Dysmorphogenesis of the mandible, zygoma, and middle ear ossicles in hemifacial microsomia and mandibulofacial dysostosis.

A review of the anatomical changes in patients with various "first arch" syndromes shows that some anomalies (e.g., micrognathia, ear defects) generally appear together. This study tested the hypothesis that the mandible, zygomatic arch, and middle ear ossicles are a developmental field (i.e., when any of these structures is anomalous, the other two will be also). The hypothesis was tested using data from 25 patients with mandibulofacial dysostosis (MFD) and 40 patients with hemifacial microsomia (HFM). Analysis of the pooled data showed that the hypothesis of character association was generally supported. However, the medians suggested that different factors probably played a role in determining how these three anatomical structures were associated in MFD and HFM. Errors in chondrogenesis may have been primarily responsible for the HFM phenotype. Alterations in Meckel and palatoquadrate cartilages would account for the size and shape changes observed in the ossicles and mandible, while changes in cranial base cartilages may explain the changes noted in the zygomatic arch. Since all three structures were equally affected in MFD, it is problematic to use an interference with chondrogenesis as an explanation for the phenotype. We conclude that although the mandible, zygomatic arch, and middle ear ossicles appear to form a "developmental field," the association between structures varies for HFM and MFD. The relatively lesser involvement of the zygomatic arch in HFM than in MFD suggests different pathogeneses for the two diagnostic groups and maybe a useful criterion for judging animal models of HFM.