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Imaging in congenital inner ear malformations—An algorithmic approach

Malformations of the inner ear are an important cause of congenital deaf‑mutism. Arrest in embryologic development of inner ear during various stages gives rise to the variety of malformations encountered. Current treatment options include hearing aids, cochlear implants, and auditory brainstem implants (ABI). With the advent of cochlear implant surgery and ABI, decent functional outcomes can be obtained provided such cases are diagnosed correctly and timely. To that end, high‑resolution computed tomography (HRCT) has a fundamental role in the assessment of these conditions, ably supplemented by magnetic resonance imaging (MRI). The purpose of this pictorial essay is to illustrate the imaging features of inner ear anomalies in children with congenital deaf‑mutism as per the latest terminology and classification and provide an algorithmic approach for their diagnosis.


Introduction
The embryologic development of inner ear occurs in a predefined sequence starting in the 3 rd week of intrauterine life. Depending upon the timing of insult in utero, anatomically different anomalies may manifest. However, only a minority (20-30%) will show radiologic abnormality on imaging. [1] This has been attributed to changes occurring at the cellular or microscopic levels which are beyond the resolving power of current imaging techniques. These children usually present with congenital deaf-mutism in the first few years of life. Often these children can have systemic abnormalities in association with inner ear malformations; hence a complete physical examination must be performed.
Currently, three treatments are available for hearing rehabilitation in such patients: Hearing aids, cochlear implants, and auditory brainstem implants (ABI). Cochlear implantation has been proven to be clinically effective and cost-effective in children with severe to profound bilateral sensorineural hearing loss. [2] Due to the widespread adoption of cochlear implantation, high-resolution computed tomography (HRCT) temporal bone and magnetic resonance imaging (MRI) have become invaluable in the imaging evaluation and characterization of inner ear anomalies. HRCT provides an excellent depiction of the bony labyrinth, osseous details as well as variant anatomy, whereas MRI demonstrates membranous labyrinth, vestibulocochlear nerve and brain pathology to good effect. [3] These findings can aid in the decision between cochlear implant and ABI. The choice between these two modalities for preoperative imaging varies among institutions, with some relying primarily on either CT or MR imaging whilst others preferring to use both these imaging modalities in a complementary fashion. [4,5] If a cochlear implant is planned, imaging findings impact the surgical approach and also the type of electrode used. [6] This review article describes the spectrum of inner ear anomalies one is likely to encounter on imaging of children with congenital deaf-mutism. The terminology used draws upon from the latest classification system of Sennaroglu. [7] Additionally, it provides an algorithmic approach for the classification of the same.

Normal Anatomy of Inner Ear
The inner ear is divided into the bony and membranous labyrinth. The bony labyrinth is well-demonstrated by HRCT and consists of vestibule, cochlea, semicircular canals, vestibular aqueduct, and cochlear aqueduct [ Figures 1 and 2].
The membranous labyrinth is housed within the bony labyrinth with intervening space bathed in the perilymph. It consists of utricle and saccule, cochlear duct, semicircular ducts, and endolymphatic duct and sac. The fluid contained within is called endolymph. MRI is useful for demonstrating the membranous labyrinth.

Internal auditory canal (IAC)
The IAC is a bony canal 2-8 mm wide that courses through the petrous bone between the cerebellopontine angle and labyrinth and transmits the facial and vestibulocochlear nerves. The cochlea lies anterior and vestibule posterior to it. The vestibulocochlear nerve divides into three branches within the IAC: Cochlear nerve, superior vestibular nerve, and inferior vestibular nerve. The IAC is divided into four compartments by a horizontal falciform crest and an incomplete vertical Bill's bar, the latter is occasionally visualized [ Figure 3]. The facial nerve and cochlear nerve occupy the anterosuperior and anteroinferior compartments, respectively. The superior and inferior vestibular nerves are housed in posterosuperior and posteroinferior compartments, respectively.

Cochlea
Anterior to the IAC is cochlea, which is a spiral canal that winds 2½ to 2¾ times around the central bony modiolus. On HRCT, the mid-modiolar view demonstrates the modiolus as a central osseous density, interscalar septa which divide the cochlea into the basal, middle, and apical turns and cochlear aperture transmitting cochlear nerve from IAC to the cochlea. Section through the round window niche, which lies just inferior to the mid-modiolar section, demonstrates the basal, middle and apical turns separately [ Figure 2]. Besides, Stenvers projection which is obtained by taking sections parallel to the long axis of petrous bone can also show normal cochlear turns to good effect. The cochlear lumen is divided by osseous spiral lamina into superior scala vestibuli and inferior scala tympani.
Cochlear aqueduct is a bony canal that is filled with loose connective tissue and connects the scala tympani with subarachnoid space adjacent to pars nervosa of jugular foramen [ Figures 1d and 4]. It runs parallel and inferior to the IAC with a wider medial opening which tapers off laterally as it courses into the petrous bone.

