What is expected of the facial nerve in michel aplasia? Anatomic variation.

We sought better understanding about the facial nerve anatomy in the rare inner ear Michel anomaly to help better define this aplasia and prevent potential complications in surgery on these patients. The data from computed tomography scans and magnetic resonance images of six Michel aplastic ears (three patients) were evaluated for a facial nerve course. Facial nerve course and anatomic landmarks were noted. Based on data obtained from this group of very rare patients, three different facial nerve anatomies were encountered. The first patient had normal-looking mastoid cells, normal middle ear ossicles, and a completely formed facial nerve canal through the middle ear. The second patient had pneumatized mastoid air cells despite an anomalous ossicular chain. This patient also had a facial nerve canal but not through the middle ear. In the third patient, although mastoid cells were present, neither ossicles nor a definite facial nerve canal could be detected. With guidance provided by the anatomy of the other parts of the ear, such as air cells and the ossicular chain, the danger zones posing a high probability of facial nerve injury can be predicted. Although all Michel aplasias may have aplastic petrous bone in common, there are some degrees of variation.