See also: Congenital hearing loss
Last updated: March 5, 2021
Temporal Bone CT scans are done routinely in persons with childhood sensorineural hearing loss. About 25% of patients with congenital hearing loss will have bony inner ear malformations (Mafong et al, 2002). About 55% of inner ear malformations are "Mondini" malformations.
The normal cochlea has two and one-half turns. A cochlear malformation consists of a membranous abnormality, a bony abnormality, or a combination of these two. There is a complex naming system used for many of the radiological findings. The material below is abstracted from Schwartz and Harnsberger (1998). A general idea is that generally speaking, labyrinthine maldevelopment occurs earlier than cochlear issues, and while cochlear problems can exist by themselves, it would be rare to have a pure labyrinthine developmental problem. Here are the basics:
- A complete labyrinthine and cochlelar aplasia is called the Michel deformity. This is extremely rare, and occurs early in development (about the 3rd week). The Michel deformity accounts for less than 1% of congenital ear malformations.
- Not quite as severe is the "common cavity" malformation, which occurs roughly at the 4-5th week. The common cavity comprises 26% of cochlear malformations. There is some neural tissue in the common cavity and sometimes cochlear implants are attempted.
- Cochlear aplasia or hypoplasia (i.e. labyrinth may be present), can occur around the 6th week. This anomaly accounts for about 15-18% of cochlear malformations. The Mondini deformity occurs from disturbances during roughly the 7th week of gestation.
- The Mondini is the most common malformation, and accounts for about 55% of all inner ear malformations. The cochlear develops only 1 1/2 turns, and lacks the interscalar septum. Often there is also a large vestibular aqueduct. By definition, basilar turn development is intact. The Mondini often also includes a cystic apex, a dilated vestibule and a large vestibular aqueduct.
The majority of Mondini cases are bilateral. Many of these are genetic in origin (Chen et al, 2012). Unilateral Mondini malformations are not generally inherited. Mondini has been sometimes reported in association with numerous other named syndromes including Klippel-Feil, Wildervanck, Alagille, Waardenburg, Pendred, and DiGeorge.
This is a radiological diagnosis, generally made with a high-resolution CT temporal bone CT scan. Note that these scans should not be performed lightly as they entail considerable radiation. In other words, you don't want to do a CT scan of the temporal bone to screen someone with, for example, a sudden hearing loss.
An example of a high-resolution CT scan of a Mondini malformation is shown on the right (Strome et al, 1998). The black arrow shows a sac-like cochlea. The white arrow shows an amorphous vestibule without any defined semicircular canals. There are various other variant malformations including cochlear aplasia and hypoplasia, as well as others -- see table above(Sennaroglu and Saatci, 2002)
Often accompanying the Mondini dysplasia is abnormal communication between the endolymphatic and perilymphatic spaces of the inner ear and subarachnoid space. It is usually caused by a defect in the cribriform area of the lateral end of the internal auditory canal. Presumably because of this abnormal channel, perilymphatic fistulae are more common in this disorder.
Nearly all persons with Mondini malformations are completely deaf on the affected side, but high frequency hearing can be present in some. While the hearing loss in these malformations is probably sensory in origin, the audiograms in these people sometimes suggest a conductive hearing loss -- this is probably related to relatively better bone conduction in some of these malformations than normal because of lower compliance between the perilymphatic and CSF compartments.This is similar to the situation in superior canal dehiscence. Patients with Mondini malformations sometimes benefit from cochlear implants (Turrini et al, 1997)
Persons with Mondini may have a progressive hearing loss during childhood, and may also be afflicted with a sudden deterioration secondary to internal rupture of cochlear membranes, in a way similar to thought occurs in Meniere's.
Nearly all persons with Mondini also have absent vestibular function on the affected side, but this cannot be relied upon. See below regarding present otolithic responses in the CHARGE variant.
Children with these malformations may also exhibit motor delay due to damage to the labyrinthine part of the inner ear (Bodensteiner et al, 2003).
A related anomaly and more severe syndrome, the CHARGE association consists of coloboma, heart disease, choanal atresia, retarded development, genital hypoplasia, ear anomalies including hypoplasia of the external ear and hearing loss. These individuals have a Mondini type deformity and absence of semicircular canals. A recent report documents that they have normal otolithic responses to off-vertical axis rotation (Wiener-Vacher et al, 1999).
There are many reports that patients with Mondini may develop recurrent meningitis as a result of ear infections communicating with spinal fluid. (e.g. Sharawat et al, 2019) They are also more prone to develop CSF leaks. (Caro-Osorio et al, 2018).
As in the Mondini malformation, the semicircular canals or otoliths may be preserved, there is a possibility of dizziness from a "sick" ear. In the author's experience, this usually occurs in early adulthood.
Patients with Mondi have less vestibular hair cells than normal subjects on autopsy (Kaya et al, 2017).
The reason for this is that accompanying the Mondini dysplasia is abnormal communication between the endolymphatic and perilymphatic spaces of the inner ear and subarachnoid space. It is usually caused by a defect in the cribriform area of the lateral end of the internal auditory canal. Presumably because of this abnormal channel, perilymphatic fistulae are more common in this disorder.
Surgeons who are planning to operate on persons with a Mondini need to be aware and plan for the possibilty of a "CSF gusher"(Sajai et al, 2000). This means that due to the open communication between the inner ear and spinal fluid compartment, cerebrospinal fluid can "gush" out of an opened inner ear. It would seem to us that fistula "repairs", using conventional soft-tissue seals, are doomed to fail. To us, it would seem that the main options are between sealing the windows with cement, or simply a complete labyrinthectomy.
As mentioned above, some patients with Mondini respond to cochlear implant. It has also been suggested that obliteration of the endolymphatic sac may stabilize the progressive hearing loss that afflicts some Mondini patients.
Symptomatically, these patients resemble Meniere's patients.
This man with congential deafness developed dizziness as an adult.
|This CT of the temporal bone shows a clearly intact cochlea and semicircular canal system on the left side of the picture (right side of the head), but the cochlear turns cannot be seen on the right side of the picture.||Hearing is normal on the right side, and absent on the left. There is a "shadow curve" on the right as very loud sounds on the right cannot be masked.|
A VEMP test shows a small response on the left side, in spite of the fact that hearing is absent on the left. This is likely due to preserved semicircular canals on the left side (which can be seen on other images). Note also that there is a contralateral VEMP response on both sides, and that the contralateral response for left sided stimulation is bigger than the ipsilateral response !
This illustrates the pitfall intrinsic to using bilateral stimulation and bilateral recording for VEMP's - -responses on one side may be due to stimulation of the opposite side.
Other hypoplastic syndromes:
|MRI read by radiologist as showing hypoplastic lateral canals. Patient with Downs syndrome.||Normal MRI of inner ear for comparison purposes.|