Neurnradiologg

Neuroradiology 11,205-207 (1976)

© by Springer-Verlag 1976

Arachnoid Cyst Simulating lntrameatal Acoustic Neuroma H. O. M. Thijssen, E. H. M. Marres and J. L. Slooff Departments of Neuroradiology, E. N. T. and Neuropathology, University of Nijmegen, Holland

Summary. A very rare cause of perceptive deafness was found in a child with an intrameatal arachnoid cyst. This case is the second reported in the literature.

Key words: Acoustic neuroma, Arachnoid cyst.

Perceptive deafness can be caused by cochlear or retrocochlear lesions. The latter may be situated in the path of the acoustic nerve which is located in the internal acoustic meatus, in the .part which passes through the pontocerebellar cistern or in the brain stem or cerebral hemisphere. In this communication our interest is focussed on lesions causing a progressive loss of hearing, which are situated between the cochlea and brain stem, therefore are located in the internal meatus and pontocerebellar cistern. The lesion most frequently found in this location originates from the nerve itself and is the acoustic neuroma. Lesions originating from the surrounding structures include meningiomas of the dura around the internal acoustic porus and tumours in the pontocerebellar cistern such as ependymomas, astrocytomas, epidermoids, metastases and arachnoid cysts. The occurrence of arachnoid cysts in the pontocerebellar cistern is well known [1, 2, 3, 4, 6]. A very rare cause of progressive retrocochlear deafness, however, is an arachnoid cyst in the internal acoustic meatus. A 7 y e a r old girl suddenly developed flattening of the nasolabial fold and drooping of the mouth on the left side of the face, together with impaired closure of the eyelids. This had occurred previously when she was one year old. At the age of six, according to her parents, her hearing was found to be normal at a school medical examination. There were no complaints of vertigo and no tinnitus.

Neurological examination revealed a complete peripheral facial paralysis on the left side. On otological examination the tympanic membranes appeared to be normal. Total left sided deafness was found on audiometric examination. Electromyography revealed complete denervation of the left facial nerve. The electronystagmographic examination showed a complete disappearance of the labyrinthic function on the left side. The cerebrospinal fluid was normal. The radiological examination revealed dilatation of the left internal acoustic meatus with its maximum immediately next to the fundus of the meatus. The height of the meatus at that point was 11.5 mm and the corresponding height of the right meatus was 3 mm (corrected for the enlargement of 1.3x; normal value 2.9 mm, with a maximum difference of 1 ram, also corrected for enlargement; 7, 8). (Figs. 1 and 2). On meatocisternography with Duroliopaque it was found that the pontocerebellar cistern was normal in shape. There was, however, no filling of the arachnoid space in the internal acoustic meatus. A slightly convex impression was visible in the contrast medium at the level of the porus (Fig. 3). A space-occupying process was diagnosed in the left internal acoustic meatus. This was therefore most probably an acoustic neuroma. A few weeks later an operation was performed via the translabyrinthine route. A cyst was found in the internal meatus. The superior and inferior vestibular nerves and the cochlear nerve could no longer be found; a few fibres of the facial nerve were still visible. The internal meatus was closed with artificial dura. On histological examination the cyst wall was found to be built up of a thin edge of loosely woven, poorly vascularized connective tissue covered with flat to cubical epithelium in one or several layers, which showed similarities to arachnoid cells. Some foam cells were found in the connective tissue (Fig. 4).

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H . O . M . Thijssen et al.: Arachnoid Cyst Simulating Intrameatal Acoustic Neuroma

Fig. 1, 2. Dilatation of the left internal acoustic meatus l

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Fig. 3. Nonfilling of the left internal acoustic meatus on meatocisternography Fig. 4. Cyst wall built up of loosely woven connective tissue with some foam cells, covered with epithelium simular to arachnoid cells. 128 x 3

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Discussion

An arachnoid cyst in the internal acoustic meatus, which gives the impression of a tumour and causes a paralysis of the facial nerve and the acoustic nerve, has been found only once before [6]. In this case, too, there was a marked dilatation of the internal meatus. The occurrence of arachnoid adhesions and cysts in the pontocerebellar cistern is equally rare [1, 2, 3, 6]. At the Mayo Clinic [4] only twenty cases with an arachnoid cyst in the posterior fossa were operated on between 1932 and 1972. In six cases the cyst was located in the pontocerebellar cistern. These arachnoid cysts may be congenital or be acquired as a result of inflammation or trauma. They can be so large that they give rise to erosion of the petrous pyramid [10]. The combination of an arachnoid cyst and an acoustic neuroma in the pontocerebellar cistern has also been established [9]. The cysts can occur unilaterally in the pontocerebellar cistern.

