Saccades and VOR gain
Two things determine the answer
Every vHIT trace tells you two things, and you need to read both. The first is VOR gain — how well the eye velocity matches the head velocity. The second is corrective saccades— whether and when the eye snapped back to the target after the VOR failed to hold it. A canal can have low gain with prominent saccades (acute uncompensated loss), low gain with subtle saccades (chronic compensated loss), or even an apparently "normal" gain that is artefactually inflated by an embedded covert saccade. Reading saccades is what separates good vHIT interpretation from numerical gain-watching.
VOR gain: what it tells you
Gain is intuitive: if the eye moves exactly as fast as the head, gain is 1.0 and gaze stays locked on the target. If the eye moves at half the speed of the head, gain is 0.5 and gaze drifts. A useful five-tier classification, broadly aligned with the vHIT literature, is:
- Normal — gain ≥ 0.80
- Mild loss — gain 0.70 to 0.79
- Moderate loss — gain 0.40 to 0.69
- Severe loss — gain 0.20 to 0.39
- Profound loss — gain < 0.20
These cut-offs are for the lateral canal in adults. The thresholds for vertical canals run slightly lower across the board. Always cross-check against any age-specific reference norms in your local system.
Corrective saccades: overt and covert
When the VOR fails to keep gaze on the target, the brain compensates with a saccade — a rapid, ballistic eye movement that brings gaze back. vHIT distinguishes two types based on timing:
- Overt saccade: occurs after the head has stopped, typically at 270 to 350 ms after impulse onset. Visible at the bedside as a corrective "catch-up" eye movement and the basis of the original bedside head impulse sign.
- Covert saccade: occurs during the head movement, typically at 80 to 200 ms after impulse onset[Weber KP 2008]. Hidden to the naked eye because it's buried inside the head impulse — the bedside examiner doesn't see it because the head is still moving. The patient's gaze ends up back on target, so the bedside head impulse test looks normal, even though canal function is impaired. This is the principal limitation of bedside HIT that vHIT solves.
Try the eye-versus-head simulator. Start with the "Normal" preset, then add an overt saccade by raising the slider. Move to "Compensated (covert)" and notice how the eye velocity briefly spikes during the head movement — that spike is the covert saccade. Then mix them: a compensating patient often shows both, with the covert saccade growing larger and the overt one shrinking as the months pass.
Saccade pattern over time
Saccades evolve. In the acute phase of a vestibular lesion, overt saccades dominate — they are large, late, and obvious. Over weeks to months, central compensation occurs and the brain re-times its corrective saccades so they begin earlier and earlier, eventually becoming covert. Manzari et al. tracked this transition longitudinally in vestibular neuritis and showed a progressive shift from overt-only at week 1 to covert-predominant by month 12[Manzari L 2013].
The clinical implication: a covert-only pattern doesn't mean the lesion has recovered, it means the central compensation has improved. The underlying canal function may still be severely deficient. SHIMP testing (next chapter) was designed in part to unmask this — it shows residual canal deficit even when HIMP looks reassuringly compensated.
Reading saccade patterns
Use the saccade trainer below to practise distinguishing overt from covert. Click on the trace where you see a saccade; the simulator judges whether you correctly placed it in the during-head (covert) zone or the after-head (overt) zone.
Pitfalls in saccade interpretation
Not every spike in the eye trace is a corrective saccade. Common false positives:
- Blink artefact — sudden, very brief drops in pupil tracking that the software may render as a saccade. Look at the eye position trace, not just velocity.
- Anticipatory saccades — eye starts moving before the head, which means the patient predicted the thrust direction. Make thrusts more unpredictable.
- Glissadic drift — slow eye drift after the head stops, sometimes confused for a slow overt saccade. True saccades have characteristic velocity profiles (rapid rise, rapid fall).
Key teaching points
- Read every trace for two things: VOR gain and corrective saccades.
- Overt saccades (after head stops) are visible at bedside. Covert saccades (during head movement) are not — and they make bedside HIT falsely normal.
- Acute lesions: overt-dominant. Compensated lesions: covert-dominant. The transition takes weeks to months.
- A "normal" gain hiding a covert saccade is not normal canal function — it is normal central compensation.
References
- 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
- Manzari L, Burgess AM, MacDougall HG, Curthoys IS. Vestibular function after vestibular neuritis. International Journal of Audiology 2013;52:713–8. doi:10.3109/14992027.2013.809485
- Mantokoudis G, Saber Tehrani AS, Wozniak A, Eibenberger K, Kattah JC, Guede CI, Zee DS, Newman-Toker DE. Compensatory saccades made to imagined targets are not affected by attempts to suppress them. Experimental Brain Research 2015;233:739–48. doi:10.1007/s00221-014-4106-7
- Alhabib SF, Saliba I. Video head impulse test: a review of the literature. European Archives of Oto-Rhino-Laryngology 2017;274:1215–22. doi:10.1007/s00405-016-4157-4
- 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