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Peripheral vestibular hypofunction can be identified by asymmetric vestibular responses to caloric irrigation of the horizontal semicircular canal or by the head impulse test.1 Whereas the first test investigates the low frequency function of the vestibulo‐ocular reflex (VOR) the latter assesses the high frequency function. The high frequency VOR function is usually more persistently impaired in unilateral vestibular lesions (eg, vestibular neuritis) than the low frequency function and thus the more sensitive parameter to detect chronic peripheral vestibular deficits.2
Contrary to this current vestibular knowledge, we present a patient with a chronic peripheral vestibular deficit showing normal high frequency but impaired low frequency VOR function. This unusual lesion pattern was caused by an intralabyrinthine schwannoma.
A 47‐year‐old patient presented with a 3 month history of repetitive episodes of vertigo, nausea and lateropulsion to his right side. The first episode lasted 1.5 h followed by short lasting (seconds to 15 min) episodes once a day. The patient reported a similar episode of vertigo 1 year previously, at that time associated with an acute hearing loss on the right side and persisting tinnitus ever since. Apart from moderate right sided hypacusis, neurological examination was normal in the asymptomatic stages during follow‐up examinations over the next 2 years. Particularly, there was no spontaneous or head shaking nystagmus and no gaze evoked nystagmus.
After written informed consent was obtained according to the Declaration of Helsinki, cochlear functions were assessed by audiogram (including speech discrimination score) and brainstem evoked response audiometry, otolith function by click evoked myogenic potentials, fundoscopy and the subjective visual vertical, semicircular canal function by caloric irrigation and scleral search coil recordings (Remmel Labs, Maryland, USA) with three dimensional coils (Skalar, Delft, the Netherlands).3 Eye‐in‐space and head‐in‐space position were used to calculate eye‐in‐head position. High acceleration (6000–9000°/s2), small amplitude (15–25°) head thrusts in the horizontal canal planes were used to analyse individual semicircular canal function. Vestibular hypoexcitability was assessed by caloric irrigation using Jongkees's formula, and the directional preponderance is given in per cent.
Follow‐up caloric irrigation tests (n=4) revealed a caloric hypoexcitability of the right side which remained largely stable over a course of 3 years (mean 55% (SD 8%); range 43–61% canal paresis). In contrast, scleral search coil recordings (fig 1A, BB)) during head impulse testing showed normal VOR gain to both sides (right 0.79 (0.07); left 0.78 (0.09)) compared with six age matched control subjects (right 0.86 (0.09); left 0.84 (0.08)).
Pure tone audiometry revealed a right sided sensorineural pancochlear hearing loss with a pure tone average of 60 dB. The patient's speech discrimination score was 50% (at 95 dB). Audiometry of the left ear was normal. Acoustic evoked potentials revealed slightly prolonged interpeak latencies I–III on the right side. Click evoked myogenic potentials, subjective visual vertical and fundoscopy were normal.
High resolution MRI showed a small lesion in the right labyrinthine vestibule with typical features of an intralabyrinthine schwannoma. A contrast enhancing mass with a diameter of 3 mm was seen on the T1 weighted image (fig 1C1C).). The tumour completely obliterated the vestibule and partially grew into the ampulla of the horizontal semicircular canal, but largely spared the rest of the vestibular nerve and the cochlea, as depicted on the T2 weighted image (fig 1D1D).). High resolution CT (slice thickness 1 mm) showed a normal bony labyrinth on both sides. Cranial MRI did not reveal intracranial lesions.
The unusual finding is selective impairment of the VOR in the low frequency range whereas the high frequency VOR remained intact. Head impulses are small amplitude movements with high acceleration that are used to identify peripheral vestibular lesions. The head impulse test1 can easily be conducted at the bedside and has a moderate sensitivity (35–45%) and high specificity (>90%). Sensitivity, however, can be improved by up to 100% when quantitative data of the VOR are obtained during the head impulse with the scleral coil technique.2 Accordingly, we can exclude a false negative head impulse test in our patient.
The widely used caloric irrigation is usually sufficient to identify vestibular hypoexcitability in the acute stage but may fail to identify vestibular hypofunction in the chronic stage.2 In vestibular neuritis, head impulses always reveal vestibular hypofunction at the time of caloric hypoexcitability but they remain capable of identifying this deficit in the chronic stage when responses to caloric irrigation may have become normal. This sign is probably related to the frequency dependence of dynamic vestibular imbalance after vestibular neuritis.
Conversely, high accelerating VOR was normal in our patient but caloric irrigation consistently revealed a unilateral vestibular hypoexcitability. This is the first report of selective impairment of unilateral low accelerating VOR in a vestibular schwannoma which grew from the schwann cell of the nerve. Evidence for an intralabyrinthine schwannoma came from the contrast enhancing mass with no signs of bony destruction in the CT scan of the temporal bone. These rare tumours arise from the proximal portion of either the vestibular nerve or the cochlear nerve. The patient's lesion involved the horizontal semicircular canal and the most proximal part of the vestibular nerve while sparing the largest portion of the nerve. A similar dissociation between low and high accelerating VOR function can occasionally be seen in Meniere's disease. Thus vestibular afferents conveying the high and low frequency VOR from the horizontal semicircular canal (ampulla) may be anatomically different and functionally dissociated before they enter the vestibular nerve. It cannot be deduced from this case whether the selective low frequency VOR deficit is related to (i) a frequency dependent vulnerability of type I hair cells,4 (ii) conduction loss of low frequency signals caused by damaged myelin or (iii) a compressive lesion of the primary vestibular afferents. A mechanical lesion might affect a subset of (eg, irregular) vestibular fibres possibly encoding the low frequency movements as high frequency responses are preserved after ablation of irregular afferents.5 Alternatively, the frequency dependence of vestibular compensation may be different in small chronic peripheral vestibular lesions of the semicircular canal which spare the vestibular nerve. The otoliths probably do not contribute as our clinical test of saccular and otolithic function was normal.
Competing interests: None.