Jackler Classification Essay

All the patients (n=72) with bilateral SNHL underwent HRCT (16-slice) & high-field MRI (1.5T). The age range of study population varied between 6 months to 20 years.

—  HRCT of temporal bone aws performed with the following parameters:

1. 0.75-mm collimation, 0.625-mm section thickness, 140 kVp, 120 mAs, pitch of 0.8, a 15-cm field of view, and a 512 x 512 matrix. The initial data sets were then reconstructed at 0.1-mm intervals.

 

—  MR scanning was performed with following sequences.

1. 3D FIESTA(Fast Imaging Employing Steady-state Acquisition) was performed with these parameters: 10-cm FOV, TR/TE of 8/4, 32 sections of 1-mm thickness, 256 x 256 matrix, flip angle of 45°, bandwidth of ± 32 kHz, two phase cycles, and an acquisition time of 2 minutes 24 seconds. Phase-cycling involved two 3D steady-state datasets acquired sequentially, each with a specific radiofrequency phase shift added at every TR.

2. For better image resolution of nerves oblique parasagittal view which was perpendicular to the plane of the internal auditory canal (IAC) is taken on both sides. The cochlea can be further evaluated by generating MIP images. The process is the same as that used in MR angiography, except the inner ear is targeted in the reformatting process.

3. For screening of brain T2W (TE 102.9ms, TR 4780ms) sequence in Axial plane is taken.

 

EMBRYOLOGY OF INNER EAR: Fig. 1

 

At approximately the third week of gestation, otic placodes arise from the surface ectoderm on each side of the rhombencephalon. The otic placodes subsequently invaginate and form otocysts, which are the otic and auditory vesicles. At around the fifth week, diverticulum buds from the otocysts form the endolymphatic sacs, followed by the cochlea and vestibules.

The membranous cochlea achieves 1 to 1 ½ turns at the end of 6 weeks, and 2 ½ turns are formed at the end of the 7th week. The semicircular canals start to develop from the utricle segments of the otocysts at 7–8 gestational weeks. The superior canals form first, followed by the posterior and then the lateral canals. The inner ear structures have an adult configuration by the end of 8 weeks (3).

Fig. 1: Image showing process of embryogenesis of inner ear from 4th week to 8th week of gestation.
References: Jackler RK et al, (1987) Congenital malformations of the inner ear: a classification based on organogenesis. Laryngoscope 97:2–14.

 

Ossification begins in the cochlea, followed by the semi-circular canals. Apposition of a shell of ossification proceeds rapidly between 18 and 24 weeks of gestation. There is very little subsequent remodelling after birth. The internal auditory canal, vestibular aqueduct, mastoid, and external auditory canal continue to grow after birth (3)

 

RADIOLOGICAL ANATOMY OF INNER EAR  

 

Fig. 2

Fig. 2: Image showing anatomy of middle and inner ear
References: Saladin KS. (2004) Anatomy and physiology- the unity of form and function. 3rd ed. New York: McGraw-Hill;

 

Bony Labyrinth The bony labyrinth consists of the vestibule, semicircular canals and cochlea  

 

Vestibule

 

The central portion of the cavity of the bony labyrinth is the vestibule. The vestibule is a relatively large ovoid perilymphatic space measuring approximately 4 mm in diameter. The vestibule is continuous anteriorly with the cochlea and posteriorly with the semicircular canals. There are cribrose areas, minute openings for the entrance of the nerve branches from the vestibular nerve on the medial wall and floor of the vestibule, where the vestibule abuts the lateral end of the internal acoustic canal. The vestibule has two other openings, the oval window (for the footplate of the stapes) and the vestibular aqueduct.

 

Semicircular Canals

 

The three semicircular canals are continuous with the vestibule. Each of the canals makes about two thirds of a circle and measures about 1 mm in cross-sectional diameter. Each canal is enlarged anteriorly by an ampulla. The nonampulated ends of the superior and posterior semicircular canals join to form the bony common crus. The perilymphatic space of each semicircular canal opens into and communicates freely with the vestibule at both ends.

 

Cochlea

 

The cochlea is a conical structure, its base facing the internal auditory canal and its apex or cupola directed anteriorly, laterally, and slightly downward. The base measures around 9 mm, and its axis height is about 5 mm. The base is perforated by numerous apertures for the passage of the cochlear nerve. The cochlea consists of a bony canal wound around a conical central core called the modiolus. The modiolus is the conical central pillar of the cochlea. Its base is broad and appears at the lateral end of the internal acoustic canal, where it corresponds with the cochlear exit of the corresponding part of the eighth cranial nerve. It is perforated by numerous orifices for the transmission of the branches of the nerve. The bony cochlear canal takes between 2 ½ and 2 ¾ turns around the modiolus. The first turn bulges toward the tympanic cavity, and this elevation on the medial wall of the tympanic cavity is known as the promontory. The cross-sectional diameter of the beginning of the canal is about 3 mm. The openings in or near the first portion of the cochlear canal include the round window, which is covered by the secondary tympanic membrane; the oval window (actually an opening of the vestibule), which is covered by the footplate of the stapes; and the cochlear canaliculus, which leads via a small canal to the subarachnoid s The bone separating one turn of the cochlea from the next is called the interscalar septum.(4)

