Inner Ear Malformations

Definition and epidemiology

Inner ear malformations are congenital structural anomalies caused by arrested or aberrant development of the membranous and bony labyrinth during embryonic life. About one in five children with congenital sensorineural hearing loss have an inner ear malformation on imaging (Jackler et al., 1987; Sennaroğlu and Bajin, 2017). The embryogenesis-based classification of Jackler et al. (1987) was used for many years, but as cochlear implantation grew, a more detailed classification that could guide surgical decisions was needed.

The most widely used classification today was developed by the Turkish otologist Levent Sennaroğlu of Hacettepe University. It first distinguished two types of incomplete partition (IP-I and IP-II) (Sennaroglu and Saatci, 2002) and, in its current form, divides malformations into eight main groups: complete labyrinthine aplasia (Michel deformity), rudimentary otocyst, cochlear aplasia, common cavity, cochlear hypoplasia, incomplete partition, enlarged vestibular aqueduct and cochlear aperture abnormalities. Cochlear hypoplasia is divided into four types and incomplete partition into three (Sennaroğlu and Bajin, 2017).

The classic term Mondini deformity now describes the combination of IP-II with an enlarged vestibular aqueduct and a dilated vestibule. Using the term indiscriminately for any cochlear anomaly is discouraged, because it hides important surgical and prognostic differences between types (Sennaroğlu and Bajin, 2017).

Symptoms and signs

The main finding is congenital hearing loss. It is usually bilateral but can be unilateral, and ranges from mild to profound. Severe malformations such as Michel deformity, cochlear aplasia and common cavity usually cause profound loss. Hearing loss in IP-II and enlarged vestibular aqueduct may start milder, fluctuate and worsen after head injury. In types such as IP-III and enlarged vestibular aqueduct, a mixed loss with a low-frequency air-bone gap may be seen; the gap arises from a ‘third window’ effect in the inner ear rather than from middle ear disease.

When the semicircular canals and vestibule are involved, vestibular function may be impaired, and these children may be late to sit and walk. In some types the bony barrier between the cochlea and the internal auditory canal or middle ear is deficient. This can allow cerebrospinal fluid (CSF) into the middle ear, causing clear fluid leakage from the ear or nose and recurrent meningitis; in some children meningitis is the first sign of the malformation (Sennaroğlu and Bajin, 2017).

Causes and risk factors

Malformations are explained by developmental arrest at different stages; the earlier the arrest, the less the structure develops (Jackler et al., 1987). In Michel deformity the otic capsule has barely formed, whereas IP-II and enlarged vestibular aqueduct arise later. Sennaroglu and Saatci (2002) showed that IP-I and IP-II differ not only in severity but in structure: in IP-I the cochlea is cystic and lacks the modiolus and interscalar septa, while in IP-II only the apical part of the modiolus and the apical septum are missing.

The genetic basis of some malformations is known. IP-III is X-linked and associated with variants in the POU3F4 gene, so it mainly affects boys (Sennaroğlu and Bajin, 2017). IP-II and enlarged vestibular aqueduct often occur with Pendred syndrome, linked to the SLC26A4 gene. Semicircular canal aplasia or hypoplasia is a typical finding in CHARGE syndrome. In many non-syndromic cases no cause is found.

Audiological and clinical assessment

The diagnosis is made by imaging; audiological assessment establishes hearing levels, auditory nerve function and the rehabilitation option. High-resolution temporal bone computed tomography (CT) shows the bony labyrinth, the width of the internal auditory canal and cochlear aperture, and the course of the facial nerve. MRI shows the membranous labyrinth and the nerves in the internal auditory canal, especially whether the cochlear nerve is present and how thick it is. An absent cochlear nerve can remove the benefit of a cochlear implant; cochlear nerve anomalies are common on the affected side in children with unilateral loss (Pollaers et al., 2020).

• ABR and OAEs: the key tests of threshold and auditory nerve function. With cochlear nerve deficiency OAEs may be present while the ABR is absent; this pattern can be confused with ANSD and shows why imaging matters.
• Behavioural audiometry: ear-specific thresholds are obtained with visual reinforcement or conditioned play audiometry as appropriate for age; hearing aid fitting is based on these thresholds.
• Tympanometry and acoustic reflexes: needed to exclude middle ear disease in mixed loss. If a child with a low-frequency air-bone gap has a normal tympanogram and present acoustic reflexes, a third window effect in the inner ear is likely, and unnecessary middle ear surgery should be avoided.
• Vestibular assessment: in a child with delayed motor development, vestibular function can be assessed with the video head impulse test (vHIT) and VEMPs.

The differential diagnosis includes hereditary non-syndromic hearing loss, congenital CMV and ANSD. The doctor decides which imaging is done and when; for cochlear implant candidacy, CT and MRI are often both requested.

Treatment and audiological rehabilitation

A hearing aid is the first option for mild to severe loss. Children who gain too little benefit and have a cochlear nerve receive a cochlear implant. In Papsin’s (2005) series of 103 children, speech perception outcomes in children with inner ear anomalies did not differ significantly from those with normal anatomy; only those with a narrow internal auditory canal or narrow cochlear canal did worse. In the same series, children with common cavity and cochlear hypoplasia had a narrower dynamic range and were harder to programme.

The type of malformation determines the surgical approach and electrode choice (Sennaroğlu and Bajin, 2017). In IP-II, surgery is usually similar to standard cochlear implantation and outcomes are generally good. In IP-I, common cavity and especially IP-III, the bony barrier between the cochlea and the internal auditory canal is deficient, so a profuse CSF leak (gusher) is expected when the cochleostomy or round window is opened; electrodes with a stopper that helps control the leak are preferred, and perimodiolar electrodes are avoided so that the electrode does not enter the internal auditory canal. In cochlear hypoplasia the small cochlea may need shorter, thinner electrodes. In severe malformations an abnormal course of the facial nerve can complicate surgery. Choice of electrode and surgical technique rests with the surgeon.

In IP-III there is no bony barrier between the internal auditory canal and the cochlea, so inner ear fluid is exposed to CSF pressure. If a mixed loss is mistaken for otosclerosis and stapes surgery is performed, a profuse CSF leak and permanent hearing loss can follow, so imaging before stapes surgery is important (Sennaroğlu and Bajin, 2017).

When there is no cochlea or cochlear nerve, a cochlear implant cannot provide hearing and an auditory brainstem implant (ABI) is considered. The main indications for ABI are Michel deformity, cochlear aplasia, cochlear nerve aplasia and cochlear aperture aplasia; in rudimentary otocyst, a hypoplastic cochlear nerve and some cases of common cavity and cochlear hypoplasia, the decision is made individually (Sennaroğlu and Bajin, 2017). In malformations with a risk of meningitis, vaccination and, where a CSF fistula is found, surgical repair are planned by the doctor.

Impact on quality of life and advice

The type of malformation shapes expectations for a child’s auditory and language development. Cochlear implant outcomes are generally good in types such as IP-II and enlarged vestibular aqueduct, whereas progress may be slower and more variable in severe malformations and in children using an ABI. Families should therefore be given realistic expectations, auditory-verbal therapy should start early and development should be monitored regularly.

Children with impaired vestibular function may reach motor milestones late; physiotherapy and play-based balance activities can help. Because head injury can worsen hearing in enlarged vestibular aqueduct and IP-II, care with contact sports is advised. In malformations carrying a risk of meningitis, families should be told to seek help promptly for symptoms such as fever and headache.

Complete your child’s vaccinations with your doctor, make sure the hearing aid or implant is worn all day, and keep follow-up appointments. If hearing drops suddenly or dizziness appears, contact your audiologist and doctor.