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Electronystagmography findings in acoustic neuromas.
The first symptoms of an acoustic neuroma are usually tinnitus, deafness and occassionally vertigo. One always endeavours to make a diagnosis of this tumour when it is relatively small; ideally, when it is still within the confines of the internal auditory canal. In such a tumour limited to the canal, it is possible to remove the tumour preserving the integrity of the cochlear and facial nerves by a middle cranial fossa approach. If the tumour is detected when slightly larger, it can still be totally removed by the translabyrinthine route preserving the integrity of the facial nerve. Here, we would like to stress that any patient suffering from an unexplained unilateral sensorineural hearing loss or tinnitus falls in the group suspected to have an acoustic neuroma. It is only by having such a high suspicion index that one can hope to detect an acoustic neuroma in its earliest stage. In the majority of cases, the tumour arises from the vestibular portion of the eighth nerve, yet it causes deafness and tinnitus more often than vertigo. This is because it is usually a very slow growing tumour and the gradual loss of vestibular function is well compensated by central mechanisms. Electronystagmography (ENG) is a sensitive, reliable, objective and valuable method of investigating the vestibular system in a patient suspected to have an acoustic neuroma. This paper deals with the ENG findings in cases of surgically confirmed acoustic neuroma. The results and significance of the various ENG tests viz. pendular eye tracking, tests for spontaneous, gaze and positional nystagmus, and the bithermal caloric test are discussed.
About 400 patients are investigated every year in our vertigo clinic for neuro-otological complaints. Each patient undergoes a thorough neuro-otologic evaluation including a detailed history, an ENT, general and neurological examination and biochemical, radiological, audiological and vestibular testing. A multi-channel machine, RACIA polygraph type ENG 74-5 has been used for ENG recording. The pendular eye tracking test and biocalibration are done using an optokinetic stimulator. It utilizes a moving point of light (LED) on a screen mounted such that the two extremes of the screen subtend an angle of 30° at the nasion when the patient is seated 1 meter away. This is followed by testing for spontaneous nystagmus with eyes closed and eyes open in the caloric position, gaze nystagmus (20° gaze deviation) and positional nystagmus in five positions-supine, sitting erect, supine with the head turned right lateral, supine with the head turned left lateral and supine with the neck hyper-extended. Recording is done in each position with eyes closed and eyes open. The bithermal caloric test is performed using 20 ml of water at 44°C and 30°C to irrigate the tympanic membrane over a period of 30 seconds. The irrigation is done through a 3 mm diameter catheter introduced into the external auditory canal with its tip lying just a millimetre away from the tympanic membrane. The caloric test results are evaluated using culmination frequency as the parameter and are plotted on the butterfly chart.' The normal values of these various ENG tests as standardized for our laboratory and for our population are shown in [Table - 1].[2],[3]
Over the last five years, 2194 patients underwent the complete battery of neuro-otological investigations. Of these, 30 were diagnosed as having acoustic neuroma. These were subsequently confirmed at surgery or post mortem examination. Thus, the incidence of acoustic neuroma in our neuro-otology clinic is 1.4% of all vertigo patients. There were 20 males and 10 females giving a male to female ratio of 2:1. Two patients (7%) had bilateral neuroma. Pendular eye tracking test: We classify pendular tracings into Types I through IV[3] [Fig. 1]. Twenty patients (66.7%) had a normal pendular tracing (Types I, II). In 7 cases (23.37c), the pendular tracing was superimposed by multiple jerky beats giving a cog wheel appearance (Type III) indicating a central lesion. The pendular tracing was grossly disorganised and ataxic in 3 cases (10%) indicating an extensive central lesion i.e. brainstem compression by a large tumour. Spontaneous nystagmus: Significant horizontal spontaneous nystagmus[2] (more than 19 beats/30 seconds) in one direction was noticed in 12 patients (40%). In five cases, the direction of the nystagmus was towards the side of the tumour; in six, away from the tumour and in one, the nystagmus was direction-changing. Seven patients had vertical spontaneous nystagmus with or without accompanying horizontal component. A failure of visual fixation suppression occurred in 2 patients (7%) who were found to have very large neuromas. The gaze test: Significant gaze nystagmus was noted in 12 patients (40%). All these patients had large neuromas pressing on the cerebellum or the brainstem as proved subsequently at surgery or post mortem. Positional test: Twenty one patients (70%) demonstrated significant positional nystagmus in at least 3 out of 5 positions tested. Their nystagmus varied as regards latency of appearance, duration, reproducibility, and fatiguability, and no definite distinction could be made between the peripheral and central types of positional nystagmus, nor was three any characteristic pattern discernable. Bithermal caloric test: Twenty-seven patients had reduced caloric responses on the tumour side. Of these, 25 had hypoactive and 2 had absent caloric responses. In 3 patients, the caloric responses elicited from the side of the tumour were within the normal range. Eleven cases had normal contralateral responses, 10 showed hyperactive contralateral responses and 9 had hypoactive contralateral responses. The significance of these findings is discussed later. It may be recalled that in 3 patients the ipsilateral caloric responses were normal. In two of these cases, the responses from the contralateral side were also within the normal range, but the tumour side showed comparatively weaker responses than the normal side. In the third patient, the caloric responses from the tumour side were normal whereas the contralateral responses were hyperactive. At surgery, this patient was found to have a fairly large neuroma arising from the inferior vestibular nerve and compressing the brain stem.