Fig. 3

 

Fig. 4: Axial CT image showing normal anatomy of internal auditory canal (green arrows), cochlea (red arrows) and vestibule (yellow arrows)
References: Radio-diagnosis and Imaging, Manipal University, Kasturba Hospital - Manipal/IN

 

Fig. 5: Coronal reconstruction CT image showing normal anatomy of cochlea (red arrows), semicircular canal (yellow arrows) and vestibule (green arrows)
References: Radio-diagnosis and Imaging, Manipal University, Kasturba Hospital - Manipal/IN


Membranous Labyrinth

 

The interconnecting spaces actually within the membranous labyrinth constitute the endolymphatic cavity. The membranous labyrinth consists of the cochlear duct, the vestibular sense organs, the endolymphatic duct and sac, the round window membrane, and the vascular system.

 

Cochlear Duct

The cochlear duct is a spiral tube lying within the cochlea and attached to its outer wall. The cochlear duct is a blind pouch; it cleaves the perilymphatic space within the bony labyrinth, dividing it into two portions, the scala vestibule and the scala tympani. The cochlear duct is triangular, its roof being formed by Reisner’s membrane, its outer wall by the endosteum lining the bony canal, and its floor by the basilar membrane and the outer part of the osseous spiral lamina. It contains the organ of Corti, which is the site of placement of the supporting and sensory (hair) cells that mediate hearing.

 

Endolymphatic Duct and Sac

 

The endolymphatic duct begins within the vestibule as a dilated portion, the endolymphatic sinus. It arises at the confluence of the utricular and saccular ducts. As it leaves the vestibule, it narrows into an isthmus and passes into the vestibular aqueduct, located near the crus common. As the aqueduct turns caudally to approach the dural opening of the vestibular aqueduct, the membranous duct (within the bony aqueduct) widens again into the flat endolymphatic sac. The intraosseous part of the sac fills most of the vestibular aqueduct. The remainder of the sac protrudes from the inferior aperture of the aqueduct and lies between the periosteum of the petrous bone and the dura mater. The sac is not one compartment but rather a complex system of connecting channels. (4)

 

IMAGING ANATOMY OF VESTIBULOCOCHLEAR NERVE:

 

Vestibulocochlear nerve carries special afferent fibers for hearing (the cochlear component) and balance (the vestibular component). It enters the lateral surface of the brainstem, between the pons and medulla, after exiting the temporal bone through the internal acoustic meatus and crossing the posterior cranial fossa. Inside the temporal bone, at the distal end of internal acoustic meatus, the vestibulocochlear nerve divides to form,                                                                                  

·         Cochlear nerve

·         Vestibular nerve

The vestibular nerve enlarges to form the vestibular ganglion, before dividing into superior and inferior parts, which distribute to the three semicircular ducts and utricle and saccule.

 

The cochlear nerve enters the base of the cochlea and passes upwards through modiolus, the ganglion cells of the cochlear nerve are in the spiral ganglion at the base of the lamina of modiolus as it winds around the modiolus. Branches of the cochlear nerve pass through the lamina of modiolus to innervate the receptors in the spiral ganglion.

   

Fig. 6

Fig. 6: Axial 3D FIESTA showing normal cochlear and vestibular nerve (white arrows) on both sides, vestibule (yellow arrow) and lateral semicircular canal (LSCC) (red arrow)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

Fig. 7

Fig. 7: Axial 3D reconstruction image showing internal auditory canal (black arrow), cochlea (red arrow), vestibule (yellow arrow), superior semicircular canal (SSCC, green arrow), lateral semicircular canal (LSCC, blue arrow), posterior semicircular canal (PSCC, purple arrow)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

Fig. 8

Fig. 8: Left oblique para-sagittal (3D FIESTA sequence) view showing vestibulo-cochlear and facial nerve in internal auditory canal.
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

 

 

 HRCT AND MRI APPEARANCE OF COCHLEAR PATHOLOGIES

 

Jackler et al., 1987 (3) classified congenital bony cochlear malformations as Michel deformity, cochlear aplasia, common cavity, cochlear hypoplasia, and incomplete partition (IP; Mondini dysplasia).

Sennaroglu & Saatci et al., 2002 (5) proposed a new classification system that divided incomplete partition (IP) into subcategories: IP type I (cystic cochlear malformation) and IP type II (classic Mondini dysplasia).

Phelps used the term pseudo-Mondini for a dysplastic cochlea with a dilated basal turn, and confined the originally described Mondini dysplasia to a normal basal cochlear turn and deficient inter scalar septum for the distal 1 ½ turns. Moreover, he emphasized that if the basal turn is intact, some hearing will remain, and there will be no risk of a major fistula (6).

