Classification of Inner Ear Malformations



Fig. 6.1a
Complete labyrinthine aplasia with hypoplastic petrous bone



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Fig. 6.1b
Complete labyrinthine aplasia without otic capsule


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Fig. 6.1c
Complete labyrinthine aplasia with otic capsule


According to radiological findings [4], three different groups of CLA are present:



  • CLA with hypoplastic or aplastic petrous bone

    In these cases CLA is accompanied by hypoplasia or aplasia of the petrous bone. The middle ear may be adjacent to the posterior fossa (Fig. 6.1a).


  • CLA without otic capsule

    In this group of CLA, formation of the petrous bone is normal, but the otic capsule is hypoplastic or aplastic. According to Donaldson [6], the endosteum receives its vascular supply from the IAC, and the enchondral and outer periosteal layers get their vascular supply from the middle ear mucosa. This may be due to the abnormal vascular supply from the IAC and middle ear, resulting in the absence of all three layers of the otic capsule (Fig. 6.1b).


  • CLA with otic capsule

    Formation of the petrous bone and the otic capsule is normal. It can be speculated that vascular supply from the middle ear is normal as the otic capsule normally develops. The facial nerve canal can be seen (Fig. 6.1c). Only in this group of CLA with otic capsule development, the facial canal is in its normal location. This shows that otic capsule formation is essential for the facial canal to obtain its normal position .



6.1.1.2 Audiological Findings


Audiological examination reveals either no response at all or profound sensorineural hearing loss (SNHL) at 125, 250 and 500 Hz at the upper limits of the audiometer which may be due to vibrotactile sensations.


6.1.1.3 Management


It is not possible to perform cochlear implant (CI) surgery in these children as there is no inner ear development. Auditory brainstem implantation (ABI) is thus the only surgical option for hearing habilitation. Although translabyrinthine, retrosigmoid, and retrolabyrinthine approaches can be used for ABI surgery, the retrosigmoid approach is preferred in children [7]: the temporal bone is much smaller in children of 2–3 years of age when compared to that of an adult. As a result, the translabyrinthine approach provides a much more limited surgical exposure than the retrosigmoid approach. In addition, in translabyrinthine approach drilling, the temporal bone to expose the brainstem requires longer surgical times compared to retrosigmoid craniotomy. Therefore, retrosigmoid approach is favored for ABI surgery in children.



6.1.2 Rudimentary Otocyst



6.1.2.1 Definition and Radiology


A rudimentary otocyst consists of incomplete millimetric representations of the otic capsule (round or ovoid in shape) without an IAC (Fig. 6.2). Sometimes parts of the semicircular canals may accompany rudimentary otocyst. This pathology represents an anomaly between a Michel deformity and common cavity. In Michel deformity, there is no inner ear development, while in common cavity (CC), there is an ovoid or round cystic space instead of a separate cochlea and vestibule. The CC communicates with the brainstem via the nerves in the IAC. The rudimentary otocyst is a few millimeters in size without the formation of an IAC.

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Fig. 6.2
Rudimentary otocyst

The inner ear is in the form of an otocyst (otic vesicle) between the third and fourth week [4]. The insult probably occurs at the beginning of the formation of the otocyst and results in rudimentary otocyst deformity.


6.1.2.2 Management


The fact that there is no connection between the otocyst and the brainstem is a contraindication to CI surgery. As a result, these patients are candidates for ABI.


6.1.3 Cochlear Aplasia



6.1.3.1 Definition and Radiology


Cochlear aplasia is the absence of the cochlea. The accompanying vestibular system may be normal (Fig. 6.3a) or enlarged (Fig. 6.3b) [1]. The labyrinthine segment of the facial nerve is anteriorly displaced and occupies the normal location of the cochlea. Cochlear aplasia with a dilated vestibule (CADV) must be differentiated from common cavity (CC). If the cochleovestibular nerve (CVN) is present, cochlear implantation can be done in CC. However, CI surgery should not be done in CADV. In some patients, it may be very difficult to distinguish between these entities.

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Fig. 6.3a
Cochlear aplasia with normal vestibular system


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Fig. 6.3b
Cochlear aplasia with dilated vestibular system

Cochlear aplasia with normal labyrinth is almost always symmetrical. The fact that similar findings are present in different patients suggests genetic etiology. In CADV, however, asymmetric development is present; pathology may be genetic or environmental. Otic capsule development is always normal.

