Mondini Deformity 

Embryology

Typically, the membranous labyrinth achieves its usual shape around the eighth week of development, and ossification of the otic capsule around the membranous labyrinth is complete by birth. Traditionally, pathogenesis of cochlear malformations was believed to be related to different stages of developmental arrest during inner-ear formation. Based on this rationale, developmental arrest at the seventh week of gestation produces a cochlea that is smaller and incompletely partitioned with only 1.5 turns.[6] However, recent evidence demonstrates that not all inner-ear anomalies result from developmental arrest at a specific date.[2]

Classification

In 1987, Jackler et al proposed a classification system of inner-ear malformations that was based on specific dates of cessation of cochlear development. Cessation of development at the fifth week caused cochlear agenesis, cessation at the sixth week produced cochlear hypoplasia, and developmental cessation at the seventh week produced a small cochlea that was incompletely partitioned with resultant 1.5 turns.[6] This classification system was created using polytomography, a form of imaging that has now been discarded and since replaced with high-resolution CT imaging, which is superior in its resolution to polytomography.

In 2004, Sennaroglu and Saatci created a modified classification system for incomplete partition of the cochlea to help differentiate different forms of incomplete partitioning.[7] Using a retrospective review of temporal bone CT scans from patients with sensorineural hearing loss, the researchers were able to ascertain two variants in the their study population.

The first variant, termed incomplete partition type I (IP-I), demonstrated a cochlea that was without the entire interscalar septum and the modiolus, resulting in a cystic appearance. While the vestibule was found to be dilated when compared to a normal temporal bone, enlargement of the vestibular aqueduct was rare.

Incomplete partition type II (IP-II) demonstrated a cochlea that had a normal basal turn; however, the apex was cystic in appearance owing to absence of the modiolus and the corresponding interscalar septum. Additionally, the vestibule was mildly dilated and the vestibular aqueduct was enlarged. The IP-II malformation is the deformity described by Carlo Mondini and is the basis for the title Mondini deformity.

A third type of incomplete partition (IP-III) has been recently described and is found in individuals with X-linked deafness. Morphologically IP-III demonstrates the presence of the interscalar septa; however, the modiolus is absent.[8]

In all three variants of incomplete partitioning, the cochleas are of normal size, which differs from the findings of Jackler et al in 1987. Additionally, Sennaroglu and Saatci state that using the term 1.5 turns for Mondini deformity is inaccurate and should be discontinued. The reasoning is that, if the cochlea has 1.5 turns, then it would technically be smaller than a normal cochlea, which would place it into the category of cochlear hypoplasia rather than incomplete partitioning.[6]

Schematic representation of the normal cochlea and cochlear malformations. A: normal cochlea, mid-modiolar section; Mo = modiolus, CA = cochlear aperture, B = basal turn, M = middle turn, A = apical turn, arrowheads = interscalar septa. B: Normal cochlea, inferior section passing through the round window niche (RWN); arrowhead = interscalar septum between middle and apical turns. C: Cochlear aplasia with normal vestibule. D: Cochlear aplasia with enlarged vestibule. E: Common cavity. F: Incomplete partition type I. G: Incomplete partition type II. H: Incomplete partition type III. I: Cochlear hypoplasia, bud type (type I). J: Cochlear hypoplasia, cystic cochlea type (type II). K: Cochlear hypoplasia, with less than 2 turns (type III).

Genetics

Mutation of the SLC26A4 gene, which codes for the membrane transport protein pendrin and functions as an iodide/chloride transporter, has been implicated in congenital hearing loss.[9, 10] Mutation of the SCL26A4 gene is very heterogeneic, with more than 100 different mutations identified, including missense, nonsense, and splice-site mutations.[11]

The SCL26A4 mutation has been associated with Pendred syndrome and DFNB4, both of which share sensorineural hearing loss associated with Mondini deformity or an enlarged vestibular aqueduct.[12, 13, 14, 15]  Typically, individuals with Pendred syndrome are homozygous for the SCL26A4 mutation and display incomplete partition type II or an enlarged vestibular aqueduct.[16]

A study by Forli et al indicated that biallelic SLC26A4 mutations are associated with worse hearing thresholds; the biallelic mutations also carry a greater likelihood that thyroid pathologies and Mondini malformations are present than is isolated enlarged vestibular aqueduct. These effects particularly accompany homozygous mutations.[17]

Not all congenital malformations of the inner ear result from SCL26A4 gene mutation.[18] This is an interesting finding because the various types of cochlear malformation have always been thought of as a continuum of defects, all due to developmental arrest at some embryological stage.

