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.
