15 Inner Ear Malformations and Implantation Implantation in patients with a malformation of the inner ear or cochlear nerve abnormality is challenging. Generally speaking, the more severe and less common the malformation the more challenging the case is. In addition, the outcome becomes less predictable and the risks of the surgery increase. In rare but severe malformations, the discussion is ongoing about how to rehabilitate: with a cochlear implant (CI) or auditory brainstem implant (ABI)? Some knowledge of the embryology is needed to understand the classification of the malformations, but more important is knowledge of the radiologic differences. The inner ear develops in the third to the tenth week of gestation. In the third week, two otocysts, on either side of the hindbrain, subdivide in two pouches. The ventral portion develops into the cochlear duct and the saccule. The dorsal segment is transformed into the endolymphatic sac and duct, the utricle, and the semicircular canals. By the tenth week the morphology of the adult inner ear can already be fully recognized. Ossification of the cartilage capsule of the cochlea and labyrinth begins in week 14 and is completed in week 23.1 Congenital inner ear malformations can be variable, and are often classified according to a suggested arrest during the development of the membranous and bony labyrinth.2–5 Cochleovestibular malformations are related to malformations of the bony internal auditory canal and/or the cochlear nerve, but also with isolated cases of cochlear nerve hypoplasia or aplasia, a narrowed internal auditory canal (IAC), or an isolated malformation of the vestibular part of the inner ear. Rare malformations, as in CHARGE syndrome or X-linked incomplete partition type 3, do not match the “arrest in development” classification, but they cannot be left out of this overview. The classification is a tool to define different groups of pathology, but in reality there seems to be gradual transition between the different groups. Classification of Malformations of the Labyrinth, the IAC, and the Cochlear Nerves Malformation of the membranous labyrinth • Complete: membranous labyrinthine dysplasia • Partial: cochleosaccular dysplasia, cochlear basal turn dysplasia Malformations of the membranous and osseous labyrinth • Incomplete partition type 2 (including enlarged vestibular aqueduct [EVA]) • Large vestibular aqueduct syndrome (LVAS) • Cochlear (vestibular) hypoplasia • Incomplete partition type 1 (cystic cochleovestibular anomaly) • Common cavity • Cochlear aplasia • Cochleovestibular aplasia (Michel deformity) • Incomplete partition type 3 (X-linked) • Labyrinthine dysplasia (isolated) • CHARGE syndrome–associated malformations Malformations of the cochlear nerve and inner ear canal • Cochlear nerve hypoplasia or deficiency • Cochlear nerve aplasia • Inner ear canal hypoplasia In this chapter we discuss and present the auditory rehabilitation options in malformations of the cochleovestibular structures, the IAC, and the cochlear nerve. • Aplasia: complete lack of development • Hypoplasia: incomplete development • Dysplasia: aberrant development The labyrinth refers to the complete inner ear, consisting of the cochlea and the vestibular system. In the era of cochlear implantation patients with isolated membranous malformations will often be implanted even though the patient and the surgeon are not familiar with this entity because it is not visible on imaging. There are no key features in the radiologic work-up and the surgical approach is not different from that in standard cases. Perhaps in the future higher-resolution imaging will show the membranous dysplasias. Complete membranous labyrinthine dysplasia is extremely rare and has been described with Jervell–Lange-Nielsen syndrome and Usher syndrome.6 There are two forms of partial membranous labyrinthine dysplasia: (1) cochleosaccular dysplasia in which the organ of Corti is partially or completely missing,7 and (2) cochlear basal turn dysplasia, in which the dysplasia is limited to the basal turn of the cochlea. The latter may be related to familial high-frequency sensorineural hearing loss (SNHL) in patients who would be candidates for electroacoustic stimulation, which is addressed in Chapter 9. (See Table 15.1.) In the Jackler classification the incomplete partition was divided into severe and mild types and enlarged vestibular aqueduct (EVA) was not strictly associated with either type of malformation. In the newer classification of Sennaroglu and Saatci the incomplete partition type 2 (IP-2) is strictly associated with an EVA while IP-1 is not. Furthermore, the distinction between mild and severe cochlear hypoplasia is abandoned, and a more