Histopathology of inner ear malformations: Do we have enough evidence to explain pathophysiology?

OBJECTIVE

To investigate the histopathology of inner ear malformations (IEMs) in order to explain their pathophysiology.

METHOD: Light microscopy was used to study 33 specimens exhibiting various IEMs in the collection of the Otopathology Laboratory at Harvard University's Massachusetts Eye and Ear Infirmary.

RESULTS

The investigation found 18 incidences of cochlear hypoplasia (CH) (3 CH-I, 10 CH-II, 5 CH-III), 11 incomplete partitions (IPs) (5 IP-I, 6 IP-II), 2 vestibular dilatations, and 2 cases of cochlear nerve aplasia. The IP-I cases had characteristic defective endosteums, while the IP-II cases showed hydropic changes in the scala vestibuli. The CH cases were small in size externally, with normal or defective internal architecture.

CONCLUSION

In combination with embryological data, these findings suggest that cases of CH-III and CH-IV are most probably genetically predetermined to be small in size, and that development of the membranous labyrinth stops at a point earlier than normal, so that it is shorter. At the time of complete ossification, this results in a cochlea with small external dimensions and normal internal architecture. In CH-I and CH-II cases, there is arrested development of the internal architecture, in addition to a small cochlea; it is most likely that in these cases, there is a severely defective vascular supply from the internal auditory canal (IAC). IP-I may be the result of a defective vascular supply from the blood vessels of the IAC. In IP-II, an enlarged endolymphatic sac (EES) appears to be the genetic abnormality that causes the other abnormalities, as it allows high pressure to be transmitted into the cochlea and vestibule. In IP-III, the pathophysiology appears to be an abnormal vascular supply from the middle ear mucosa, caused by a genetic abnormality and resulting in a thinner otic capsule and the absence of the modiolus.