Phylogeny and embryology of the facial nerve and related structures. Part II: Embryology.

A precise description of the developmental anatomy of the facial nerve and associated ear structures, augmented by an appreciation of phylogenic history, has proven extremely helpful intraoperatively. Predictions of facial nerve position can be made with reasonable accuracy when they are based on a proper analysis of developmental anomalies of other ear structures. In cases in which the facial nerve canal has not developed, it may be impossible to obtain accurate localization of the facial nerve radiographically prior to surgery. In such cases, judgment based on an understanding of embryology may be the surgeon's only guide to the position of the facial nerve. Although these principles have proven valid and reliable so far, it is not prudent for the surgeon to consider them gospel, of course. Extreme caution must always be exercised when operating on an ear with congenitally abnormal anatomy. Nevertheless, it is not necessary for the surgeon to approach a congenitally malformed ear with the fear that "You never know where the facial nerve is going to be." Understanding embryology allows otologic surgery to be planned with reasonable accuracy and carried out safely and expeditiously.