Bilateral dilatation of the auditory canal without acoustic neurinoma. So-called "patulous canal".

Four cases of typical “Patulous Canal” are presented. Two of the patients had von Recklinghausen's disease. Patulous canal is defined as bilateral dilatation of the internal auditory canal without an accompanying acoustic neurinoma.Bilateral perceptive deafness may rarely accompany this syndrome.Etiologies of bone changes of von Recklinghausen's disease may be summarized as:1. Changes in the bone secondary to mass effect of neurofibromas.2. Intrinsic factors in the initial stage of ontogenesis.Among the latter, abnormalities of mesodermal tissues are found frequently.Ontogenitically the skull, especially the temporal bone including the internal auditory canal, are derived from the peri-otic capsule of the cartilaginous neurocranium which is of mesenchymal origin.The inner ear is derived from the otic vesicle which separates from the epidermal ectoderm.The presence of bone changes such as dilatation of the internal auditory canal in von Recklinghausen's disease and in other illnesses may suggest that bony changes are accompanied by mesenchymal abnormalities ontogenetically.The rarity of the complication of deafness may be explained by the low incidence of coincidental involvement of both mesenchymal and ectodermal tissues in congenital and intrinsic abnormalities.Attention has been focused on the occurrence of similar bony changes, such as dilatation of the internal auditory canal in von Recklinghausen's disease. The possible mechanisms causing these changes are discussed from the embryological viewpoint.