michel anomaly

Axial CT scan shows right petrous bone aplasia with absence of inner ear structures. The medial wall of the middle ear is flattened (arrow), being in close contact with the infratentorial nervous structures. Note normal differentiation of the malleus.
Coronal CT scan shows a normally developed external and middle right ear. The long process of the incus (arrow) leans against the medial middle ear wall. The oval window, the stapes, and the second portion of the facial nerve are absent. A small dehiscence of the medial wall of the middle ear (arrowhead), located at the IAM, probably corresponds to the entrance of the facial nerve.

http://www.ajnr.org/cgi/content/full/20/2/281/F1