It does not appear to be possible to differentiate between an acoustic neuroma and an arachnoid cyst located in the pontocerebellar cistern on the basis of the clinical data. According to Wilner [9], differentiation can be arrived at only by means of radiological data. He maintains that a widening of the internal acoustic porus is always the result of an ocoustic neuroma. According to Metzger [5], however, 5 % of all acoustic neuromas originate from the cisternal part of the acoustic nerve (Brunner type), and these do not always erode and widen the internal porus. Non-filling of the internal acoustic meatus and the pontocerebellar cistern on meatocisternography, with a normal internal porus, attests to the presence of an arachnoid cyst in the pontocerebellar cistern. When there is bilateral nonfilling of the internal meatus in this examination, there is a strong indication of diffuse arachnoiditis, possibly accompanied by cyst formation. The unilateral widening of the porus and internal acoustic meatus is a strong argument for the presence

H. O. M. Thijssen et al.: Arachnoid Cyst Simulating Intrameatal Acoustic Neuroma

of an acoustic neuroma. The fact that this is not absolute, however, is illustrated by a patient [2] without any loss of hearing, but with a striking unilateral widening of the meatus and porus, in whom a completely normal filling of the arachnoid space up to the fundus of the meatus was demonstrated by meatocisternography. Our case, like that published by Sumner [6], also proves that this widening can be due to other causes than an acoustic neuroma. According to Wilner [9], we can imagine that no changes in the osteal boundaries of the porus and internal acoustic meatus occur in the case of arachnoid cysts located in the pontocerebellar cistern; as soon as the arachnoid cyst exists in the internal meatus itself, however, changes in these structures can occur. The combination of widening of the internal meatus and nonfilling during meatocisternography, which occurred in our case, is a certain indication of the presence of a tumour in the meatus. We wondered whether it is possible, in retrospect, to differentiate between an intrameatal acoustic neuroma and an arachnoid cyst which is also located intrameatally. It is just as impossible to make this differentiation on clinical grounds when these turnouts are located in the meatus itself as it is, according to Wilner [9], to distinguish between an arachnoid cyst in the pontocerebellar cistern and an acoustic neuroma. Is it possible to find any difference in the radiological signs between the intrameatal arachnoid cyst and the similarly located acoustic neuroma? Both cases with an arachnoid cyst in the meatus, our patient and the one described by Sumner [6], have some findings in common: a) The maximum widening is immediately next to the fundus; the cysts have therefore developed deep in the meatus. b) The dilatation of the internal meatus is abnormally large; in one case the maximum height is 10.7 mm and in the other 11.5 mm (corrected for enlargement). c) The wall boundary of the widened meatus is intact, indicating an expansive development of the tumour. In our view, these findings are not specific for the intrameatal arachnoid cyst. The acoustic neuroma can also develop immediately beside the fundus of the meatus and cause a similar deformation of the internal meatus [5]. Acoustic neuromas can develop in several directions from their place of origin, generally in the upper compartment of the internal meatus: in the caudal or

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cranial direction, in the direction of the porus or the petrous pyramid. The question of whether the location immediately beside the fundus in the two cases described with an arachnoid cyst in the meatus is a preferred site cannot be answered until more extensive material is available.

Conclusion In cases of progressive retrocochlear deafness accompanied by a widening of the acoustic meatus and nonfilling on meatocisternography, the possibility of an intrameatal arachnoid cyst must be considered along with the possible presence of an acoustic neuroma. No differentiation can be made either on clinical or on radiological grounds.

References 1. Bengochea, F. C., Blanco, F. L.: Arachnoidal cysts of the cerebellopontine angle. J. Neurosurg. 12, 66-71 (1955) 2. Frazer, R. A. R., Carter, B. L.: Unilateral dilatation of the internal auditory canal. Neuroradiology 9, 227-229 (1975) 3. Baker, H. L.: Cerebellopontine angle myelography. J. Neurosurg. 36, 614-624 (1973) 4. Little, J. R., Gomez, M. R., Mac Carty, C. S.: Infratentorial arachnoid cysts. J. Neurosurg. 39, 380-386 (1973) 5. Metzger, J., Sterkens, J. M., Dorland, P., Pertuiset, B., Dufour, M.: Les neurinomes de l'acoustique et leur diagnostique diffdrentiet. Trait6 de Radiodiagnostique, Tome 17-1. Paris: Masson 1974 6. Sumner, T. E., Benton, C., Marshak, G.: Arachnoid cyst of the internal auditory canal producing facial paralysis in a three-year-old child. Radiology 114, 4 2 5 4 2 6 (1975) 7. Valvassori, G. E.: Benign tumours of the temporal bone. Radiol. Clinics N. A. 12, 533-543 (1974) 8. Valvassori, G. E.: referred in Table Ronde du Colloque d'Anatomie radiologique de la t6te. J. Radiol. Electrol. 54, 765-797 (1973) 9. Wilner, H. I., Kashef, R.: Unilateral arachnoidal cysts and adhesions involving the eighth nerve. Amer. J. Roentgenol. 115, 126-132 (1972) 10. Wolfowitz, B. L., Solomon, A.: Erosions of the petrous temporal bone. S. Afr. Med. J. 48, 1799-1802 (1974) Received: March 31, 1976

Dr. H. O. M. Thijssen Department of Neuroradiology Institute of Diagnostic Radiology Geert Grooteplein Zuid 18 Nijmegen, The Netherlands

Arachnoid cyst simulating intrameatal acoustic neuroma.

Neurnradiologg Neuroradiology 11,205-207 (1976) © by Springer-Verlag 1976 Arachnoid Cyst Simulating lntrameatal Acoustic Neuroma H. O. M. Thijssen,...
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