This
type of malformation has been described as a flattened cochlea with
development only of the basal Coil. Instead of two and one-half turns in
the bony cochlea, there may be only one and one-half turns, with the
middle and apical turns occupying a common space or cloaca.
The agenesis is rarely symmetrically bilateral, but the other ear
always shows some degree of
malformation. The vestibular structures and their associated neural
elements may be similarly underdeveloped.
There appears to have been an interruption in the development of
the cochlea at the stage in which it is represented by a short, curved
tube (about the 6th week of gestation).
Vestibular
structures may be recognizable in some cases; in others there is no
membranous labyrinth. Occasionally
some sensory epithelium may be present; this implies the possibility of
function. An enlarged endolymphatic duct and sac and a deficient
utriculoendolymphatic valve are commonly found associated with collapse of
Reissner’s membrane or an absence of endolymphatic hydrops.
Lesions of the end-organ of Corti may be present or absent.
The stria vascularis and spiral ligament are often normal.
Mondini’s deformity may be associated with an absence of the oval
and round windows along with other aplastic lesions of the middle ear.
The
Mondini type of deafness may first become manifest in childhood or early
adult life. Auditory function
ranges from marked deafness to normal hearing.
Mechanisms of hearing loss in Mondini’s aplasia may be explained
on the basis of pathologic findings in the inner or middle ears, including
dysgenesis of the end organs and associated neural elements sufficient to
cause a sensorineural hearing loss ; aplasia or the oval or round windows,
which might explain manifest sensorineural hearing loss in the presence of
normal-appearing organs of Corti; and aplasia or infection of the middle
ear causing a conductive loss.
This left ear
shows the midmodiolar view of the cochlea (C) with one and one half turns.
|
