Module 9 · Disease pages

Vestibular schwannoma

Slowly progressive unilateral vestibular loss
Clinical vignette
A 58-year-old man presents with two years of progressive right-sided hearing loss and recent intermittent unsteadiness. There has been no rotational vertigo. Audiometry shows an asymmetric high-frequency sensorineural loss on the right with reduced word recognition. vHIT shows reduced gain in the right lateral canal (0.62) with covert saccades, and a more marked reduction in the right posterior canal (0.55). Anterior canals are preserved bilaterally. MRI confirms a 14 mm right-sided cerebellopontine angle mass.
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What it is

Vestibular schwannoma is a benign tumour arising from Schwann cells on the vestibular nerve, typically within the internal auditory canal (IAC), with extension into the cerebellopontine angle as it grows. Annual incidence is around 1 per 100,000; bilateral schwannomas are pathognomonic of neurofibromatosis type 2. The presenting symptom is almost always asymmetric sensorineural hearing loss, sometimes accompanied by tinnitus or unsteadiness. True spinning vertigo is uncommon because the tumour grows slowly enough for central compensation to keep up.

The vHIT pattern

Schwannomas produce slowly progressive, often canal-selective vestibular loss on the affected side. The pattern depends on which nerve fascicles the tumour preferentially compresses or infiltrates. Most schwannomas arise from the inferior division, so the posterior canal is often the most severely affected canal, sometimes with relative preservation of the lateral and anterior canals — the opposite of what you see in superior vestibular neuritis.

Right vestibular schwannoma — affected side
0100200R latg=0.620100200R antg=0.810100200R postg=0.45
Right ear in a moderately sized vestibular schwannoma. Posterior canal most severely affected (inferior nerve origin), with lateral canal moderately reduced. Covert saccades dominate — slow tumour growth has allowed central compensation. Anterior canal relatively preserved.

Why the saccades are usually covert, not overt

Acute lesions show overt saccades because the brain has had no time to adapt. Schwannomas grow over years and the VOR reorganises continuously: the brain pre-programs corrective saccades to start during the head movement rather than after it. By the time most schwannomas are diagnosed, covert saccades dominate the trace[Weber KP 2008]. A patient with a large CPA mass may have remarkably few clinical complaints because their central compensation has done such an effective job. This is one reason vestibular schwannoma is so often missed until imaging is obtained for asymmetric hearing loss.

Side-finding when calorics and vHIT disagree

Schwannomas can produce dissociated peripheral testing: calorics may show severe reduction on the affected side, while vHIT shows mild or moderate reduction. The mechanism is different from Meniere disease (where vHIT is the preserved side) — here, the chronic timecourse simply allows the high-frequency machinery to recover or compensate more fully than the low-frequency system. Either way, severity is best judged by integrating both tests rather than relying on one.

Audiogram

Asymmetric high-frequency sensorineural hearing loss is the most common audiometric pattern, though any unilateral SNHL in an adult should raise the question of schwannoma. Word recognition is often disproportionately poor compared to the pure-tone thresholds — the "rollover" phenomenon — and is a useful supplementary clue.

2505001k2k4k8k020406080100120Frequency (Hz)Hearing level (dB HL)○ right× left
Asymmetric high-frequency sensorineural hearing loss on the right, with normal hearing on the left.

Key teaching points

  • Slow growth means central compensation has had time to develop; covert saccades dominate.
  • vHIT pattern is canal-selective and reflects which nerve fascicles the tumour compresses; posterior canal often most affected.
  • Pre-op vHIT gain predicts the severity of post-resection vestibular crisis; relevant for counselling and rehab planning.
  • Any unilateral progressive SNHL in an adult needs MRI to exclude schwannoma — vestibular testing supports but does not replace imaging.

References

  1. Halmagyi GM, Chen L, MacDougall HG, Weber KP, McGarvie LA, Curthoys IS. The video head impulse test. Frontiers in Neurology 2017;8:258. doi:10.3389/fneur.2017.00258
  2. Curthoys IS, Manzari L. Clinical application of the head impulse test of semicircular canal function. Hearing, Balance and Communication 2017;15(3):113–26. doi:10.1080/21695717.2017.1353774
  3. MacDougall HG, McGarvie LA, Halmagyi GM, Curthoys IS, Weber KP. The video head impulse test (vHIT) detects vertical semicircular canal dysfunction. PLoS One 2013;8:e61488. doi:10.1371/journal.pone.0061488
  4. Weber KP, Aw ST, Todd MJ, McGarvie LA, Curthoys IS, Halmagyi GM. Head impulse test in unilateral vestibular loss: vestibulo-ocular reflex and catch-up saccades. Neurology 2008;70:454–63. doi:10.1212/01.wnl.0000299117.48935.2e
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