clearly defined new entity, the incomplete partition type 1 (IP-1), is proposed.2,4 Subtypes of a certain malformation can also be classified8 and will be mentioned in the following overview. This is the most common malformation: it is thought of as an arrest in development in the seventh week of gestation. IP-2 consists of three components: a cystic cochlear apex with a normal basal turn, a dilated vestibule, and an enlarged vestibular aqueduct. The cochlea consists of 1.5 turns, in which the middle and apical turns unite to form a cystic apex. There is no interscalar septum between the middle and apical turns. Combined with a dilated vestibule and an enlarged vestibular aqueduct, this is known as the classical Mondini deformity,9,10 but is better referred to nowadays as incomplete partition type 2. EVA is defined as an enlargement of the aqueduct exceeding the diameter of the posterior semicircular canal on an axial image in which both structures are shown.5 Other definitions are those of Valvassori,11 Phelps,12 and Zheng,13 and the most often used definition is the measurement of the vestibular aqueduct at its broadest diameter in the middle of the long axis of the aqueduct, exceeding 1.5 mm (Fig. 15.1 and Fig. 15.2). Fig. 15.1 An enlarged vestibular aqueduct seen on CT in an axial view. Fig. 15.2 In this case it is obvious that there is an enlarged vestibular aqueduct, but it is a good example to explain how to measure the mid duct diameter: (a) Create a “virtual” line (black line) along the posterior cranial fossa dura, the duct cannot be seen/measured beyond this point. (b) Create in the middle of the duct a line between the most anteromedial point and the posterolateral point (yellow arrow). (c) The middle of this line (red cross mark) is the best position to measure the diameter of the vestibular aqueduct in millimeters: in this case between the two red arrows.
15.1 Introduction
15.2 Embryology
15.3 Classification of Malformations
To define the malformations we use the terms:
15.3.1 Malformation of the Membranous Labyrinth
Complete Membranous Labyrinthine Dysplasia
Partial Membranous Labyrinthine Dysplasia
15.3.2 Malformations of the Membranous and Osseous Labyrinth
Incomplete Partition Type 2 (Including LVAS)
Classification
Incomplete partition type 2 (IP-2)
Enlarged vestibular aqueduct (EVA)
Jackler 19872 | Sennaroglu 20024 |
Incomplete partition (mild)/Mondini deformity | Incomplete partition type 2 (IP-2) |
Cochlear hypoplasia (mild) |
|
Incomplete partition (severe) | Cochlear hypoplasia |
Cochlear hypoplasia (severe) | Incomplete partition type I (IP-1) |
Common cavity | Common cavity |
Cochlear aplasia | Cochlear aplasia |
Michel deformity | Michel deformity |
Fig. 15.3 An enlarged vestibular aqueduct (EVA) shown on T2W MRI (CISS). An enlarged vestibule (red arrow) together with the EVA and cochlear cystic apex (see next image) make the classic incomplete partition type 2 (as described by Mondini in 1791).
Fig. 15.4 Incomplete separation of the cochlear turns; the middle and apical turns form a cystic structure without an interscalar septum between the middle and apical turns (the normal location of this septum would have been at the red arrow).
Fig. 15.5 An incomplete partition of the cochlea in a coronal view: the middle and apical turns form a cystic structure. The insert shows the normal aspect of cochlear partition.
Large vestibular aqueduct syndrome (LVAS)
The enlarged vestibular aqueduct is also suggested to occur as a solitary malformation, without a concurrent cochlear dysplasia.14 This would then be a patient with SNHL and an EVA, which is referred to as the large vestibular aqueduct syndrome. It remains doubtful whether this is a separate entity as more recent research has shown that the modiolus is abnormal in almost all cases with an EVA.15 Most likely there is a gradual dysplasia of the apical modiolus and interscalar septation in association with an EVA.
Clinical Features
The classical presentation is progressive or fluctuating sensori-neural hearing loss, beginning in childhood. Sudden deafness can be present after minor head trauma, but is not present in all cases with IP-2.16
Incomplete partition type 2 is associated with Pendred syndrome: all patients with Pendred syndrome have an IP-2.
IP-2, or at least the EVA, can also be associated with BOR (branchio-oto-renal) syndrome, Waardenburg syndrome, and several other syndromes.17 Patients with IP-2 and hearing loss, without a proven genetic cause, are considered to have the LVAS, and thus an EVA and hearing loss.
(See Fig. 15.1, Fig. 15.3–Fig. 15.5.)
Cochlear (Cochleovestibular) Hypoplasia
It is thought that this is due to an arrest in development in the sixth week of gestation.
