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[Diploic epidermoid on the petrous bone: a case report].

An unusual case of a congenital petrous bone epidermoid which resulted in a sudden deafness is reported. The patient was a 25-year-old man who suddenly developed a right deafness and tinnitus. Neurological examination on admission revealed slight hearing disturbance in the high sound area. Plain craniogram showed a destructive bone lesion in the middle portion of the petrous bone on the right side. Bone-window CT scan disclosed a bony destructive mass in the petrous bone, and enlargement of the internal auditory canal. The mass lesion was almost isodense on plain CT scan and was not enhanced by contrast media. T1 weighted imaging of MRI demonstrated an isointense mass in the petrous bone, and T2 weighted imaging demonstrated hyperintensity. Gd-DTPA MRI revealed no enhancement. Angiogram did not reveal abnormal findings. Right lateral suboccipital craniectomy was performed to remove the posterior wall of the internal auditory canal. The tumor existed extradurally originating from the dipole of petrous bone. The tumor appeared pearly and fragile. The pathological examination of the specimen was diagnosed as epidermoid. The postoperative course was uneventful except for right hearing loss on the 4th postoperative day. Epidermoid arising in the middle portion of the petrous bone is quite rare, so this case was hereby reported with some references.