In persons with IP-III, X-linked deafness has been found to be related to the pathogenesis. Males with X-linked deafness are expected to suffer from severe hearing loss, while females are expected to experience more mild-to-moderate hearing loss.[19]

Current estimates of the prevalence of congenital SNHL are between 1 and 6 newborns per 1,000. 20-30% of patients with congenital hearing loss demonstrate some sort of radiographically anomalous inner ear.[6, 19, 3] A study of 63 patients with SNHL and radiographically documented abnormalities found that 55% of these patients had incomplete partitioning of the cochlea.[6]

The prognosis of Mondini deformity varies. Depending on the degree of malformation of the cochlea, sensorineural hearing loss can range from nonexistent to profound.

Most newborns with congenital sensorineural hearing loss have impaired hearing at birth and are usually identified with infant hearing screening. In persons with Mondini deformity, the variability of hearing acuity in individuals is wide, ranging from normal hearing to profound sensorineural hearing loss.

Of the 25 patients studied in their 1987 study, Jackler et al found that the average hearing level in patients with incomplete partition was 75 dB with a speech discrimination score of 44%.[6]

In patients with congenital cochlear malformation, otoscopy findings are normal, with intact middle ear structures present behind the tympanic membrane.

Hearing evaluation

Using a 512-Hz tuning fork, the Weber test can be performed to assess gross hearing. In patients with Mondini deformity, sound should localize to the ear opposite to the congenitally malformed ear.

In patients with congenitally acquired anomalous inner ears, high-resolution computed tomography (CT) scanning and magnetic resonance imaging (MRI), of the temporal bone, are the diagnostic tools of choice in diagnosing the disorder. (See the CT scan below.)[20]

Incomplete partition anomalies of the cochlea. A and B: Incomplete partitions. Cochlea (C), without the modiolus and interscalar septa, resembles an empty cystic structure (A) and is accompanied by dilated vestibule (v) (B). C and D: Incomplete partition type II. The apical part of the modiolus and the corresponding interscalar septa are defective (black arrow), giving the apex of the cochlea a cystic appearance due to the confluence of middle and apical turns (C). This is accompanied by minimally dilated vestibule (v) and large vestibular aqueduct (white arrow). E: Incomplete partition type III. Cochlea has interscalar septa (black arrow), but the modiolus is completely absent. Published by Cochlear Implants International.

High-resolution CT scanning of the temporal bone should be obtained to assist in diagnosis and can be particularly useful in classification of the cochlear malformation. Assessing the width of the internal auditory canal and course of the facial nerve is easily performed using high-resolution CT scanning.

MRI is useful in assessing soft tissues of the temporal region, specifically in determining the presence or absence of the eighth cranial nerve in the internal auditory canal.

Because of the root cause of the sensorineural hearing loss in patients with Mondini deformity, surgical therapy is the only treatment consideration.

Cochlear implantation has been described as an effective tool for treating sensorineural hearing loss due to incomplete partitioning.[4, 5]

Prior to surgery, imaging (specifically MRI) of the affected temporal bone is important to ascertain the presence of sufficient cochlear lumen for electrode placement and to assess for hypoplasia/aplasia of the eighth cranial nerve.[21]

Another concern for surgery is whether there is any aberrancy in the course of the facial nerve. Sixteen percent of individuals with inner-ear malformations were found to have an aberrant facial nerve.[20] “Gushers,” or oozing of cerebrospinal fluid (CSF) due to bony abnormalities found in the cribriform plate separating the internal auditory canal and the cochlea, have also been described in patients with incomplete partitioning. Oozing CSF more seems to be more common in patients with Mondini deformity.[22]

As for electrode selection for patients with Mondini deformity, because the basal turn is normal and research has shown that the majority of spiral ganglion cells reside in the basal turn, all types of electrodes should theoretically provide sufficient stimulation.[22]

Chen et al found that following cochlear implant surgery in 545 children (aged 7-36 mo) with severe to profound, prelingual hearing loss, auditory skills in the 31 patients with Mondini dysplasia developed similarly to those in the 514 patients with radiologically normal inner ears. No significant difference was found at 12-, 24-, and 36-month follow-up in the mean auditory skill scores for both groups.[23]

Patients diagnosed with Mondini deformity should consult with a neurootologist to ascertain whether cochlear implantation is warranted. Additionally, an audiologist and speech/language pathologist should be consulted.

Oozing of CSF due to the abnormal bony architecture of the inner ear has been reported among patients with Mondini deformity who have undergone cochlear implantation. Because these patients now have an inner ear in which CSF is exposed to perilymph, meningitis is an increased risk, so these patients should receive routine vaccinations.[22]