7The Incomplete Partitions: IP-I, IP-II and IP-III

FOne external shape, three internal failures

An incomplete partition (IP) is a cochlea of near-normal external dimensions whose internal architecture — the modiolus and/or the interscalar septa — is deficient; this distinguishes it from cochlear hypoplasia (small cochlea) and from common cavity/cochlear aplasia. The modiolus is the central bony cone that the cochlear nerve climbs and through which Rosenthal's canal carries the spiral ganglion; the interscalar septa are the bony shelves that separate the turns. IP types are defined by which of these is missing. Sennaroglu and Saatci first split IP into types I and II in 2002; type III was added later, giving the three-pattern scheme now in routine use. Because the spiral ganglion and nerve depend on the modiolus, the amount of modiolar bone present is the single fact that most shapes both gusher risk and the auditory prognosis across the three types.[2002][2017]

TIP-I and IP-II: the cystic cochlea and the classic Mondini

IP-I is a cystic cochlea: the modiolus and the interscalar septa are entirely absent, so the cochlea images as an empty cyst sitting beside an often-cystic vestibule. The defective partition between cochlea and internal auditory canal (IAC) carries a high intraoperative cerebrospinal-fluid (CSF) gusher risk. IP-II is the lesion Carlo Mondini described in 1791: a cochlea of about 1.5 turns in which the basal turn is normal but the apical turns coalesce into a single cystic cavity because the apical modiolus and the upper interscalar septum are deficient. IP-II frequently travels with an enlarged vestibular aqueduct and a dilated vestibule, the triad once loosely called the Mondini deformity; the intact basal modiolus is why its gusher risk is far lower than IP-I and why the array seats normally. Because the basal turn and its spiral ganglion are preserved, cochlear-implant outcomes in IP-II are usually good and approach those of a radiologically normal cochlea once the first year of use has passed.[2002][2017][2009]

TIP-III: the X-linked gusher ear

IP-III is the rarest pattern and the most dramatic: the modiolus is completely absent and the interscalar septa are present, so the cochlear turns sit directly on the fundus of the IAC with no bony plate between the perilymph and the CSF. It is the anatomical substrate of X-linked deafness DFNX2, caused by mutations in the transcription factor POU3F4; affected boys present with mixed hearing loss and the historic stapes-gusher phenomenon on stapes surgery. Because the cochlea opens straight into the subarachnoid space, a CSF gusher occurs in essentially 100% of cases at cochleostomy, and the electrode can mis-insert into the IAC rather than the scala — both must be anticipated and planned for, not discovered. Distinguishing the three on CT: IP-I is a featureless cyst; IP-II keeps a normal basal turn with an apical cyst plus an enlarged vestibular aqueduct; IP-III shows the bulbous IAC-cochlea continuity and absent modiolus with septa retained.[2017][2002]

CWhat each type asks of the surgeon

IP-II usually accepts a standard array through a standard approach; counsel for a possible small perilymph ooze but expect good speech outcomes. IP-I and IP-III demand a planned gusher strategy: head-up positioning, a well-fitted cochleostomy or round-window seal, fascia/muscle packing, and a CSF-diversion plan; many surgeons choose an array designed to limit over-insertion. In IP-III the operative goal is to keep the electrode in the cochlea and out of the IAC, where it neither hears nor is safe; intraoperative imaging or impedance/telemetry checks help confirm placement. Across all three the cochlear nerve must be confirmed before surgery — a malformed but nerve-bearing IP cochlea can implant well, whereas the partition anomaly itself is rarely the reason an implant fails.[2017][2009][2020]