Vestibular schwannoma
A slow tumour produces a slow asymmetry — and an SVV that the audiogram cannot predict.
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Vestibular schwannoma is the SVV's hardest customer. A patient may have a sizeable tumour, a clear caloric weakness, and yet a near-normal static SVV. Understanding why is the lesson.
Schwannomas grow over years to decades. They produce a slowly increasing vestibular asymmetry. The brain rebalances slow asymmetries almost completely. Static SVV — a tonic measurement — gets compensated away. Dynamic vestibular function does not compensate as well.
The patient's clinical picture confirms this. Most schwannoma patients report progressive unilateral hearing loss and word recognition difficulty rather than vertigo. The vestibular system has been compensating quietly in the background for years.
Static SVV tilt magnitude correlates poorly with tumour size or caloric weakness. What it correlates with is which fibres are being compressed. A small tumour on the superior vestibular nerve can produce a measurable SVV tilt. A larger tumour confined to the cochlear or inferior vestibular fibres may leave it alone.
Dynamic SVV — centrifugation or off-vertical-axis rotation — unmasks the residual asymmetry. A static SVV of one degree may produce a dynamic SVV of five or six degrees. Where available, dynamic SVV is the more sensitive otolith test in suspected schwannoma.
Surgical removal of the tumour typically ablates the affected vestibular nerve, including the utricular afferents. The static SVV jumps to acute peripheral lesion magnitudes — eight to ten degrees ipsiversively — and then decays over weeks as compensation resumes.
Expected SVV signature
Static SVV in vestibular schwannoma is often near-normal even when caloric testing shows substantial unilateral weakness. Slow tumour growth allows central compensation to rebalance the tonic graviceptive asymmetry — so the magnitude of the static SVV tilt correlates poorly with tumour size, with audiometric loss, and with caloric weakness[5].
| Feature | Finding |
|---|---|
| Direction | Ipsilesional when tilted; often normal |
| Static magnitude | Typically 0–4°; can be larger with rapid growth or brainstem compression |
| Dynamic SVV | Unmasks larger residual asymmetries (5–8°) in compensated cases |
| Time course | Stable across visits; jumps acutely after surgical resection |
| Variability | Low; not a cerebellar pattern |
What does correlate is the proportion of utricular (superior vestibular nerve) fibres affected. A small tumour growing along the superior vestibular nerve may produce a measurable SVV tilt; a larger tumour confined to the cochlear or inferior vestibular nerve may leave the static SVV intact.
Why central compensation hides the lesion
Vestibular schwannomas grow over years to decades. The slow rate of afferent loss allows the central nervous system to compensate almost completely — the asymmetry signal that SVV measures is repeatedly rebalanced. This is why patients with sizeable schwannomas often report no vertigo, only progressive unilateral hearing loss and mild imbalance.
Dynamic SVV (eccentric rotation, or off-vertical axis testing) unmasks the compensated asymmetry — a 1° static tilt may become a 5° dynamic tilt. Where available, dynamic SVV is the more sensitive test for vestibular schwannoma.
Clinical use
- Pre-operative baseline — knowing whether utricular function is preserved informs the patient about post-surgical recovery time.
- Post-operative tracking — surgical removal of the tumour typically ablates the affected vestibular nerve. The SVV tilt jumps to acute UVL magnitudes (8–10°) and then decays over weeks as compensation resumes.
- Differentiating from Ménière’son the ipsilateral side: the schwannoma SVV is stable across visits, Ménière’s fluctuates.
Companion findings
- Asymmetric sensorineural hearing loss with poor word recognition out of proportion to the pure-tone loss
- Caloric weakness on the affected side (often the earliest vestibular sign)
- oVEMP reduced or absent (superior vestibular nerve)
- cVEMP often preserved unless the tumour involves the inferior vestibular nerve
- MRI with internal auditory canal protocol — diagnostic