The present paper describes a case demonstrating an abnormality of a large internal auditory meatus (LIAM) together with other inner ear anomalies. The otic capsule was affected by a widened, bulbar internal auditory meatus with loss of the bony wall dividing the lateral end of the meatus from the cochlea. The vestible was also abnormally dilated. The child suffered a progressive loss of cochlear function with apparent fluctuation of hearing and some episodes of sudden hearing loss.
The now well-recognised appearance of the large internal auditory meatus has previously been described in Australia by Beaumont and Whyte (1994) and Voyvodich et al (1996). The case here presented is that of a child with bilateral large internal auditory meatus (LIAM), absence of the crista falciformus, widening of the aperture at the base of the modiolus implying possible dehiscence of the lamina cribrosa, and mild dysplasia of the vestibule and cochlea. The clinical picture is of interest in that the child had a congenital hearing loss that progressively fluctuated and deteriorated.
When the child (D.R.) was six months old, her mother suspected a hearing loss. This was fully investigated by both behavioural assessment and brainstem evoked response audiometry by the age of two years. These tests indicated she had a severe-profound sensori-neural hearing loss (SNHL) and that the right ear was worse than the left. She was fitted with hearing aids. Prenatal and postnatal history were uneventful except that at eighteen months she did have a right ear infection with discharge. At age 4 years she was noted to have a sudden decrease in hearing, possibly following a fall, and since then her parents noticed that her hearing seemed to fluctuate and continued to deteriorate. Table I shows the progressive hearing tests. Electrocochleography (Ecochg) was performed which confirmed the profound hearing loss, with the right ear being worse than the left. Table 2 shows the Ecochg results, performed when the child was 5 years old. The child has been using both sign and oral communication.
The C.T. scan showed abnormally enlarged IAMs which were oval shaped in the axial plane (Figures 1, 2). There was an abnormal connection between both the cochlea and vestibule with the IAM on both sides. The crista falciformus (horizontal crest) and Bill's bar (the vertical crest) were missing (Figure 3). The connection between the LIAM and the base of the cochlea at the modiolus appeared dilated. The vestibule appeared slightly dilated and the cochleas showed a mild mondini type deformity with at least two turns present. The child also had normal vestibular aqueducts on both sides. A CT scan, performed when she was two years of age, showed hypoplasia of the vestibulo-cochlea complex with widened, symmetrical IAMs.
A right cochlear implant was inserted at age 6 years and 3 months. At the time of surgery, no perilymphatic gusher was found. She was implanted with the Cochlear(R) 124 and has subsequently performed well with the device.
Congenital hearing loss is accompanied by radiologically demonstrable bony inner ear abnormalities in only 15% of cases. Abnormalities of the IAM are seen in association with congenital hearing loss. An IAM which is narrowed to less than 2nun is significant as it is often associated with absence or hypoplasia of the acoustic nerve (Valvassori 1995). When considering a child for a cochlear implant, a very narrow IAM to this degree requires an MRI to check the integrity of the cochlear nerve.
Most reports in the literature concerning the dilated JAM refer to expansion secondary to an acoustic neuroma or other lesion occupying the meatus. However, Tomura et al (1995) point out that the size and shape of the IAM is known to be variable. They regarded diameters of more than 0.8mm in the mid portion of the meatus in the horizontal plane to be abnormal. The incidence of an enlarged IAM, greater than 0.8mm in the mid portion, was 2.3% in their series. They also emphasised that the presence of the crista falciformis and a normal cortical outline are two factors that are important in differentiating a "normal variant" from an "abnormally enlarged" IAM. The present case illustrates a markedly widened IAM with lack of the crista falciformis. The lateral bony wall, in some views, appeared non existent and certainly thinned in others. It is this combination of abnormal IAM enlargement with lateral fundus dehiscence which should be looked for when investigating a child with progressive hearing loss.
The development of the IAM post natally is thought only to progress in the media] portion of the meatus (Sakashita and Sando 1996). Sakashita and Sando studied the development of the internal acoustic meatus and found that the lateral portion of the fundus, lateral to the foramen singulare, does not grow. This area includes the region of the transverse crest and Bill's bar. The authors suggested that this was due to the area being surrounded by the otic capsule which is known to attain adult size by week 21 of gestation. The medial portion of the canal lengthens until about age 10 years of age. They described the IAM as oval if the mid portion was 0.5mm larger than both the porus and fundus diameters. Our case had an oval IAM, with the mid portion of the canal, measured on the axial plane, being 0.9cm and the porus measuring 0.6cm. The lack of further development of the lateral portion of the IAM suggests the presence of an associated congenital abnormality which may predispose the child to progressive and/or fluctuating hearing loss.