Vestibule and semicircular canals
The vestibule lies posterior to the IAC and joins with the cochlea, semicircular canals, and vestibular aqueduct. It houses the utricle and saccule. The vestibular aqueduct is a bony canal containing endolymphatic duct which connects the utricle and saccule with endolymphatic sac in the epidural space of posterior cranial fossa [ Figures 1b and 4]. It courses postero-lateral and parallel to the posterior semicircular canal. Its normal diameter is 1.5 mm or less and is nearly the same as that of the posterior semicircular canal. The three semicircular canals (superior, lateral, and posterior ) house the semicircular ducts which join the vestibule. The posterior part of the superior semicircular canal joins with the upper part of the posterior semicircular canal to form the crus commune.

CT technique
HRCT of the temporal bone using an MDCT scanner (64 slices or more) provides good anatomic details for the evaluation of inner ear.
• Scan plane: Axial scans through the temporal bone are acquired (512 × 512 matrix) from the top of the petrous apex to the inferior tip of the mastoid bone • Slice thickness: The raw axial image dataset is reconstructed with a section thickness of 0.6 mm. Coronal images are reconstructed from the anterior margin of the petrous apex to the posterior margin of the mastoid. Sagittal images can be reformatted whenever required • Windowing and centering: Images are displayed at window centering level of 700 HU and a window width of 4000 HU. 3D surface reconstruction and volume-rendered images can also be obtained.

MR technique
3T MR imaging system provides excellent images for inner-ear examinations.  Figure 6C] • Imaging for the brain: The examination must be supplemented by routine imaging of the brain in all patients to exclude lesions of the central nervous system responsible for hearing loss; contrast may be administered when deemed necessary.

Imaging Spectrum
According to current classification, inner ear malformations are divided into eight types. Each group of malformation presents with similar clinical findings and has similar treatment options and prognostic implications.

Complete labyrinthine aplasia
Also known as Michel aplasia, it is the most severe end of the spectrum resulting from developmental arrest during the 3 rd week of gestation. [8] Imaging findings • Complete absence of inner ear structures [ Figure 7] • Can be unilateral or bilateral. When unilateral, the contralateral inner ear is often malformed [9] • The petrous bone and otic capsule may be hypoplastic or aplastic. [10]

Treatment
Since there is no inner ear, a cochlear implant is contraindicated and ABI is the only available treatment.

Rudimentary otocyst
A rudimentary otocyst represents an incomplete millimetric otic capsule (round or ovoid in shape) with the absence of internal auditory canal. [7] Like complete labyrinthine aplasia, ABI is the only treatment option.

Cochlear aplasia Imaging findings
• There is complete agenesis of the cochlea, with or without the affliction of other inner ear structures [ Figure 8]. Two subgroups are described: Cochlear aplasia with a normal labyrinth and cochlear aplasia with a dilated vestibule (CADV) • The labyrinthine segment of facial nerve shows anterior displacement occupying the site where normal cochlea should have been.

Imaging differential
Labyrinthitis ossificans: Absence of bulge of the cochlear promontory in the medial wall of middle ear differentiates cochlear aplasia from labyrinthitis ossificans. [9]

Treatment
The absence of cochlea precludes a cochlear implant, ABI being the sole treatment option. Common cavity Imaging findings • Cochlea and vestibule are not identified as separate structures but are fused together to form a single cavity [ Figure 9] • Associated malformations of semicircular canals are common.

Imaging differential
Cochlear aplasia with dilated vestibule (CADV): In the common cavity, the internal auditory canal enters at its center whereas the vestibule is positioned postero-lateral to the fundus of the internal auditory canal in CADV. While a cochlear implant can be attempted in the common cavity, it is contraindicated in CADV. However, it may be difficult to differentiate between these two entities. [7] Treatment Cochlear implant is the primary treatment option; ABI is attempted in case of neural deficiency or poor response to cochlear implant.

Imaging findings
• Cochlea and vestibule can be differentiated but cochlear external dimensions are less than normal with internal architecture often not made out [7] • It may appear bud-like or cystic hypoplastic with absent or shortened modiolus and interscalar septa. The number of cochlear turns may be reduced [ Figure 10] • Four subtypes are described based on cochlear morphology [7] • CH-I: cochlea appears small bud-like • CH-II: cochlea appears cystic with small dimensions; however, external outline is normal • CH-III: cochlea has reduced number of turns (<2) with small dimensions; however, external outline is normal • CH-IV: cochlea has a normal basal turn; however, middle and apical turns are hypoplastic.

Imaging differential
Incomplete partition deformity: The cochlear dimensions are normal in the incomplete partition deformity as opposed to reduced dimensions in cochlear hypoplasia.

Treatment
It varies from stapedotomy, hearing aids and cochlear implants. If the cochlear nerve is deficient ABI is the only available option.

Incomplete partition (IP)
In this group, cochlea and vestibule are seen as distinct structures but show variable internal architecture deformities. Cochlear size is normal, unlike cochlear hypoplasia. It is subdivided into three groups: IP-I, IP-II, and IP-III.