 

 Relative incidence of cochlear malformations:

 

Malformation

Incidence (%)

Incomplete partition (Mondini's dysplasia)

55

Common cavity

26

Cochlear hypoplasia

15

Cochlear aplasia

3

Complete labyrinthine aplasia (Michel's aplasia)

1

 

Malformation of the inner ears may be associated with normal hearing. This is especially in the case for semi-circular canal (SCC) anomalies. Vestibular symptoms, which occasionally are severe, are present in approximately 20% of patients.

A wide variety of morphologic patterns of inner ear malformation has been observed radiographically and may involve the cochlea, SCCs, or vestibular aqueduct (VA). Other anomalies cannot be explained by a premature arrest in development alone and appear to arise from an aberrant embryologic process. An example of this type of anomaly is a cochlea of normal length but abnormal size or coiling geometry. In humans, the inner ear is of adult size at birth and shows strikingly little variation in size among individual patients. Between the fourth and fifth weeks of development, the spheric otocyst develops three buds that ultimately form the cochlea, SCCs, and VA. An inner ear malformation may be limited to one of these anlages, may involve a combination of two, or may even affect all three.

Co-existence of deformities involving the cochlea, SCCs, and VA has several possible explanations: (1) the anomaly is genetically predetermined; (2) an insult to the embryo occurred before the fifth week; or (3) each of the buds was susceptible to some teratogenic influence at a later stage of development. A majority of inner ear malformations are bilateral and symmetrical. In cases in which radiographs detect an anomaly on only one side, the opposite “normal” inner ear has a hearing loss in approximately 50% of cases.

 

MALFORMATION OF INNER EAR:

 

MICHEL APLASIA:

Also known as complete labyrinthine aplasia is a rare congenital inner ear abnormality, accounting for approximately 1% of cochlear bony malformations. This condition is defined as complete absence of inner ear structures and is caused by developmental arrest of otic placode early during the third week of gestational age.

Fig. 9

Fig. 9: Axial 3D FIESTA showing Michel’s deformity on right side (red arrow) in the form of complete absence of inner ear structures.
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

COCHLEAR APLASIA:

Failure of cochlea development late in the third week of gestation results in this condition.

 

COMMON CAVITY:

In common cavity malformation, developmental arrest occurs at the fourth week of gestation and is defined as a single cavity that represents the undifferentiated cochlea and vestibule.

 

Fig. 10

Fig. 10: Image A: Axial 3D FIESTA showing common cavity deformity on left side (red arrow), Image B: Axial CT of the same patient showing common cavity deformity (yellow arrow) on left side.
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad, 2012

 

INCOMPLETE PARTITION I:

Incomplete partition I is also known as cystic cochlea vestibular malformation, where the cochlea has no bony modiolus, resulting in an empty cystic cochlea This is accompanied by a dilated cystic vestibule with developmental arrest at the fifth week of gestation (9).

 

INCOMPLETE PARTITION II:

In this group, the cochlea consists of 1 ½ turns; the apical and middle cochlea turns are undifferentiated and form a cystic apex . The vestibule is normal while the vestibular aqueduct is always enlarged. Developmental arrest occurs at the seventh week of gestation.

 

Fig. 11

Fig. 11: Axial 3D FIESTA with superimposed reconstructed 3D image showing Mondini’s deformity (red arrows) and small dysplastic vestibule (yellow arrows) on both sides.
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

LABYRINTHITIS OSSIFICANS:

Labyrinthitis ossificans represents pathologic ossification of the membranous labyrinth as a response to an insult to the inner ear. It occurs as the result of a healing reaction, and generally occurs secondary to an inflammatory process such as infection (post meningitis), trauma or any other tumorous processes. This condition leads to a sensorineural hearing loss, and may prevent or complicate cochlear implantation. (10,11)

 

Fig. 12

Fig. 12: Image A:Axial 3D FIESTA showing loss of normal fluid filled spaces of membranous labyrinth on both sides (red arrows), Image B:Axial CT scan of same patient showing ossification of cochlea and vestibule on both sides (yellow arrows)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

ANOMALIES OF THE EIGHTH NERVE:

Hypoplasia and aplasia of the eighth nerve are often, but not inevitably, associated with congenital narrowness or even absence of the IAC. Similarly, although eighth nerve maldevelopment frequently accompanies malformation of the inner ear, the presence of a normal cochlea and semicircular canals does not guarantee normal development of the audiovestibular nerve. High-resolution, thin-section MRI with T2-weighted sequences currently is the best means of assessing the fine anatomy of the eighth nerve in the IAC. The internal auditory canal frequently is normal.

 

Fig. 13

Fig. 13: Image A: Axial 3D FIESTA showing absent vestibulo-cochlear nerve (VCN) on both sides (white arrows), Image B: Axial CT scan showing narrowed internal auditory canal (IAC) on both sides (red arrows), Image C and D: Right and left oblique parasagittal (3D FIESTA) views showing absent VCN (yellow arrows)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

SEMICIRCULAR CANAL ABNORMALITIES:

The most commonly recognised semicircular canal abnormality is a short lateral semicircular canal being confluent with the vestibule.

