Video Head Impulse Test (vHIT): Covert vs Overt Saccades and Clinical Interpretation
In short
The Video Head Impulse Test quantifies high-velocity VOR function across all six semicircular canals; distinguishing covert from overt saccades directs targeted therapy.
In this guide (8 sections)
Biomechanics of High-Frequency Canal Evaluation
The Video Head Impulse Test (vHIT) has transformed clinical neuro-otology by enabling rapid, quantitative, non-invasive assessment of all six individual semicircular canals (horizontal, anterior, and posterior canals bilaterally). Traditional caloric testing stimulates only the horizontal canals at an unphysiologically low frequency (~0.003 Hz). In contrast, vHIT tests the vestibulo-ocular reflex at natural physiological frequencies (2.0 to 5.0 Hz) and high accelerations (1000 to 4000 degrees/sec^2) typical of real-world walking and head turns.
A lightweight high-speed infrared video camera (250 Hz) and inertial sensors track pupil velocity and head velocity simultaneously. In a healthy canal, VOR gain (eye velocity divided by head velocity) is approximately 1.0 (normal range 0.8 to 1.2 for horizontal; >0.7 for vertical canals). When a canal is impaired, VOR gain drops significantly, and the eyes fail to stay on target during head rotation, requiring the brain to generate corrective saccades to restore foveal fixation.
Domestic Context: Diagnostic Accuracy in Indian Healthcare
In Indian clinical practice, vHIT testing is increasingly accessible in tier-1 and tier-2 medical centres. It provides objective physiological proof for patients who have suffered unexplained unsteadiness for months and undergone multiple normal brain MRIs, cervical spine X-rays, and blood panels.
Understanding vHIT reports is essential for physiotherapists. A patient returning home from an ENT hospital with a diagnosis of 'inner ear weakness' can be specifically rehabilitated based on whether their vHIT shows an isolated horizontal canal deficit, a posterior canal drop, or a mixed pattern, allowing exercise therapy to target the exact functional plane of disability.
Clinical Testing: Saccadic Timing and Waveform Analysis
When VOR gain is insufficient, two distinct types of refixation saccades appear on vHIT velocity traces.
- Overt Corrective Saccades: Occur AFTER the head movement has completely finished (latency >150 ms); visible to the examiner's naked eye as a distinct flick of the eye back to the target.
- Covert Corrective Saccades: Occur DURING the active head movement (latency <100 ms); generated before the head stops moving, completely invisible to the naked eye and detectable only by high-speed vHIT cameras.
- Saccadic Grouping and Organisation: Well-grouped, tightly clustered covert saccades reflect successful central neuroplastic compensation, reducing visual oscillopsia during functional tasks.
- RALP and LARP Vertical Canal Testing: Testing Right Anterior-Left Posterior (RALP) and Left Anterior-Right Posterior (LARP) planes isolates vertical canal deficits that caloric tests miss entirely.
Stage-by-Stage Functional Rehabilitation Protocol
- Stage 1: VOR Gain Adaptation (For Residual Gain >0.5): If vHIT demonstrates residual canal gain, deliver high-repetition VOR x1 adaptation drills in the specific plane of the affected canal to drive synaptic plasticity.
- Stage 2: Saccadic Substitution Training (For Severe Loss <0.4): If vHIT shows profound gain loss with scattered overt saccades, train early, organized compensatory saccades (eye-head saccadic substitution drills) to shift overt saccades into covert saccades.
- Stage 3: Functional Dynamic Locomotion: Incorporate head turns at velocities matching the patient's daily functional demands, using dynamic balance challenges to restore community confidence.
Clinical Exercise Protocol: Plane-Specific Saccadic Substitution Drill
Active Eye-Before-Head Saccadic Substitution Drill
Objective: Trains the brain to fire anticipatory covert saccades, stabilizing gaze when head velocity exceeds residual VOR canal capacity.
- 1Tape two target letters on a wall 1 metre apart at eye level, standing 1.5 metres away.
- 2Look directly at Target 1 with both head and eyes aligned.
- 3Move only your eyes rapidly to Target 2, ensuring your gaze locks securely onto it while your head stays still.
- 4Turn your head smoothly to align with Target 2 while keeping your gaze pinned firmly on the target; repeat the cycle back toward Target 1.
Clinical Comparison: vHIT Metrics Across Semicircular Canals
| Canal Tested | Normal VOR Gain Range | Pathological Threshold | Typical Etiology |
|---|---|---|---|
| Horizontal (Lateral) Canal | 0.80 to 1.20 (mean ~1.0) | Gain < 0.78 with corrective saccades | Vestibular neuritis (superior division), AICA stroke |
| Anterior (Superior) Canal | 0.70 to 1.10 (mean ~0.9) | Gain < 0.70 with corrective saccades | Vestibular neuritis (superior division), canal plugging |
| Posterior Canal | 0.70 to 1.10 (mean ~0.85) | Gain < 0.70 with corrective saccades | Inferior vestibular neuritis, PICA stroke, trauma |
| Covert Saccades Present | Zero in healthy young individuals | High amplitude during head impulse | Compensated unilateral or bilateral vestibulopathy |
| Overt Saccades Present | Zero in healthy young individuals | High amplitude after head impulse stops | Acute, uncompensated unilateral vestibular loss |
Red Flags: Normal vHIT in Acute Continuous Vertigo (Stroke Screen)
The most critical clinical insight provided by vHIT is the central stroke paradox: in a patient presenting with Acute Vestibular Syndrome (constant spinning, vomiting, nystagmus), a completely NORMAL vHIT result across all canals is a red flag indicating a cerebellar or brainstem stroke. Because an acute inner ear infection must damage canal gain, a normal vHIT confirms the inner ear is healthy, proving the acute vertigo originates centrally in the posterior circulation.
Frequently Asked Questions
What is the difference between an overt and a covert saccade on my vHIT report?
An overt saccade is a visible catch-up eye flick that happens after your head has stopped moving. A covert saccade happens invisibly during the head movement, which is a sign of your brain learning to compensate.
Why did my vHIT show normal results when my caloric test showed a 30% weakness?
Calorics test your ear at very slow, low frequencies (0.003 Hz), while vHIT tests at natural walking speeds (2 to 5 Hz). In many recovering inner ear conditions, high-frequency function recovers while low-frequency caloric response remains reduced.
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