Imaging findings
A. Incomplete partition-I [ Figure 11] • Previously called cystic cochleovestibular malformation, cochlea appears cystic with the absence of modiolus and internal architecture but seen separate from the vestibule The vestibule is often enlarged and dysplastic, however, the vestibular aqueduct is usually not dilated. The internal auditory canal is usually enlarged • The cochlear nerve may or may not be present • These patients are at an increased risk of meningitis and CSF gusher during cochlear implant surgery [7,11] • It may be associated with syndromes such as Klippel Feil syndrome [ Figure 12]. B. Incomplete partition-II [ Figure 13] • The cochlear apex assumes a cystic appearance due to a defect in the apical part of modiolus and interscalar septa leading to confluent middle and apical turns. Basal turn is normally developed • The term Mondini deformity is used when this deformity is present with a mildly dilated vestibule and an enlarged vestibular aqueduct • The cochlear nerve is always present. C. Incomplete partition-III [ Figure 14] • Associated with X-linked deafness, it is the rarest show an overall small cochlea with less than two turns (black arrows) suggesting cochlear hypoplasia type III. Stenosis of IAC is also seen (white arrow). Cochlear hypoplasia type II (E) reveals an overall small cochlea with cystic appearance (square arrow)

A B
C D E of the partition anomalies of cochlea in which the interscalar septa are present; however, modiolus is completely absent • The external dimensions of the cochlea are more or less normal. There is a bulbous enlargement of internal auditory canal with defective lamina cribrosa • The cochlear nerve is always present.

Imaging differential
Cochlear hypoplasia: It can be differentiated from incomplete partition abnormality on the basis of reduced cochlear dimensions in cochlear hypoplasia. [7] Treatment It varies between hearing aids and cochlear implants. ABI is performed in cases of poor response to a cochlear implant or nerve deficiency (IP-I).

Imaging findings
• The vestibular aqueduct is said to be enlarged if its width exceeds 1.5 mm at the midpoint between posterior labyrinth and operculum [ Figure 15] or when it exceeds that of the adjacent posterior semicircular canal [12] • It can be bilateral in upto 90% of patients [13] • The cochlea, vestibule, and semicircular canals are normal • Pöschl projection can be used to demonstrate the entire course of the vestibular aqueduct on a single section which is not possible on axial sections due to its oblique orientation [ Figure 16]. It is a 45° oblique reformat perpendicular to the long axis of the petrous bone. More commonly used for the evaluation of superior semicircular canal dehiscence, it is also useful for the assessment of vestibular aqueduct particularly in cases of borderline enlargement. [12,14]

Treatment
The severity of hearing loss dictates management, hearing aids being used for mild to moderate hearing loss and cochlear implants for severe cases.

Abnormalities of cochlear aperture Imaging findings
• Cochlear aperture transmits the cochlear nerve from the internal auditory canal to cochlea. Hypoplasia of the cochlear aperture is defined by width less than 1.4 mm and aplasia is characterized by replacement of cochlear aperture with bone [7] • Concomitant narrowing of the internal auditory canal may also be present [ Figure 17] • The cochlear nerve may be hypoplastic or absent.
Cohlear nerve hypoplasia is diagnosed if its size is less than ipsilateral normal facial nerve or contralateral normal cochlear nerve. Cochlear nerve hypoplasia or aplasia may occur without abnormality of cochlea or cochlear aperture.

Treatment
Hypoplastic cochlear aperture with cochlear nerve hypoplasia is initially treated by hearing aid followed by cochlear implant in case of a suboptimal response. ABI is the only possible treatment in cases of cochlear nerve aplasia.

Abnormalities of semicircular canals
Malformations of semicircular canals may be seen in isolation or in combination with cochlear and vestibular anomalies.
• They can be absent, hypoplastic, or dilated • Any of the three canals may be affected, and it is not uncommon for them to be affected in combination. The most frequently involved is the lateral semicircular canal (as it is the last to develop embryologically) wherein the lateral canal is assimilated within a dilated dysplastic vestibule, the so-called vestibule-lateral semicircular canal dysplasia [ Figure 18].

Abnormalities of IAC
IAC abnormalities may occur as part of other inner ear malformations or in isolation. Normal diameter is stated to be between 2 and 8 mm.
• Stenosis is defined as an IAC diameter of less than 2 mm. It is frequently associated with cochlear nerve deficiency or absence • Dilated IAC has been found to be associated with IP-I malformation but may also be seen in isolation • Duplication of IAC [ Figure 19] is a relatively uncommon condition in which two bony canals are seen within the petrous bone on HRCT of which one continues as the facial nerve canal and the other terminates in cochlea and vestibule. [15,16] An algorithmic approach to classification and diagnosis of inner ear anomalies is presented in Figure 20.

Conclusion
HRCT and MR imaging have evolved as indispensable imaging modalities for the assessment of congenitally deaf-mute children. Inner ear malformations, when present, can be classified using this algorithmic approach as described above. It is imperative to use standard and correct terminology while describing these malformations because it can have a significant impact on the management and prognosis of these children. Sound knowledge of inner ear anatomy, appropriate use of classification terminology and

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Conflicts of interest
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