Ears with malformations limited to the vestibular system often have normal or near-normal hearing. When the cochlea also is abnormal, sensory hearing levels tend to be impaired, to a variable degree.

The SCC dysplasia appears to have an association with conductive hearing loss, presumably because of inner ear micromechanical factors rather than stapes fixation (12). The LSCC malformation is associated with SNHL and conductive hearing loss (CHL) varied from mild to profound but did not correlate with the severity of LSCC malformation.

 

Fig. 14

Fig. 14: Axial 3D FIESTA showing absent lateral semicircular canal (SCC) and superior SCC on left side (red arrow)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

ENLARGED VESTIBULAR AQUEDUCT (VA):

Enlargement of the VA is the most common radiographically detectable malformation of the inner ear (3,5). Enlargement of the VA is diagnosed when its diameter exceeds 2 mm, although enlarged VAs may exceed 6 mm in width. In many cases, VA enlargement accompanies malformation of the cochlea or SCC. It also may be the sole radiographically detectable abnormality of the inner ear in a child with hearing loss. This condition is commonly referred to as the large VA syndrome. The advent of high-resolution CT in the axial plane has made assessment of the VA much easier.

Normally the vestibular aqueduct is not seen or is appreciated as only a lucent line. The vestibular aqueduct was considered enlarged if the external pore was greater than 2 mm.

 

 

  Fig. 15

Fig. 15: Axial 3D FIESTA showing dilated endolymphatic duct and sac on both sides (red arrows)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

INTERNAL AUDITORY MEATUS:

It can be narrow or wide.

A congenitally large canal may be an incidental finding in healthy individuals When a large internal auditory canal (IAC) (larger than 10 mm in diameter) accompanies a malformation of the inner ear, it does not, as an independent variable, correlate with the level of hearing.

Primary importance of detecting enlargement of the IAC is its association with spontaneous CSF leak and the occurrence of gusher during stapes surgery.

A narrow IAC may indicate a failure of eighth cranial nerve development. When a patient has normal facial function and an IAC less than 3 mm in diameter, it is likely that the bony canal transmits only the facial nerve . A narrow IAC may accompany inner ear malformations or may be the sole radiographically detectable anomaly in a deaf child. A narrow IAC was considered a relative contraindication to cochlear implantation, because it suggests that the eighth nerve may be insufficiently developed to conduct an auditory signal,  but now with the help of MRI the cochlear nerve can be studied carefully before planning for surgery.

 

Fig. 16

Fig. 16: Axial CT scan showing narrowed internal auditory canal (IAC) on both sides (yellow arrows)
References: Department of Radiology, B J Medical College and Civil Hospital/Gujarat University, Ahmedabad 2012

 

 

HRCT depicts the minute details of dysplastic or ossified cochlea, enlarged vestibular aqueduct, high-riding jugular bulb, aberrant facial nerve course and abnormal bony structures. However absence of vestibule-cochlear nerve (VCN) cannot be detected accurately on HRCT.

 

MRI allows visualization of fluid-filled spaces of the inner ear, early stages of fibrosis, VCN, and brain. However, complete ossification of cochlea and labyrinth cannot be detected well on MRI.    

 

Following are the list of imaging findings what the cochlear implant surgeon wants to know from the reporting radiologist.

 

Table 1: Check list for reporting CT and MRI of a patient prior to cochlear implant
References: Radio-diagnosis and Imaging, Manipal University, Kasturba Hospital - Manipal/IN

Indian Journal of Radiology and Imaging, Vol. 19, No. 2, April-June, 2009, pp. 99-106

Head & Neck Radiology

Pictorial review of MRI/CT Scan in congenital temporal bone anomalies, in patients for cochlear implant

Santosh S Gupta1, Shailendra R Maheshwari1, Milind V Kirtane2, Nitin Shrivastav2

1 Department of Radiology , P.D. Hinduja National Hospital and Medical Research Centre, Mumbai, India
2 Department of ENT, P.D. Hinduja National Hospital and Medical Research Centre, Mumbai, India

Correspondence Address:
Santosh S Gupta
Department of Radiology (MRI), P.D. Hinduja Hospital National Hospital and Medical Research Centre, Mahim, Mumbai-400 016
India
drsantoshg@rediffmail.com


PMID: 19881062

DOI: 10.4103/0971-3026.50825

Abstract

High-resolution CT scan (HRCT) and MRI are routinely performed prior to cochlear implant surgery. These modalities help assess the status of the inner ear structures. A few patients have significant anomalies, which need to be assessed and understood in detail. We present a pictorial essay of these anomalies and described our HRCT and MRI techniques in patients being imaged prior to surgery.

