The pathological-anatomical change in the inner ear with congenital perceptive hearing loss is, in some cases, a malformation of the cochlea, where the modiolus is developed in such a manner that only the basal turn of the cochlea is present, while the upper turns form a common cavity—the scala communis. There can also be varying degrees of changes in the pars inferior of the membranous labyrinth and slight changes in the pars superior. This defect (Mondini defect) can be demonstrated by tomography in the axial-pyramidal projection. The Mondini defect is thought to be an arrest malformation occurring in the seventh fetal week, as a nonspecific reaction to a large number of endogenous and exogenous stimuli. It can occur spontaneously, or it can be hereditary, possibly as a part of Pendred's syndrome, where it occurs frequently. Chemical or infectious stimuli can be the cause, and the defect can be found as an isolated phenomenon or as a component of widespread malformation.
Illum P. The Mondini Type of Cochlear Malformation: A Survey of the Literature. Arch Otolaryngol. 1972;96(4):305–311. doi:10.1001/archotol.1972.00770090481002
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