Video Head Impulse Test

vHIT: Introduction & Principle - Reflex Gaze Game

• Video Head Impulse Test (vHIT): An objective bedside test assessing the Vestibulo-Ocular Reflex (VOR).
• Underlying Physiology (VOR):
  • Reflex maintaining stable vision during rapid head movements.
  • Eyes move opposite to head direction with equal velocity.
  • VOR Gain: $Eye Velocity / Head Velocity$; normal is close to 1.
• Purpose of vHIT:
  • To assess the function of each of the six semicircular canals (SCCs) individually.
  • Identifies peripheral vestibular loss by analyzing VOR at high frequencies (physiological head movements).

⭐ vHIT assesses the function of all six semicircular canals individually at high frequencies, unlike caloric tests which assess only horizontal canals at very low frequencies (0.003 Hz).

vHIT: Procedure & Technique - Head Thrust How-To

• Patient Setup: Seated, fixating on a target. Goggles with high-speed camera track eye movement.
• Head Impulse:
  • Small (10-20 degrees), rapid (150-250 deg/s) head turns.
  • Unpredictable timing & direction.
• Canal Testing:
  • Lateral: Horizontal thrusts (yaw).
  • LARP (Left Anterior, Right Posterior):
    • Turn head ~30° Right.
    • Pitch: Down for LA, Up for RP.
  • RALP (Right Anterior, Left Posterior):
    • Turn head ~30° Left.
    • Pitch: Down for RA, Up for LP.
  • 📌 Mnemonic: Head turn opposite to Anterior canal tested (e.g., Right turn for Left Anterior).

⭐ Head impulses must be unpredictable, rapid, and of small amplitude to accurately assess VOR.

vHIT: Interpretation of Results - Reading the Ripples

• Normal Vestibulo-Ocular Reflex (VOR) Gain:

  • Lateral canals: $≈1.0$ (typically $>0.8$).
  • Vertical canals: typically $>0.7$.

• Catch-up Saccades: Indicate vestibular hypofunction in the tested canal.

  • Overt: Visible, occur after head movement.
  • Covert: Not visible (subclinical), occur during head movement.

    ⭐ Covert saccades are generated during head movement and are not visible to the naked eye, while overt saccades occur after head movement stops and are visible.

• Key Findings & Interpretation:

  Feature	Normal vHIT	Abnormal vHIT (Canal Hypofunction)
  VOR Gain	Lateral $>0.8$ Vertical $>0.7$	↓ (Lateral <0.8, Vertical <0.7) for tested canal
  Saccades	Absent	Present (Overt and/or Covert)
  Implication	Intact canal function	Hypofunction of the specific canal tested

vHIT: Clinical Applications - Diagnostic Edge

• Differentiates: Peripheral (e.g., vestibular neuritis) vs. Central (e.g., stroke) lesions; core of HINTS exam.
• Diagnoses: Unilateral & bilateral vestibulopathy.
• Assesses:
  • Vestibular neuritis: ↓ VOR gain in affected canal(s).
  • Meniere's disease: Variable; may be normal or show canal dysfunction.
  • Superior Canal Dehiscence (SCD): Often normal vHIT.
• 📌 HINTS: Head Impulse, Nystagmus, Test of Skew. ⭐ > In acute vestibular syndrome, a normal vHIT (negative Impulse) with direction-changing nystagmus or skew deviation strongly suggests a central cause (e.g., stroke).

vHIT vs. Caloric Test

High‑Yield Points - ⚡ Biggest Takeaways

• vHIT assesses Vestibulo-Ocular Reflex (VOR) for all six semicircular canals (SCCs).
• Measures VOR gain (normal ≈ 1); ↓ gain indicates SCC hypofunction.
• Detects covert & overt catch-up saccades, signs of peripheral vestibular loss.
• Crucial in HINTS exam (Head Impulse, Nystagmus, Test of Skew) for differentiating stroke from vestibular neuritis.
• Tests high-frequency VOR, complementing low-frequency caloric tests.
• Pinpoints specific affected canal(s) and side of lesion.