Keywords: Cochlear implant, HRCT, MRI, temporal bone

Introduction

Over the last decade there has been tremendous growth in the number of cochlear implants being performed and, consequently, there has also been a steady increase in the imaging, done as a part of the preoperative workup of these patients. High-resolution computed tomography (HRCT) and MRI of the temporal bones provide vital information; these are baseline investigations and are necessary in all patients posted for cochlear implant surgery. MRI is now increasingly being used to study the membranous labyrinth and the cranial nerves; it provides accurate information and exquisite anatomical detail.[1]

This paper is a pictorial essay on the various congenital temporal bone anomalies seen in patients being investigated prior to cochlear implant surgery. There are several complex congenital anomalies that are encountered while imaging such patients. The radiologist needs to follow a clinically oriented classification of these anomalies, which helps the implant surgeon plan the correct management strategy. A classification widely used by otolaryngologists is the one described by Sennaroglu and Saatci,[2] and we too have used this system with a few modifications.

CT and MRI Techniques

HRCT

HRCT scans are performed on a 64-slice volume scanner (LightSpeed VCT, GE, Milwaukee, USA) in a straight axial plane (kV: 140, mA: 350, matrix: 512 x 512, slice thickness: 0.625 mm/10.63, 0.531:1, scan field of view (FOV): 32 cm, display FOV: 9.6 cm). The original isometric volume data is used to obtain coronal reformatted images. The images are reviewed with a high-resolution bone algorithm, using a small FOV for separate right and left ear documentation.

MRI

MRI scans are performed on 1.5-T MR machine (Excite Twin Speed, GE, Milwaukee, USA) with an 8-channel head coil. Sedation is used in most patients. A 3D-FIESTA (fast imaging enabling steady-state acquisition) axial sequence (TR: 5.5, TE: 1.7/Fr, FOV: 16 x 16, slice thickness: 1.0/−0.5, matrix: 320 x 320, NEX: 6.0) is performed followed by coronal reformations along with 3D maximum intensity projection (MIP) reconstructions. A 3D-FIESTA sequence is also acquired in a direct oblique sagittal plane (TR: 6.7, TE: 2.1/Fr, FOV: 12 x 12, slice thickness: 1.0/−0.5, matrix: 384 x 320, NEX: 6.0) perpendicular to the VII-VIII nerve complexes. With this technique we are able to obtain better resolution than with reformations from an axial sequence; this enables better delineation of the nerves [Figure - 1]. A routine T2W axial sequence through the brain is obtained in all patients.

Discussion

Congenital malformations of the inner ear are rare anomalies; they can be identified on imaging with HRCT and/or MRI in about 20% of patients with congenital sensorineural hearing loss.[3]

Based on the site of abnormality, congenital inner ear anomalies can be classified into:

(a) cochlear malformations, (b) vestibular malformations, (c) malformations of the semicircular canals, (d) vestibular and cochlear aqueduct malformations, (e) cochlear nerve deficiency, (f) isolated attenuated modiolus, and (g) isolated cochlea. A classification commonly used by ENT surgeons is the one described by Sennaroglu and Saatci[2] ; we have used this system with some modifications since we encountered some additional anomalies on MRI (such as cochlear nerve deficiency) With the resolution provided by the newer CT scan and MRI equipments, it is now possible to see minute internal structures of the cochlea such as the interscalar septum [Figure 2a] which divides the major cochlear turns; this can be seen on both HRCT and MRI 3D-FIESTA sequences [Figure 2b]. The osseous spiral lamina, a thin membrane within each turn of the cochlea that separates the scala vestibuli from the scala tympani,[4] can also be well seen on MRI as a thin, linear structure [Figure 2b]. The MRI 3D sequence data can be used to obtain a 3D MIP reconstruction [Figure 2d], which gives a good outline of the inner ear structures, especially when complex anomalies need evaluation. The membranous labyrinth contains endolymph and is surrounded by the perilymph which, in turn, separates it from the otic capsule or bony labyrinth. The cochlea consists of two and one half turns, which extend into the vestibule. The three semicircular canals arise from the vestibule in arches along all three planes. Laterally the otic capsule is called the promontory and is thickest over the basal turn of the cochlea; posteriorly, it is perforated by the round and oval windows [Figure 2c].

The vestibular aqueduct represents an osseous aperture in the bony labyrinth; it is about 5 mm in length and is located along the medial aspect of the pyramid. Although its lumen is lined by squamous and cuboidal epithelium, it houses an extension of the membranous labyrinth-the endolymphatic duct. On axial CT scan, the vestibular aqueduct is seen as a small slit running medial and parallel to the plane of the posterior semicircular canal. Its distal, external funnel-like opening, much like the external opening of the cochlear aqueduct, can usually be visualized on a CT scan, when it can be seen opening into a linear ridge of bone-the foveate impression.[5]

The seventh-eighth nerve complexes are well seen on MRI. On the 3D-FIESTA sequence, it is possible to identify further divisions of the eighth nerve [Figure 1a]. The cochlear nerve is well seen on axial 3D-FIESTA images [Figure 1b], extending into the modiolus. The anatomy and delineation of the nerves is better appreciated in an oblique sagittal plane, perpendicular to the plane of the internal auditory canal.[6] With this technique [Figure 1c], the superior and inferior vestibular nerves are seen in the posterior quadrants and the cochlear nerve in the anteroinferior quadrant, while the facial nerve is seen in the anterosuperior quadrant.