Classification
In cochlear (cochleovestibular) hypoplasia the cochlea and vestibule and vestibular structures are separated but less developed than in IP-2. All structures seem smaller (hypoplastic) than in a normal labyrinth. The cochlea is less than 4 mm in height or has fewer than 2.5 turns. The intracochlear organization can be normal or dysplastic. No EVA is seen in these malformations, the IAC is normal or has a hypoplastic diameter, and the semicircular system is hypoplastic or aplastic.18 Hypoplastic cochleae are sometimes subclassified as type I, budlike; type II, cystic hypoplasia; and type III, cochlea with fewer than 2 turns.
Clinical Features
These patients mostly present with a congenital severe to profound sensorineural hearing loss.
(See Fig. 15.6–Fig. 15.10.)
Fig. 15.6 In this CT scan of a child with a cochlear hypoplasia, the apical and middle turns are confluent and the basal turn is visible.
Fig. 15.7 The MRI CISS sequence shows a clear partition of the basal turn, which makes the difference from an IP-1.
Fig. 15.8 The CT images show the cochlear apical malformation as a rounded lumen instead of a more flattened middle and apical turn. This example would still be classified as type III: “less than two turns” cochlear hypoplasia.
Fig. 15.9a, b In the MRI CISS (a) and the CT image (b), the labyrinth malformation is seen with a small bony island within the dysmorphic small lateral semicircular canal. This type of labyrinth is not specific for cochlear hypoplasia and could exist in IP-1 or in cochlear hypoplasia.
Fig. 15.10a–d Schematic representation of the normal cochlea (a), IP type 1 (b), IP type 2 (c), and cochlear hypoplasia (d). The differences between IP-1, IP-2, and cochlear hypoplasia are the size and the inner structures (modiolus and septi) of the cochlea. In an IP-1 the cochlea is cystic and no interscalar septi are visible. Also the modiolus is absent. In a cochlear (vestibular) hypoplasia and IP-2 the cochlea is smaller but still has interscalar septi and a modiolus is present. The best way to distinguish the differences between these entities is on MRI.
Incomplete Partition Type 1 (Cystic Cochleovestibular Anomaly)
It is thought that this is due to an arrest in development in the fifth week of gestation.
Classification
In this malformation (IP-1) a cystic cochlea is connected to a cystic vestibule. The cochlea does not have interscalar septi and the modiolus appears to be completely absent. In fact it is an unpartitioned cochlea despite the name given to this type of malformation. The cochlea looks like an empty cystic space. This is accompanied by grossly dilated vestibule. Separate structures of the cochlea and the vestibule are still recognizable (this in contrast to the common cavity malformation). The lateral canal is mostly cystic, while the other semicircular canals can also be malformed. The cribriform area between the cochlea and the IAC can be open, with a direct connection of the empty vestibule to the cerebrospinal fluid in the IAC.
(See Fig. 15.11–Fig. 15.13.)
Common Cavity
The common cavity malformation is thought of as an arrest in development in the fourth week of gestation.
Classification
In a common cavity there is one single cavity, representing cochlea and vestibule, without any differentiation between cochlear and vestibular structures. If the cavity is located at the posterior side of the IAC there is no distinction between common cavity and a cochlear aplasia. It seems that all common cavities have an abnormal diameter of the IAC, directly correlated with the size of the common cavity.4,19,20 Furthermore, the cavity has an open connection with the IAC, but a membranous separation may be apparent. A bulging stapes footplate can also be associated with a common cavity.21
Fig. 15.11 The cystic cochleovestibular anomaly of the labyrinth can be distinguished as two separated but dysplastic structures (this image and Fig. 15.12). It differs from a common cavity malformation, which consists of only one cystic structure.
Fig. 15.12 If the cystic cochlea contains some form of partition it is a cochlear hypoplasia, but if it is unstructured, without intracochlear separation or modiolus, it is an IP-1. The best way of analyzing the partition and the scalar organization is on MRI. (See Fig. 15.13, MRI of the same cochlea).
Fig. 15.13 In the cochlea of an IP-1 case there is no partition, only a cystic cavity This can best be seen on MRI (See Fig. 15.10b.)
Fig. 15.14 Sometimes it is hard to distinguish a cochlear aplasia from a common cavity. On this MRI image of another patient, the IAC is not very clear and either a cochlear aplasia or a common cavity can be seen Only the connection and the size of the internal auditory canal will divide the cochlear aplasia case from a common cavity case. There is no “gray area” in this definition.
It is important in these cases to look for the neural structures connecting to the cavity on MR imaging. This will provide essential information in case of candidacy for a CI. If the neural connection is not clearly visible on MRI, a connection can still be possible.
