The head impulse test (HIT) is a bedside examination used to identify problems in the inner ear, specifically a peripheral vestibular deficit.
Understanding the Head Impulse Test
The HIT is a quick and effective way for clinicians to assess the function of the vestibular system, which plays a critical role in maintaining balance and spatial orientation. Here's a breakdown of what it involves:
How It's Performed:
The clinician performs the following steps during a HIT:
- Patient Positioning: The patient is typically seated, and their head is held by the clinician.
- Head Rotation: The clinician briskly and rapidly turns the patient's head to one side in a small amplitude.
- Observation: The clinician observes the patient's eyes for any movements.
Interpreting the Results:
The key to interpreting the HIT lies in observing the eye movements following the head rotation. Here's what different responses indicate:
-
Normal Response: In a healthy individual, the eyes will remain fixed on the target during the head rotation. The brain will compensate with an eye movement that matches the speed and direction of the head movement.
-
Abnormal Response (Positive HIT): If there is a problem with the vestibular system (such as a semicircular canal paresis), the eyes will initially move with the head and then be followed by a rapid corrective eye movement known as a "catch-up saccade". This means the eyes will appear to lag briefly behind the head movement. This "overt" catch-up saccade is the key indicator of a positive HIT.
Why is it Important?
- Identifies Peripheral Vestibular Deficit: As highlighted in the reference, the HIT is a useful examination to identify peripheral vestibular deficits, such as in patients with vestibular neuritis (VN).
- Distinguishes Central vs. Peripheral Vertigo: The HIT, alongside other clinical findings, can help differentiate between vertigo caused by issues in the inner ear (peripheral) and vertigo caused by problems in the brain (central).
Example
- If a patient’s head is turned briskly to the left, and the clinician observes a small catch-up saccade to the right, this would indicate an issue with the left lateral semicircular canal.
Head Impulse Test Summary
| Feature | Normal Response | Abnormal Response (Positive HIT) |
|---|---|---|
| Eye Movement during turn | Eyes remain fixed on target | Eyes move with the head briefly |
| Eye Movement after turn | No corrective movement required | "Catch-up saccade" to regain target focus |
| Indication | Healthy vestibular system | Semicircular canal paresis, likely peripheral vestibular deficit |
In conclusion, the head impulse test is a valuable diagnostic tool that allows clinicians to quickly assess the integrity of the vestibular system at the bedside. The presence of "overt" catch-up saccades after a brisk head rotation indicates a peripheral vestibular problem, such as vestibular neuritis.