The important congenital anomalies that are encountered when imaging patients prior to cochlear implant surgeries are discussed in detail below. Some of the malformations, such as those of the semicircular canals, have not been discussed, since they do not impact the surgery or management.

Michel deformity : In this deformity, there is absence of the entire cochlea and the vestibular structures, i.e., complete labyrinthine aplasia[2] It may be bilateral or unilateral [Figure - 3]. The internal auditory canals (IACs) are small in size on both sides [Figure 3a] and [Figure 3b]. Cochlear nerve deficiency will be seen [Figure 3e], on MR 3D-FIESTA images.

Cochlear aplasia: In this condition, the cochlea is completely absent[2][Figure - 4]. The vestibule and semicircular canals may be normal, dilated [Figure 4a],[Figure 4b], or hypoplastic. Dense otic bone is present at the site of the cochlea. The appearance may simulate complete labyrinthitis ossificans, in which normal-sized bone is seen anterior to the IAC as also the bulge of the cochlear promontory produced by the basal turn of cochlea; both features are absent in cochlear aplasia [Figure 4d].

Common cavity deformity: In this condition there is no differentiation between the cochlea and the vestibule, both together forming a cystic cavity[2][Figure - 5]. This occurs due to developmental arrest in the fourth week of gestation, when differentiation into the cochlea and vestibule has not yet taken place.

Incomplete partition type I (IP-I): In this condition, the cochlea lacks the entire modiolus and the cribriform area and appears cystic, along with a large cystic vestibule[2][Figure - 6]. The dimensions of the cochlea and the vestibule are normal but the internal architecture, including the modiolus, is missing, giving it an empty cyst-like appearance. The modiolus is completely absent in its entire length from the base to the apex. One needs to note the striking difference from the IP-II (Mondini) deformity, where only the middle and the apical turns form a cystic cavity due to fusion. This pathology represents a form of common cavity that is one step more organized and differentiated than the previously described common cavity malformation, the developmental arrest occurring in the fifth week.[2]

Cochleo-vestibular hypoplasia: In this group, the cochlear and the vestibular structures are separate from each other and more differentiated than in IP-II, with failure of development occurring in the sixth week.[2] Both the cochlea and the vestibule are small in size, with the hypoplastic cochlea seen as a small bud, coming off the IAC.[2]

Incomplete partition type II (IP-II) (Mondini deformity): This condition represents developmental arrest occurring at a later stage than in IP-I (seventh week of gestation), with the size of the cochlea and vestibule appearing normal and internal organization being more developed.[2] The cochlea consists of 1.5 turns, with the middle and apical turns coalescing to form a cystic apex (due to a defect in the interscalar septum), along with a dilated vestibule and enlarged vestibular aqueduct [Figure - 7]. The modiolus is present in the basal turn where the ganglion cells and nerve endings are usually seen[2],[7] and, therefore, these patients are more likely to regain hearing after cochlear implantation than in patients with IP-I.

Dilated vestibular aqueduct: This may occur in isolation [Figure - 8] or in combination with other inner ear malformations. However, Lemmerling et al,[8] in their series found that all ears with large vestibular aqueducts have an associated modiolar deficiency. Hence the term, isolated dilated vestibular aqueduct seems obsolete. IP-II (classic Mondini) is always associated with a dilated vestibular aqueduct. An isolated dilated vestibular aqueduct has also been observed and reported by other authors.[9] Valvassori et al .[10] , who were the first to describe an association between an enlarged vestibular aqueduct and sensorineural hearing loss, coined the term ′large vestibular aqueduct syndrome.′ Interestingly, this may be a part of a syndrome, such as Pendred syndrome and distal renal tubular acidosis,[11] and one needs to look for this possibility in the appropriate setting. Bamiou et al , in their series of patients with sensorineural hearing loss evaluated with HRCT, found that 60% of their patients had an isolated dilated vestibular aqueduct.[12] The criterion that they used for a dilated vestibular aqueduct was a middle-third diameter of the duct of more than 1.5 mm.[12]

Cochlear nerve deficiency: We have used the term cochlear nerve deficiency to encompass both absent [Figure - 3] and [Figure - 9] as well as hypoplastic cochlear nerves, based on the study by Glastonbury et al .[13] This study and another by Kim et al.[6] showed that the cochlear nerve is larger than either the superior or inferior vestibular nerves in 90% of normal cases and it is of almost the same size [Figure 1C] or larger than the facial nerve in 64% of cases. The cochlear nerve size is thought to be correlated with the spiral ganglion cell population and, therefore, determination of the nerve caliber may prove to be helpful in predicting the outcome of cochlear implantation.[14] An appreciably thin cochlear nerve, as seen in some of our cases, may still effectively transmit impulses to allow hearing;[15] therefore, MRI depiction of a small nerve is only a relative contraindication to cochlear i mplantation.

