Mondini Type of Genetically Determined Deafness

GENETICALLY determined deafness has been divided into two groups— those with structural developmental anomalies and soft tissue changes, and those involving only soft tissue changes. Within the latter group, it is often difficult, if not impossible, to determine definitely whether the pathological changes are a result of genetic heritage, or whether they are the result of intrauterine infection, drug toxicity, radiation, or other as yet unknown factors that may affect the foetal hearing organ during its development. The Mondini type of malformation is believed to show a dominant hereditary trait and involves the cochlear bony structure as well as, occasionally, the length of the membranous cochlea. The usual defect is an absence of the interscalar septum in the upper part of the cochlea with formation of a scala communis. An outstanding feature of most of the reported cases has been marked distension of the endolymphatic duct and sac. The extreme dilatation of these structures is in contrast to their uninvolvement, or mild involvement, in cochleo-saccular degeneration without bony malformation of the cochlea. For an excellent detailed review of inherited nerve deafness in man and animals, the reader is referred to the articles by Altmann (1950, 1964).