(See Fig. 15.14 and Table 15.2.)
Table 15.2 Classification of inner ear anomalies and their definitions
Structure | Malformation | Definition |
Cochlea | Michel deformity | Complete absence of all cochlear and vestibular structures |
| Cochlear aplasia | Complete absence of the cochlea |
| Common cavity deformity | Cystic cavity representing the cochlea and vestibule without showing any differentiation into cochlea and vestibule |
| IP-1 | Cochlea of normal size but cystic appearance with no internal structures identified |
| Cochlear hypoplasia | Small cochlea with either intact or incomplete partition. |
| IP-2 | Cochlea of normal size is shortened to 1.5 turns with an intact basal turn |
Vestibule | Dilated vestibule | Axial width is larger than 4.7 mm and coronal width is larger than 3.8 mm |
| Vestibular hypoplasia | Axial width is smaller than 3.6 mm and coronal width is smaller than 2.6 mm |
SCC | Isolated lateral SCC dysplasia | Bony island width (axial) is smaller than 2.7 mm and bony island width (coronal) is smaller than 2.3 mm or bony island of the LSC is fused to the vestibular wall on coronal section |
| Other SCC dysplasia | All SCC anomalies except for the above two deformities |
IAC | Narrow IAC | Simultaneous smaller measurements at the opening; IAC at the posterior cranial fossa (PCF) (axial) <4.7 mm, coronal opening <3.2 mm, and midpoint width coronal <3.2 mm |
| Shortened IAC | IAC length shorter than 7.1 mm |
Fundus | Opened fundus | Absence of bony septum between the IAC and cochlea or vestibule (cochlear aperture) |
| Enlarged vestibular aqueduct | The vestibular aqueduct orifice is wider than the diameter of PSC lumen on the axial slice, displaying both structures simultaneously; or the vestibular aqueduct is wider than 1.5 mm at midpoint |
Source: modified from Shim et al 200620 and Purcell 200322
Fig. 15.15 The first of five images of the same patient with a left-sided cochlear aplasia (Fig. 15.15–Fig. 15.19). In this axial CT image, at the position of the cochlea, there is no promontory, no oval window niche, and no cochlea visible. The first visible structure in the anterior labyrinthine bone is mostly the facial nerve or the internal auditory canal.
Fig. 15.16 In this axial image the IAC can be seen as a narrow hypoplastic canal in a more posterior direction. Anterior to the lateral end of the IAC no cochlear structure is visible. The incus and malleus seem to have normal dimensions but the course of the facial nerve is often aberrant.
Fig. 15.17 The lateral canal seen on this axial CT image is dysplastic with a tiny bony island.
Fig. 15.18 In this axial MRI image a slightly different axial angle (compared with the CT scan, Fig. 15.17) of the labyrinth is seen. There is no cochlea seen anterior to the internal auditory canal. It may be that the rudimentary cochlea is embedded within the complete malformation, which makes differentiation from a common cavity difficult.
Fig. 15.19 A coronal image showing a dysplastic lateral semicircular canal (LSC) and a dysplastic superior semicircular canal (SSC). In cochlear aplasia the vestibular part can be dysplastic or partly hypoplastic. The facial nerve is well visible on this image; the vestibulocochlear nerve is less visible on other images.
Cochlear Aplasia
Cochlear aplasia is thought as an arrest in development at the fourth week of gestation.
Classification
Cochlear aplasia is only slightly different from the common cavity, and one could debate whether it is a separate entity. Anterior to the lateral part of the IAC there is no structure that could be distinguished as a (rudimentary) cochlea. A vestibular structure is present, but is mostly a dysplastic vestibule or lateral semicircular canal.
One has to be sure that it is not a completely ossified cochlea, in which the bony area would be of normal dimensions and a promontory would be present. The promontory would be absent in a malformation and the labyrinthine part of the facial nerve would be at the position of the cochlea.
(See Fig. 15.15–Fig. 15.19.)
Fig. 15.20 Example of a cochleovestibular aplasia. Although there appears to be some spongy bone formation adjacent to the middle ear, the small internal auditory canal does not end in a “labyrinth-like” structure Note that the “ice cream cone” ossicles are normally shaped.
Fig. 15.21. MRI image of the same patient as in Fig. 15.21 There seems to be only one nerve in the IAC and the spongy bone formation does not incorporate any open fluid-filled structure.