In almost all patients with congenital cochlear nerve deficiency, the IACs are small in size. As per the criteria described in literature, an abnormal IAC is diagnosed if it is < 4 mm in either the vertical or transverse diameters, is irregularly shaped, or is appreciably smaller than the IAC of the contralateral side.[13] In a study by Valvassori et al. , the IAC was found to be virtually symmetric in healthy individuals, with a difference of < 1 mm in 99% of patients and 1-2 mm in 1% of patients.[16]

Isolated cochlea: The term isolated cochlea [Figure - 10] is used when the cochlear aperture (which is a small canal at the fundus of the IAC [Figure 10B], through which the cochlear nerve passes to enter the cochlea, is absent and filled with bone.[13] This has also been described as hypoplasia of the bony canal of the cochlear nerve.[17]

Attenuated modiolus: This may be an isolated finding or may be seen in association with other malformations. The "modiolus" (taken from the latin word ′hub of a wheel′) is an irregular conical (or trapezoidal)-shaped porous bone within the cochlea, from which the osseous spiral lamina of the cochlea projects out, which supports the organ of Corti[8] [Figure 1B],[Figure 2A].

We label a modiolus as attenuated on the basis of subjective criteria [Figure - 11], although the more accurate method is the measurement of its area at the point of its maximum size. According to Naganawa et al ., the size of the modiolus on MRI in normal volunteers is about 4.0 mm 2 .[18]

Surgical implications for implantation in cochlear malformations

It is important to understand the surgical approach and strategy in patients with cochlear malformations, especially so since there was a time when cochlear implantation was thought to be contraindicated in many of these cases.

Apart from the technical difficulties associated with the surgery, the expectations for improved auditory performance after cochlear implants in patients with inner ear malformations are relatively low; this may be due to a substantially reduced population of spiral ganglion cells[19] and other coexisting abnormalities. It is clear that the approach in each case should be tailored according to the type of malformation. Michel deformity is obviously a contraindication to cochlear implantation, and auditory brainstem implantation may be considered as an option in these cases. In common cavity malformations, the exact location and amount of neural tissue are not definitely known, and the surgeon can use full-banded implants rather than the half-banded ones oriented towards the modiolus. In such cases, as suggested by McElveen et al ., the use of a precurved electrode may help in avoiding the risk of the electrode entering the IAC.[20] In a hypoplastic cochlea, due to the lack of space, the electrode may enter the IAC if a full insertion is attempted. As reported by Tucci et al ., the reason for this is that the small space of a hypoplastic cochlea does not allow the electrode to curl within the cavity. In cases of IP-II, the modiolus and the basal turn are present and the surgical approach is similar to the one used in normal cases.[20]

The transmastoid facial recess approach is usually the standard technique for electrode array placement in the normal cochlea; this, however, may not be the best approach for patients with common cavity deformities or cochlear hypoplasia. These patients usually have a thin or absent cribriform area between the IAC and the common cavity and are at high risk for an intraoperative cerebrospinal fluid (CSF) gusher or postoperative CSF leak. An intraoperative CSF leak can be also seen with an IP-II deformity or in the presence of an enlarged IAC.

Cochlear nerve deficiency is not a contraindication for surgery. In those cases where the cochlear nerve is not seen on MRI, it is possible that the nerve is so thin that it is beyond the resolution of a 1.5 T MRI scanner; alternatively, it may be that the cochlear nerve fibers traverse along the vestibular nerve and hence are not detected on MRI. These patients should undergo a hearing aid trial and periodical audiological evaluation by expert audiologists/therapists before a decision is taken on their candidacy for cochlear implant surgery. In all such cases, the side with the more normal-appearing inner ear and with the relatively larger cochlear nerve should be selected for implantation; this is the logical approach. It is also recommended that when the cochlear nerve is not well seen, intracochlear electrical stimulation to determine the auditory nerve action potential and auditory brainstem response may be a valuable test before performing cochlear implantation.[21]

Conclusion

HRCT and MRI are baseline investigations that need to be done prior to cochlear implant surgery. Both these modalities provide exquisite anatomical details and information. The radiologist should have a clear understanding and knowledge of the various malformations and should follow a simple, clinically oriented, classification that can be easily understood and interpreted by the implant team. There needs to be special emphasis on identifying cochlear malformation and cochlear nerve deficiency since these have a significant impact on cochlear implant surgery and its outcome.

Acknowledgement

Dr. Tanvi Jakhi for her contribution of the excellent drawings.