The early diagnosis of acoustic neuroma is important. Even though the majority of the tumours arises from the vestibular rather than the cochlear nerve, the number of reports in the literature on details of ENG findings in acoustic neuromas are relatively few. Linthicum and Churchill[4] reported the results of vestibular function tests in 200 cases of acoustic neuroma. Electronystagmography was used in 149 cases. They found spontaneous nystagmus in 14 cases and positional nystagmus in 32 cases. On caloric testing, 82% showed a reduced vestibular response of 3070 or more. They reported that the frequency of the reduced vestibular response was directly related to the size of the tumour. Shiffman et al[6] reported the results of the caloric test in 31 cases of confirmed acoustic neuromas. They used the Fitzgerald-Hallpike method in 5 patients and ENG in the rest. Twenty-nine patients had a reduced ipsilateral caloric response. Linthicum et al[5] reported that 82% of patients with surgically verified acoustic tumours showed a reduced ipsilateral vestibular response of 30% or more on caloric testing. They stressed the importance of both cold and warm water irrigation. However, most of the reports in the literature stress on the caloric test findings alone in acoustic neuromas. We feel that ENG is valuable in the diagnosis of acoustic neuromas. Though the bithermal caloric test is the most important and informative ENG test, the other tests provide a fund of information and merit equal mention. In our series of 30 cases, 16 were found to have large neuromas compressing the cerebellum or brainstem as proved on surgical exploration or at autopsy. The ENG showed evidence of a central lesion in these patients in the form of an abnormal pendular eye tracking test, an abnormal gaze test, failure of visual fixation suppression or hyperactive contralateral caloric response. The pendular eye tracking test, if abnormal, (Types III & IV-see [Fig. 1]) indicates a central lesion. This was seen to occur in 10 patients, each of whom had a large acoustic neuroma compressing the cerebellum and/or the brain stem. Significant spontaneous nystagmus was recorded in only 12 patients (40%). Linthicum et al[5] reported recordable spontaneous nystagmus in only 10% of their cases. In our series, the direction of the spontaneous nystagmus beat was nonlocalising, beating towards the side of the tumour in 5 cases and away from the tumour in 6 cases. An abnormal gaze test suggests a central lesion. This occurred in 12 of our patients (40%) and was subsequently correlated with the presence of a large neuroma compressing the cerebellum or, the brainstem. The positional nystagmus test should be interpreted with a certain amount of caution. Though 70% of our patients domostrated a significant positional nystagmus, it was not possible in every case to draw a definite conclusion regarding its peripheral or central origin. It was also difficult in a given case of peripheral type of positional nystagmus to correlate the direction of the nystagmus with the side of the tumour. Hence, we consider the presence of positional nystagmus as a nonspecific evidence of a vestibular system disorder with little localising value and also stress its common occurrence in cases of acoustic neuromas. The bithermal caloric test is the most important and valuable test of the entire ENG test battery. Twenty seven patients (90%) had the anticipated ipsilateral, hypoactive, warm and cold, caloric responses. Attention, however, must be paid to the caloric responses elicited from the normal ear. Normal contralateral caloric responses are to be expected [Fig. 2]. They were seen in 11 cases (36.7%), all of whom had small or moderate-sized neuromas. Interestingly, 10 cases (33.3%) demonstrated hyperactivity of both warm and cold caloric responses on the contralateral side [Fig. 3]. These patients were found to have fairly large neuromas compressing the brain stem which probably caused compression of the cerebello-vestibular fibres to the contralateral vestibular nuclei. These cerebello-vestibular fibres passing through the inferior cerebellar peduncle cross in the midline close to the restiform body and are mainly inhibitory. A lesion in these fibres as is caused by a large acoustic neuroma leads to a release of the contralateral vestibular nuclei from the inhibitory influences of the cerebellum, thus explaining the noninhibition of the contralateral caloric responses. Bilateral hypoactivity [Fig. 4] of caloric responses was found in 9 patients (30%). Of these, only 2 patients had bilateral acoustic neuromas. A variety of factors could explain the contralateral hypoactivity in the remaining 7, viz., associated raised intracranial tension, lack of alertness during the test, and, drugs like barbiturates. Although all our patients were instructed to stop all drugs for 3 days prior to the ENG, some were unable to do so and were forced to take anticonvulsant drugs as they had undergone diagnostic neurosurgical procedures or a shunt operation. A normal caloric response does not rule out an acoustic tumour. A neuroma situated medial to the porous acoustious [Fig. 5] and arising from the inferior vestibular or cochlear nerve may give a normal caloric response since the impulses resulting from lateral semicircular canal stimulation pass in the superior vestibular nerve which may yet be free from compression. This occurred in 3 of our patients (10%). Thus, in conjunction with audiological and radiological findings, ENG is a valuable aid in establishing a definitive diagnosis of an acoustic neuroma and in evaluating its probable size and extent. Hence the complete ENG test battery should be performed in every case suspected to have an acoustic neuroma.
We are grateful to the Dean, Seth G.S. Medical College and K.E.M. Hospital for permission to use the hospital records. We are thankful to Dr. Ravi Ramakanthan and the Departments of Radiology and Neurology for their help and co-operation.
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