5Too Little Cochlea: Aplasia and Hypoplasia
One step less severe than Michel aplasia, these anomalies share a theme of cochlear shortfall. In cochlear aplasia the cochlea is absent while a vestibule survives; in cochlear hypoplasia a small cochlea forms but is reduced in turns, size, or both. The clinical fork is sharp: aplasia gives nothing to implant, while many hypoplastic cochleae can be implanted with short or special arrays - and outcome follows the surviving neural tissue.
FCochlear aplasia: absent cochlea, present vestibule
Cochlear aplasia is complete absence of the cochlea with a present (often normal or dilated) vestibule and semicircular canals - the feature that separates it from Michel aplasia, where everything is absent. It reflects arrest around the fifth gestational week, after the otocyst has begun to differentiate a vestibular component but before a cochlear duct forms. On CT the promontory lacks any coiled cochlear structure while vestibular structures remain visible. Because there is no cochlea, there is no spiral ganglion to host an array; the cochlear nerve is typically absent or rudimentary.[1987][2010]
FCochlear hypoplasia: a small cochlea, types I-IV
Cochlear hypoplasia is a cochlea that has formed but is too small - reduced in number of turns, in size, or both. Sennaroglu subdivides it into four types: bud-like (CH-I), cystic hypoplastic (CH-II), a cochlea with fewer than the normal turns (CH-III), and a hypoplastic cochlea with a normal basal turn but reduced middle and apical turns (CH-IV). The cochlea separates from the vestibule (unlike a common cavity) but contains less sensory and neural tissue than normal. An enlarged vestibule and semicircular-canal anomalies frequently accompany hypoplasia, and hearing ranges from near-normal to profound.[2017][2010][1987]
TThe modiolus and cochlear nerve - the variables that decide outcome
The presence and integrity of the modiolus and the bony cochlear-nerve canal (cochlear aperture) vary across hypoplasia types and predict the neural substrate. A present modiolus and a normal-calibre cochlear aperture suggest a populated spiral ganglion and a present cochlear nerve - the conditions for benefit. A deficient modiolus or stenotic aperture warns of cochlear-nerve deficiency, which must be confirmed on MRI before counselling. Imaging therefore answers two questions: is there a cavity to receive an array, and is there a nerve to carry the signal centrally?[2010][2012]
CImplantation and outcomes
Cochlear aplasia is not implantable - with no cochlea there is no place for an array; absent nerves push toward ABI evaluation. Many hypoplastic cochleae are implantable, but a full-length array will not fit a short cochlea, so short or custom arrays are chosen to match the available length. Insertion depth and electrode count are tailored to the actual cochlear dimensions to avoid kinking, extrusion, or non-auditory stimulation. Outcomes track the surviving neural tissue: when the nerve and ganglion are adequate, hypoplastic ears can do well, though on average results trail those of normally formed cochleae.[2012][2005][2017]
What is the most appropriate management of the hearing loss?
What single imaging feature most cleanly separates cochlear aplasia from Michel aplasia?
In cochlear hypoplasia, which two structures are the key variables predicting implant benefit?
Why is a full-length cochlear-implant array usually inappropriate in cochlear hypoplasia?