References

1.Parry DA, Booth T, Roland PS. Advantages of magnetic resonance imaging over computed tomography in preoperative evaluation of pediatric cochlear implant candidates. Otol Neurotol 2005;26:976-82.  Back to cited text no. 1  [PUBMED]  [FULLTEXT]
2.Sennaroglu L, Saatci I. A New Classification for Cochleovestibular Malformations. The Laryngoscope Lippincott Williams and Wikins, Inc., Philadelphia ©, Luxford 2002. The American Laryngological, Rhinological and Otological Society, Inc. Laryngoscope 2002;112:2230-41.  Back to cited text no. 2    
3.Jackler RK, Luxford WM, House WF. Congenital malformations of the inner ear: A classification based on embryogenesis. Laryngoscope 1987;97:2-14  Back to cited text no. 3  [PUBMED]  
4.Curtin HD, Sanelli PC, Som PM. Temporal bone: Embryology and Anatomy Chapter (19) In: Som PM, Curtin HD. Head and Neck Imaging (4th Edition) Volme 2, Mosby; 2003. p. 1057-75.  Back to cited text no. 4    
5.Chat V, Sultan B, Mohammed S. Radiography of normal ear [chapter 8] In: Taveras JM, Ferrucci JT Radiology Diagnosis-Imaging-Intervention Volume 3 Lippincot; 1986. p. 1-14.  Back to cited text no. 5    
6.Kim HS, Kim DI, Chung IH, Lee WS, Kim KY. Topographical relationship of the facial and vestibulocochlear nerves in the subarachnoid space and internal auditory canal. AJNR Am J Neuroradiol 1998;19:1155-61.  Back to cited text no. 6  [PUBMED]  [FULLTEXT]
7.Slattery WH, Luxford WM. Cochlear implantation in the congenital malformed cochlea. Laryngoscope 1995;105:1184-7.  Back to cited text no. 7    
8.Lemmerling MM, Mancuso AA, Antonelli PJ, Kubilis PS. Normal modiolus: CT appearance in patients with a large vestibular aqueduct. Radiology 1997;204:213-9.  Back to cited text no. 8  [PUBMED]  [FULLTEXT]
9.Jakler RK, De La Cruz. The large vestibular aqueduct syndrome. Laryngoscope 1989;99:1238-43.  Back to cited text no. 9    
10.Valvassori GE, Clemis JD. The large vestibular acqueduct syndrome. Laryngoscope 1978;88:723-8  Back to cited text no. 10  [PUBMED]  
11.Berrittini S, Forli F, Bogazzi F, Neri E, Salvatori L, Casani AP, et al . Large vestibular aqueduct syndrome: Audiological, radiological, clinical and genetic features. Am J Otolaryngol Head Neck Med Surg 2005;26:363-71.  Back to cited text no. 11    
12.Bamiou DE, Phleps P, Sirimanna T. Temporal bone computed tomography findings in bilateral sensorineural hearing loss. Arch Dis Child 2000;82:257-60.  Back to cited text no. 12    
13.Glastonbury CM, Davidson HC, Harnsberger HR, Butler J, Kertesz TR, Shelton C. Head and neck: Imaging findings of cochlear nerve deficiency. AJNR Am J Neuroradiol 2002;23:635-43.  Back to cited text no. 13  [PUBMED]  [FULLTEXT]
14.Nadol JB Jr, Xu WZ. Diameter of the cochlear nerve in deaf humans: Implications for cochlear implantation. Ann Otol Rhinol Laryngol 1992;101:988-93.  Back to cited text no. 14  [PUBMED]  
15.Ylikoski J, Savolainen S. The cochlear nerve in various forms of deafness. Acta Otolaryngol 1984;98:418-27.  Back to cited text no. 15  [PUBMED]  
16.Valvassori GE, Pierce RH. The normal internal auditory canal. AJR Am J Roentgenol 1964;92:773-81.  Back to cited text no. 16    
17.Fatterpekar GM, Mukherji SK, Alley J, Lin Y, Castillo M. Hypoplasia of the bony canal for the cochlear nerve in patients with congenital sensorineural hearing loss: Initial observations. Radiology 2000;215:243-6.  Back to cited text no. 17  [PUBMED]  [FULLTEXT]
18.Naganawa S, Ito T, Iwayama E, Fukatsu H, Ishigaki T, Nakashima T, et al . Head and neck imaging: MR imaging of the cochlear modilus: Area measurement in healthy subjects and in patients with a large endolymphatic duct and sac. Radiology 1999;213:819-23.  Back to cited text no. 18  [PUBMED]  [FULLTEXT]
19.Khalessi MH., Zarandi M. Motesaddi, Borghei P, Abdi S. Cochlear implantation in patients with inner ear malformations. Acta Medica Iranica 2004;42:188-97.  Back to cited text no. 19    
20.McElveen JT, Carrasco VN, Miyamoto RT, Linthicum FH Jr. Cochlear implantation in common cavity malformations using a transmastoid labyrinthotomy approach. Laryngoscope 1997;107:1032-6.  Back to cited text no. 20    
21.Maxwell AP, Mason SM, O'Donoghue GM. Cochlear nerve aplasia: Its importance in cochlear implantation. Am J Otol 1999;20:335-7.  Back to cited text no. 21  [PUBMED]  

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