The more commonly recognised causes of progressive fluctuating hearing loss in children include autoimmune disease, perilymphatic fistulae and the large vestibular aqueduct syndrome (Gibson 1997). The widened IAM in the mid portion together with a lateral abnormal connection between the IAM and the cochlea and vestibule may allow CSF pressures to be transmitted to the membranous cochlea, although the lack of a perilymphatic gusher at time of cochlear implantation implied that there was no clearly direct communication between the CSF and the cochlear perilymph in the present case. Nevertheless, some degree of pressure transmission through abnormally thin bone or soft tissue may still be a reasonable hypothesis and cause damage to the fragile abnormal hair cells of the inner ear. In this regard, Crain et al (1990) reported a case of bilateral IAM enlargement in a patient with Paget's disease and believed that this was due to the pulsating CSF on the soft Pagetoid bone lining the IAM. This patient had experienced bilateral hearing loss. We therefore believe that where there exists a large IAM with abnormal communication between the IAM and the inner ear, a similar pressure transmission to the inner ear may be present and result in a fluctuating and eventually deteriorating hearing loss. Certainly a large internal auditory meatus with associated lateral dehiscence and enlargement of the vestibule needs to be excluded when assessing congenital hearing loss. These findings may help to predict cases which will experience further progressive hearing deterioration.
BEAUMONT G.D., WHYTE A.M. (1994) Radiology of the temporal bone. Aust. J. Otolaryng. 1:6, 513-526.
CRAIN M., DOLAN K. (1990) Internal auditory canal enlargement in paget's disease appearing as bilateral acoustic neuromas. Ann. Otol. Rhino. Laryng. 99,833-834.
GIBSON W.P.R. (1997) Scott-Brown's Otolaryngology. 6th Edn. A. Kerr, Chapter 11, pp 3/25/1-3/25/20.
SAKASHITA T., SANDO 1. (1996) Postnatal development of the internal auditory canal studied by computer-aided three-dimensional reconstruction and measurement. Ann. Otol. Rhinol. Laryngol. 104, 469-475.
TOMURA N., SASHI R., KOBAYASHI M., HIRANO H., HASHIMOTO M.N WATARI J. (1995) Normal variations of the Temporal Bone on High-Resolution CT: Their Incidence and Clinical Significance. Clinical Radiology 50, 144-148. VALVASSORI G. (1995) The internal auditory canal revisited. The High-Definition Approach. Otolaryngol. Clin. North Am. 28, 431-451.
VOYVODICH F., WHYTE A., BEAUMONT G.D. (1996) Bilateral internal auditory canal enlargement in the absence of intracanalicular turnout. Aust. J. Otolaryng. 2:4, 437-439.
A Hyperostosis is always situated at the inner end of te meatus, is of conical shape, never pedunculated, of ivory-like hardness, immovable on pressure, often found independent of other diseases of the ear, and when such are present they are not a cause of it. It is frequently multiple, and may exist with normal hearing. When deafness is due to the presence of this growth, it is the result of mechanical obstruction, either directly, or from its interfering with the escape of discharges from within, so that complete occlusion of the meatus follows, with danger to the structures behind it. It is only under the later circumstances that a removal of the growth is justifiable.
Exostosis, on the contrary, appears before the canal is completely ossified, situated near the junction of the bony with the cartilaginous meatus, is of a variety of shapes and always pedunculated. Before it is completely ossified it can be pierced to varying depths, and it is slightly movable on pressure, even after ossification is complete. It is nearly always complicated with other diseases, and attended with difficult hearing. The tumor is single.
The causes of these growths are variable. Hyperostosis is a result of gouty or rheumatic diathesis and perhaps syphilis; whereas exostosis is due to inflammatory changes of long standing.
ALBERT H. TUTTLE
Catherine S. Birman^ M.B.B.S., F.R.A.C.S. William P.R. Gibson^* M.D., F.R.A.C.S., F.R.C.S.
^ New Children's Hospital, Royal Alexandra Hospital for Children Westmead 2145
* University of Sydney, Department of Surgery Sydney 2001
Dr. Catherine Birman
E.N.T. Department, RAHC
P.O. Box 3515
Parramatta NSW 2124
Tel: 02 9845 0000 Fax: 02 9845 2102
Copyright Australian Society of Otolaryngology Head & Neck Surgery Ltd. Jul 1999
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