After the development of the otic vesicle at the end of the fourth week, the membranous labyrinth develops in three areas: the cochlea, the vestibule, and the endolymphatic duct [4]. Cochlear aplasia is the absence of the cochlear duct, where vestibular and endolymphatic structures may develop normally. The time of the insult must be around the fifth week .


6.1.3.2 Audiological Findings


Typically, these patient will have no response at all or profound hearing loss at low frequencies. Collectively, these findings in complete labyrinthine aplasia, otocyst deformity, and cochlear aplasia demonstrate that profound hearing loss at low frequencies is purely a vibrotactile response and should not be interpreted as hearing in CI candidates with other pathologies.


6.1.3.3 Management


As there is no inner ear development, ABI is the only feasible surgical option to provide hearing in children with cochlear aplasia.


6.1.4 Common Cavity



6.1.4.1 Definition and Radiology


A common cavity is defined as a single chamber, ovoid or round in shape, representing the cochlea and vestibule (Fig. 6.4). Theoretically, this structure has cochlear and vestibular neural structures. There may be accompanying semicircular canals (SCC) or their rudimentary parts. The IAC usually enters the cavity at its center. Cases with vestibular dilatation are occasionally termed as “vestibular common cavity”; however, this is not a correct term.

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Fig. 6.4
Common cavity

Common cavity (CC) should be differentiated from cochlear aplasia with vestibular dilatation (CAVD) [1]. Cochlear aplasia with vestibular dilatation (CAVD) (Fig. 6.3b) usually has a vestibule and semicircular canals at their usual location at the posterolateral part of the IAC fundus. The external outline resembles the normal labyrinth. The vestibule is at its expected location. The accompanying SCCs may be enlarged or normal. A common cavity (CC) (Fig. 6.4), on the other hand, is an ovoid or round structure. SCCs or their rudimentary parts may accompany a common cavity. The IAC usually enters the cavity at its center. The location of a CC may be anterior or posterior to the normal location of the labyrinth. It is very important to differentiate these malformations from each other, because cochlear implantation in a CC may result in acoustic stimulation, whereas in CAVD, no functional stimulation will occur with CI. In spite of these factors, it may sometimes be difficult to differentiate between the two malformations.

CC contains cochlear and vestibular neural elements. This represents development arrest before there is a clear differentiation into the cochlea and vestibule: it is in between rudimentary otocyst and cochlear aplasia and usually occurs around the fourth to fifth week [4].

At the time of insult, the CC is only millimetric in size, as a developed otocyst. The CC may have small or large dimensions: usually, a CC with a diameter of 1–3 cm is encountered. This shows that its capacity to differentiate into the cochlea and vestibule may terminate, but it can still enlarge; so a CC larger than an initial otocyst may be encountered. IAC may be normal or narrow in a large CC. It appears that there is no relationship of the size of the IAC (length and width) and the size of the CC .


6.1.4.2 Audiological Findings


These patients can have detectable hearing thresholds only at low frequencies and at the maximum limits of the audiometer.


6.1.4.3 Management


Cochleovestibular nerve (CVN) should be demonstrated with high-resolution 3-tesla MRI before discussing management options with the family. At the present time, there is no test to determine the amount of cochlear fibers in the CVN [7]. If a behavioral audiometric response or language development is present with hearing aid use, it can be assumed that a meaningful population of cochlear fibers exists and the patient may benefit from a CI. The surgical approach is via a transmastoid labyrinthotomy as described by McElveen [8] with a straight (non-modiolar-hugging) electrode. This will have a position on the periphery of the CC with better contact with the neural tissue. A pre-curved electrode will have the contacts located medially and may not stimulate the periphery of the CC efficiently.

If the CVN cannot be demonstrated with MRI or there is a very narrow or long IAC, where the presence of cochlear fibers is questionable, an ABI may be a more appropriate option from the outset.

As the postoperative hearing cannot be accurately predicted before CI surgery, it is advisable to counsel the family that contralateral ABI may be necessary in case of limited language development with CI. This decision should be done as early as possible .


6.1.5 Hypoplasia and Incomplete Partitions


In these groups of malformations, there is a clear differentiation into a cochlea and vestibule.


