Incomplete partition type II, historically referred to as Mondini malformation or Mondini dysplasia, is the most common type of inner ear malformation, often concurrent with enlarged vestibular aqueduct (30.7% - 50%).
Etiology
Incomplete partition type II is an autosomal dominant genetic disorder that can affect one or both ears. The exact cause and mechanism are unclear, but it may result from teratogenic factors causing the cochlea to stop developing at the 14th week of gestation. The apical and middle turns of the cochlea are not separated, while the basal turn is usually present and of normal size.
Clinical manifestations
The main symptoms include severe hearing impairment or complete hearing loss in the affected ear, with no auditory brainstem response. If vestibular dysfunction is present, symptoms such as vertigo may occur. Hearing loss can be present at birth or develop between 1 - 2 years of age or even later. Increased pressure in the affected inner ear over time can lead to resorption and thinning of the stapes footplate, making it prone to rupture. This can result in cerebrospinal fluid (CSF) leakage through the tympanic cavity and eustachian tube into the nasal cavity, causing CSF otorrhea/rhinorrhea. CSF leakage can lead to recurrent meningitis.
Diagnosis
Diagnosis relies on audiological findings and imaging studies. High-resolution CT shows fusion of the apical and middle turns of the cochlea into a cystic cavity, with no separation, while the basal turn is present, normal in size, and separated from the middle turn. The cochlea appears to have 1 - 1.5 turns, with an enlarged vestibule that may form a common cavity with the basal turn, and semicircular canals are partially developed or absent. Inner ear MRI can reveal fluid-filled membranous labyrinth, absence of separation between the middle and apical turns, and abnormalities of the semicircular canals and vestibule. Recently, 3D MRI of the inner ear has been used to observe the membranous labyrinth from different angles.
Figure 1 CT and MRI of incomplete partition type II
A. Axial CT of the temporal bone, with white arrows indicating fusion of the apical and middle turns into a cystic cavity, with separation from the basal turn; B. Axial T2-weighted MRI showing cystic apical and middle turns.
Treatment
If residual hearing is present, use of hearing aids with speech rehabilitation is recommended.
For those with no residual hearing or severe hearing loss, cochlear implantation is advised.
If CSF leakage is present, a CSF leak repair surgery should be performed.