies) is probably explained by the selection bias of the patients and also by the systematic application of this novel radiological technique. We propose to include this protocol in all CT scans of the temporal bone, particularly when investigating symptoms consistent with a syndrome of Minor or the Tullio phenomenom. This system of classification makes it possible to describe the fistula and to specify its location. This should prove to be a valuable aid for pre-operative planning and intra-operative localisation of the fistula.