Module 4 · Normal traces

Normal traces

Expected gain by canal, age, and head velocity
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What "normal" means here

A normal vHIT trace looks deceptively simple: the head trace and the eye trace lie on top of each other, mirror images about the time axis. Eye velocity rises and falls with head velocity, scaled by the VOR gain. There are no corrective saccades, because the VOR did its job and gaze stayed on the target throughout the head movement.

The signature panel below shows expected normal traces in all six canals for a healthy young adult. The trace colours follow Otometrics/Interacoustics conventions: head velocity in solid amber, eye velocity in dashed blue, both plotted as positive deflections (the eye velocity is opposite in direction to the head and is plotted with its sign flipped, so they overlap when the VOR is working).

0100200R latg=0.960100200L latg=0.950100200R antg=0.880100200L antg=0.890100200R postg=0.860100200L postg=0.87
Normal six-canal panel in a healthy young adult. All gains within reference range, no corrective saccades, eye trace overlies head trace.

Expected gain by canal

VOR gain is not the same in every canal. The lateral canals consistently produce the highest gain — typically 0.90 to 1.00 in healthy adults under 60. The vertical canals produce slightly lower gain, with the anterior canals running 0.85 to 0.95 and the posterior canals 0.80 to 0.92. The geometric reasons are interesting but not bedside-relevant: what you need to remember is that "normal" is canal-specific, and a posterior canal gain of 0.82 is normal whereas a lateral canal gain of 0.82 deserves a second look.

Age-dependent norms

The reference values above hold reasonably stable across adulthood. The most cited normative study is McGarvie et al. 2015[McGarvie LA 2015], which sampled 130 healthy subjects aged 18 to 89 and reported:

  • Lateral canal: mean 0.96 (SD 0.08) below age 70; mean 0.92 (SD 0.10) above 70.
  • Anterior canal: mean 0.89 (SD 0.10) below 70; mean 0.85 above 70.
  • Posterior canal: mean 0.87 (SD 0.10) below 70; mean 0.82 above 70.

The thresholds used to define abnormal VOR gain therefore depend on age. A practical bedside rule: lateral gain < 0.80 is abnormal in any adult; anterior gain < 0.70 is abnormal; and posterior gain < 0.70 is abnormal. Below age 50, slightly tighter thresholds apply.

Asymmetry between sides

Sometimes both sides have gains within range but show a clear asymmetry. Gain asymmetry — calculated as |right − left| / (right + left) × 100% — should be less than about 8% for the lateral canals in healthy subjects. Higher asymmetry, particularly with corrective saccades on the lower-gain side, is suspicious for unilateral pathology even when each side is individually within range.

A common scenario: lateral gains 0.78 right and 0.95 left, with small overt saccades on the right. The right gain is technically in the "mild loss" range but the asymmetry (10%) and the saccades together point to a real lesion. The numbers tell you to look at the traces; the traces tell you what's happening.

Key teaching points

  • Normal lateral gain: ~0.95. Normal anterior: ~0.89. Normal posterior: ~0.87.
  • Below age 70 lateral < 0.80 is abnormal; for older adults, slightly lower thresholds apply.
  • Side-to-side asymmetry > ~8% is suspicious even if each side is within reference range.
  • The eye trace should overlie the head trace; no corrective saccades; no eye lead or anticipation.

References

  1. McGarvie LA, MacDougall HG, Halmagyi GM, Burgess AM, Weber KP, Curthoys IS. The video head impulse test (vHIT) — age-dependent normative values of VOR gain in healthy subjects. Frontiers in Neurology 2015;6:154. doi:10.3389/fneur.2015.00154
  2. MacDougall HG, Weber KP, McGarvie LA, Halmagyi GM, Curthoys IS. The video head impulse test: diagnostic accuracy in peripheral vestibulopathy. Neurology 2009;73:1134–41. doi:10.1212/WNL.0b013e3181bacf85
  3. Wiener-Vacher SR, Wiener SI. Video head impulse tests with a remote camera system: normative values of semicircular canal vestibulo-ocular reflex gain in infants and children. Frontiers in Pediatrics 2017;5:67. doi:10.3389/fped.2017.00067
  4. 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
  5. 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
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