Fig. 15.22 Another example of a cochleovestibular aplasia. The only structure recognizable at the location of the inner ear is the horizontal tympanic part of the facial nerve (red arrow).
Cochleovestibular Aplasia (Michel)
It is thought that this is due to an arrest in development before or at the third week of gestation.
Classification
There is complete absence of all cochlear and vestibular elements. It can be associated with absence of the IAC. A normal middle ear and ossicles can be present.
(See Fig. 15.20–Fig. 15.22.)
Incomplete Partition Type 3 (X-linked)
This malformation is also known as X-linked deformity.23,24
Classification
The most important aspect of the deformity is deficiency of bone between the lateral end of the bulbous IAC and basal turn of the cochlea.
Clinical Features
There is an association of X-linked mixed deafness with a stapes gusher (a profuse flow of cerebrospinal fluid if the stapes is disturbed). This distinct type of deformity differs between male and female subjects. Most predominantly a typical IP-3 malformation is seen in some severely deaf males.24 In females with IP-3 there is a slight hearing loss and the anomaly has a milder form.
The absence of the bony cochlear aperture would make the insertion of a multichannel electrode risky, as the deficient bone would almost certainly mean that the electrode would enter the IAC rather than staying in the cochlear coils.
Genetic studies have shown that there are at least two types of X-linked deafness,24,25 some of which have normal inner ears as shown by CT.
(See Fig. 15.23–Fig. 15.25.)
Labyrinthine Dysplasia (Isolated)
Classification
A true classification of this type of malformation does not exist. The simplest definition would be a malformed labyrinth with a radiologically normal cochlea. Measurements can be used to define a normal and a malformed labyrinth. Most prominent in commonly malformed is the lateral semicircular canal (LSC). (See Fig. 15.26.)
CHARGE Syndrome–Associated Malformations
Classification
CHARGE is an acronym for coloboma of the eye, heart defects, atresia of the nasal choanae, retardation of growth and/or development, genital and/or urinary abnormalities, and ear abnormalities and deafness. These features are no longer used in making a diagnosis of CHARGE syndrome, but the name persists. CHARGE syndrome is the leading cause of congenital deaf-blindness. Very few people with CHARGE will have all of its recognized features.
Genetically, ~60% of those tested have a mutation of the CHD7 gene.
Clinical Features
Hearing loss is mostly profound sensorineural, but conductive hearing loss, mixed hearing loss, or mild to moderate SNHL are also seen. Aside from deafness, the most common ear anomaly seen in CHARGE syndrome is the abnormal appearance of bowl-shaped and concave ears, known as “lop ears.” On imaging, substantial abnormalities of the inner ear including hypoplasia or aplasia of the semicircular canals and abnormalities of the cochlea and vestibule are seen. Abnormalities are frequent and multiple, mostly of the facial nerve, the venous system, the cochlear aperture, and the cochlear nerve.
Fig. 15.23 The cochlear aperture is open with a direct connection between the cochlea and the IAC. This increases the intralabyrinthine pressure and also the pressure on the ossicular chain resulting in mixed hearing loss. During implantation it is common to experience a CSF gusher.
Fig. 15.25 A CT scan of a patient with an X linked IP-3 malformation. The wide cochlear aperture makes a CSF gusher during implantation inevitable.
Fig. 15.26 In this patient with unilateral sensorineural hearing loss the difference between the left and right lateral semicircular canal (LSC) was obvious. In the insert one can appreciate the rotated left LSC, compared with the right LSC. The diameter of “the bony island” within the LSC (yellow arrows) is a good measure for defining malformed or not malformed.
Fig. 15.27 This and the following images (Fig. 15.28–Fig. 15.32) are of a girl with CHARGE syndrome. On this CT image of the right side, the hypoplastic LSC and the narrow cochlear aperture are visible. The cochlea itself appears to be normal in size.
The facial nerve is more commonly affected in children with CHARGE syndrome (and VACTERL association), displaced anteriorly adjacent to or within the round window niche.26 Also frequently described are venous displacement or malformations within the temporal bone.27 Cochlear aperture narrowing or atresia is seen on CT, but even with profound SNHL some have normal cochlear apertures. Absent or deficient cochlear nerves have been associated with CHARGE syndrome.28,29 Often the mastoid and middle ear is hypoplastic and also the round window can be hypoplastic or even aplastic.29
(See Fig. 15.27–Fig. 15.32.)
Fig. 15.29 The cochlear nerve is clearly visible in the internal auditory canal (IAC) on the right side.
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