vHIT Atlas

What this chapter covers

vHIT has two distinct roles in surgical practice: pre-operative assessment of remaining vestibular function (which guides counselling and rehab planning) and post-operative monitoring of compensation. This chapter walks through the patterns expected after three common procedures: translabyrinthine schwannoma resection, surgical labyrinthectomy, and intratympanic gentamicin treatment for Meniere disease.

Translabyrinthine schwannoma resection

Translabyrinthine surgery sacrifices hearing and ablates the labyrinth on the operated side. Immediately post-operatively, vHIT shows profound loss in all three canals on the operated side, with prominent overt saccades. The intact side, of course, remains normal. Over the following weeks to months, central compensation develops and the saccadic pattern shifts from overt to covert^{[Weber KP 2008]}.

Six weeks after right translabyrinthine resection

Profound right-sided loss across all three canals with large overt saccades. Recovery of vestibular function is not expected; compensation will develop over months.

Twelve months post-op — same patient

Canal function is unchanged. The saccades, however, have migrated from overt to covert as central compensation has reorganised the corrective response. Bedside HIT now looks much less abnormal.

Pre-operative vHIT predicts the post-operative crisis

Patients whose pre-operative vHIT shows substantial residual canal function will experience a marked vestibular crisis after translabyrinthine resection — sudden severe vertigo, nausea, ataxia — because the brain has had no time to compensate. Patients whose tumours have already destroyed most of the canal function pre-op tolerate surgery much better because they have effectively already compensated.

This is clinically useful in counselling: pre-op vHIT lateral canal gain ≥ 0.6 predicts a significant post-op vestibular crisis with 1–2 weeks of severe symptoms; gain < 0.3 predicts a much milder post-op course. Some centres offer pre-habilitation with daily gaze-stabilisation exercises in the weeks before surgery for patients with high residual function.

The compensation timecourse

After surgical or pharmacological ablation of one labyrinth, the timecourse of compensation is fairly consistent across patients:

0–7 days: severe symptoms, spontaneous nystagmus toward the intact side, dramatic overt saccades on vHIT.
1–4 weeks: spontaneous nystagmus subsides, gait improves. Static compensation largely complete.
1–6 months: dynamic compensation. Overt saccades on vHIT progressively replaced by covert. Functional recovery for activities of daily living.
6–12 months: covert saccades dominate. Bedside HIT may look near-normal. Provocative tests (oscillopsia challenge, dynamic visual acuity) reveal persistent deficit.

SHIMP testing (see HIMP vs SHIMP chapter) is particularly informative beyond month 6: HIMP gain may have improved 0.1–0.2 above the immediate post-op nadir, but SHIMP reveals the true persistent canal deficit through absent anti-compensatory saccades. For rehab-graduating decisions, SHIMP is more honest than HIMP.

Intratympanic gentamicin for Meniere disease

Intratympanic gentamicin is increasingly used as a vestibular-ablative treatment for refractory Meniere disease, with the goal of producing enough chemical labyrinthectomy to stop episodic attacks without rendering the patient bilaterally areflexic. vHIT is the ideal monitoring tool: serial measurements every 2–4 weeks during a titration course allow the clinician to stop at the point where lateral canal gain on the treated side has fallen to ~0.4–0.5 — usually sufficient for symptom control. Higher cumulative doses, which fully ablate the canal, are now avoided because the resulting oscillopsia is often worse than the original disease.

Cochlear implantation and the vestibular consequences

Cochlear implantation can affect vestibular function on the implanted side. Reported rates of post-implant vestibular hypofunction range from 10–40% depending on surgical technique and electrode design. Routine pre-operative vHIT is now common in many centres, and the information feeds into ear-of-implant decisions for bilateral candidates — implanting the worse-vestibulated side first preserves the better-vestibulated one as a safety net.

Key teaching points

Post-ablation, expect profound ipsilateral loss across all three canals with overt saccades initially; compensation develops over 6–12 months.
Saccades migrate from overt to covert over the year; canal function does not recover.
Pre-op vHIT predicts post-op crisis severity; pre-habilitation may help high-function patients.
Intratympanic gentamicin titration: use serial vHIT to stop at gain 0.4–0.5 on the treated side, avoiding bilateral ablation.
Cochlear implantation candidates: pre-op vHIT informs ear-of-implant choice for bilateral candidates.

Post-surgical vHIT