Current literature
Developmental anomalies of inner ear depend on timing of insult exposure
during embryonic development. Degree of hearing loss will depend on the
severity of the malformation. A number of classifications have been
proposed by various authors over the years (5,6,7).
Our first case is a 3 years old autistic child with bilateral cochlear
hypoplasia type IV with normal audiological parameters. Minimal
anomalies can be associated with normal hearing or may even be missed on
routine radiological imaging. In our case, the apical and middle turns
were severely hypoplastic which doesn’t correlate with the audiology. As
per current evidence, the number of turns of the cochlea plays a limited
role in causing hearing loss (3). Polvogt et al performed an interesting
study to correlate structural changes of cochlea and hearing. Temporal
bone dissection of seventeen subjects who had normal hearing in their
life was performed and a variety of cochlear anomalies were found. The
authors stated that several other features such as length of basal turn,
length and height of upper turn need to be measured in cases of cochlear
hypoplasia. The arrest of cochlear development prematurely will cause a
decrease in height from base to apex of cochlea, which can lead to
hearing loss (8). The mechanism of decrease in hearing is by reduction
of displacement of basement membrane vibration (8,9). It is claimed that
this decrease in displacement is maximum when the cochlea is the
shortest. In our case, the cochlea was severely malformed which should
have caused measurable hearing loss. On the contrary, we observed all
audiological parameters to be normal. Additionally, the cochlear nerve
and auditory tract were normal. In Autistic Spectrum Disorder, the
auditory abnormalities can be either peripheral or central. The most
common auditory deficit is the inability to filter auditory input due to
abnormalities in auditory brainstem. However, a normal waveform pattern
on BERA merits some explanation. Mohammed AS et al in 2017 reported a
case of a two- year old with incidentally detected cochlear hypoplasia
on one side and cochlear nerve aplasia on the other (10). The authors in
their description of cochlear hypoplasia mention that hearing in such
ears is variable and maybe remarkably good. The variability maybe
attributed to the degree of membranous labyrinthine development within
the truncated cochlear lumen.
Mondini dysplasia presents with specific features such as flat cochlea,
immature semicircular canals and vestibule and severe to profound
hearing loss (5,7). Limited evidence exists regarding cases with Mondini
dysplasia and a normal hearing. (7). In our scenario, the point of
interest is presence of normal hearing till adulthood followed by a
progressive, fluctuating hearing loss.
It may thus be re-emphasized that apart from number of turns of cochlea
and presence/absence of cochlear nerve there are certain parameters
which are yet to be studied in detail which influence the hearing status
of malformed ears.