6.1.5.1 Incomplete Partition of the Cochlea: Definition and Radiology


Incomplete partition anomalies represent a group of cochlear malformations with normal external dimensions and various internal architecture defects. Incomplete partitions constitute 41 % of inner ear malformations in the database of Hacettepe University Department of Otolaryngology. There are three different types of incomplete partition groups according to the defect in the modiolus and the interscalar septa.


6.1.5.2 Types of Incomplete Partition Groups



Incomplete Partition Type I (IP-I)

This type was termed as “cystic cochleovestibular malformation ” in 2002 by Sennaroglu and Saatci [9]. These represent approximately 20 % of inner ear malformations. In this group, the cochlea lacks the entire modiolus and interscalar septa (Fig. 6.5a), giving the appearance of an empty cystic structure. IP-I cochlea has external dimensions (height and length) similar to normal cases [10]. It is accompanied by an enlarged, dilated vestibule (Fig. 6.5b). Vestibular aqueduct enlargement is very rare. There may be a defect between the IAC and the cochlea due to developmental abnormality of the cochlear aperture and the absence of the modiolus (Fig. 6.5c), and CSF may completely fill the cochlea. The cochlea is located in its usual location in the anterolateral part of the fundus of the IAC.

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Fig. 6.5a
Incomplete partition type I without modiolus and interscalar septa


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Fig. 6.5b
Incomplete partition type I, with grossly dilated vestibule


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Fig. 6.5c
Incomplete partition type II with defective modiolus, minimally dilated vestibule, and a large vestibular aqueduct

Recent histopathology study suggests that IP-I may be due to endosteal development abnormality as a result of defective vascular supply coming from the IAC [4].


Audiological Findings

The majority of IP-I patients have severe to profound SNHL. They are almost always candidates for CI.

During CI surgery gusher is very common which necessitates special precautions. Facial nerve abnormalities can also be seen as a result of abnormal development of the labyrinth. This may necessitate modification of the surgical approach.

As it is possible to have CN aplasia in IP-I, some patients may not be a candidate for CI surgery. Therefore, an ABI is indicated in IP-I patients with aplastic CN. Four patients with IP-I and an aplastic CN have received ABI in our department.

Recurrent meningitis can occur in IP-I patients even prior to their CI surgery or in their nonoperated ear. High CSF pressure filling the cochlea disrupts an often thin stapes footplate, leading to a CSF fistula at the oval window and meningitis. Several cases of this have been reported in the literature [1114]. Spontaneous CSF fistula and recurrent meningitis can be seen although less frequently in cochlear hypoplasia type II. This is because both IP-I and CH-II have endosteal developmental anomaly leading to defective footplate development. If CSF fills the cochlea with high pressure, it may lead to a fistula. If the cochleostomy is not properly sealed, this may also lead to a CSF fistula with recurrent meningitis. It is interesting to note that IP-III cases almost always have a high-volume CSF gusher during CI surgery, but meningitis is very rarely reported in these patients [11, 14]. This is most likely due to the fact that the stapes footplate is normally developed because in IP-III pathology is in the outer two layers of the otic capsule and endosteum is normal. Therefore, a defect in the footplate is very unlikely.

All patients with IP-I and recurrent meningitis who have normal tympanic membranes but fluid filling the middle ear and mastoid should have an exploration of the middle ear with special attention to the stapes footplate.


Incomplete Partition Type II (IP-II)

In IP-II, the apical part of the modiolus is defective (Fig. 6.5d). This anomaly was originally described by Carlo Mondini and together with a minimally dilated vestibule and a large vestibular aqueduct (LVA) (Fig. 6.5d) constitutes the triad of the Mondini deformity . It is very important to use this particular name only if the above mentioned triad of malformations is present [1, 9, 11, 15]. The apical part of the modiolus and the corresponding interscalar septa are defective. This gives the apex of the cochlea a cystic appearance due to the confluence of middle and apical turns. The external dimensions of the cochlea (height and diameter) are similar to that seen in normal cases [10]. Therefore, it is not correct to define this anomaly as a cochlea with 1.5 turns [10]. This description should only be used for cochlear hypoplasia.
Oct 3, 2017 | Posted by in OTOLARYNGOLOGY | Comments Off on Classification of Inner Ear Malformations
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