Can the pathogenesis of auditory ossicular malformations be explained by the branchial-based theory?--Evaluation by the consecutive distribution of embryologic foci in 87 cases.

Background: Development of the auditory ossicles is initiated by induction of the cartilage primordium of each ossicle between the fifth and seventh fetal week. It is well known that primordium of the upper part of the ossicular chain is derived from the first branchial arch and that of the lower part develops from the second branchial arch. Previous studies have suggested that auditory ossicular malformations are caused by deficiencies in the early period of induction of the cartilage primordium. However, auditory ossicular malformations can occur at any developing stage, and their clinical features are very complicated.

Objective: The precise embryologic foci of auditory ossicular malformations were estimated by evaluating the consecutive distribution of these anomalies, and the pathogenesis of these malformations was discussed using new aspects.

Study Design: Retrospective study.

Patients: Eighty-seven ears of 78 patients that underwent surgical treatment after the diagnosis of an auditory ossicular malformation in Niigata University Hospital between 1998 and 2012.

Main Outcome Measures: The types of malformations were roughly divided into 4 groups based on intraoperative findings: Type A--ankylosis of the malleus head or short process of the incus; type B--defect in the connection between the incus and stapes; type C--fixation of the footplate; and type D--a complex lesion combining types B and C. Additionally, the consecutive points of the malformation were precisely evaluated, and a distribution map of the malformation was made. Foci of the malformations of each type were then estimated.

Results: Type A malformations were observed in 8 ears, type B in 33 ears, type C in 32 ears, type D in 6 ears, and unclassified anomalies in 8 ears. A deformity was observed in the malleus handle, which is located in the lower part of the ossicle, in 5 of 7 ears with type A malformations, which suggests that the pathogenesis of ankylosis of the malleus head or short process of the incus could not simply be explained by the branchial-based theory. The focus of the type B malformation was located on the long process of the incus and not onthe I-S joint. We suggest that pathogenesis of the defect inthe connection between the incus and stapes could not be explained by a conjugation deficiency in the cartilage primordium but could be explained by an ossification deficiency after the conjugation period. The focus of the location of the defect was shifted more medially in type D malformations than in type B malformations, and this was significantly different, which suggests that this type of malformation is not caused by 2 independent anomalies but is inducted by a monogenic abnormality.

Conclusion: The foci of auditory ossicular malformations were highly variable, which suggests that the pathogenesis of these malformations could not be simply explained by a branchial-based theory. Auditory ossicular malformations occur at various developmental stages of the auditory ossicles.