Skip to content

LESSON 159

Video head impulse test (vHIT)

Year 2
Spring semester
Vestibular Assessment I
Vestibular assessment
Duration: 11:22

This lesson covers the video head impulse test (vHIT): how VOR gain is calculated, why lateral canal gain is normally above about 0.8, covert and overt saccades, and how the six canals are tested in the lateral, LARP and RALP planes. Artifacts and example response patterns are also included.

Watch on YouTube

Follow along

Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn the video head impulse test, or vHIT.
We will see how VOR gain is calculated, what covert and overt saccades are, and how the six canals are tested.
The vestibulo-ocular reflex, or VOR, keeps the image stable on the retina while the head moves.
When the head turns right, the eyes turn left at the same speed, so the target stays in place.
This reflex is very fast; it begins only a few milliseconds after the head starts to move.
It is this reflex that lets us see clearly while walking or turning the head quickly.
When the VOR is impaired, a person may notice the image slipping or blurring during head movements.
The head impulse test is a bedside test described by Halmagyi and Curthoys in 1988.
While the patient looks at a target ahead, the head is turned through a small angle, very fast and unpredictably, to one side.
If the VOR is intact, the eyes stay on the target throughout the movement.
If the VOR is weak, the eyes are carried with the head, and the patient makes a fast eye movement to regain the target.
This eye movement is called a corrective saccade; the weak side is the side toward which the head was turned.
Some saccades occur during the head movement and cannot be seen by the naked eye; the video method was developed for this reason.
Each semicircular canal senses head rotation in its own plane.
A head impulse stimulates the canal on the side toward which the head turns; a rightward impulse tests the right lateral canal.
In fast movements the opposite canal contributes little, so the response comes mainly from the stimulated canal.
This is why the test allows each canal to be assessed separately.
The stimulus is high-frequency, like everyday head movements, unlike the low-frequency stimulus of the caloric test.
The video head impulse test, or vHIT, uses a high-speed camera mounted on lightweight goggles.
While the camera records eye movement, a motion sensor in the goggles measures head velocity.
For each head impulse, the software compares eye velocity with head velocity.
The test is usually quick, and patients tolerate it more easily than the caloric test.
In this way, saccades that cannot be seen by the naked eye are also recorded.
VOR gain is the ratio of eye velocity to head velocity.
If the eyes fully compensate for the head movement, the gain is close to one.
For the lateral canal, gain is normally expected to be above about 0.8.
For the vertical canals the normal cutoff is usually lower, for example about 0.7.
Some devices calculate gain from the area under the curve, others from velocities at set moments, so device-specific norms are used.
Gain alone is not enough; it is always interpreted together with the saccades.
Now let us look at a purely example results table.
In this example, right lateral canal gain is 0.95, while left lateral canal gain is 0.52.
Left lateral canal gain is clearly low, and corrective saccades are also seen on leftward impulses.
Left anterior canal gain is also low, at 0.55.
The left posterior canal, however, is within normal limits.
This pattern fits involvement of the canals served by the superior vestibular nerve, with the inferior division spared.
Corrective saccades fall into two groups according to their timing.
A covert saccade occurs while the head is still moving.
So it cannot be seen by the naked eye at the bedside and is detected only on video recording.
An overt saccade occurs after the head has stopped and can be seen by the examiner.
Both types of saccade show that the VOR on that side is insufficient.
A bedside test that looks only for overt saccades may judge a patient with covert saccades to be normal.
After a unilateral vestibular loss, the brain tries to make up for it over time; this is called compensation.
In this process saccades may move into the head movement, that is, earlier, and become covert saccades.
Saccades that appear at a similar time on every impulse, called grouping, have been linked with good compensation.
During compensation, the patient's complaints may lessen even if the gain stays low.
For this reason, serial vHIT recordings are useful for following recovery and compensation.
For the lateral canals the head is pitched about thirty degrees down, so the lateral canals become parallel to the ground.
The patient usually looks at a target about one meter away.
The head is pushed very quickly to the right or left through a small angle of about ten to twenty degrees.
The direction and timing of the impulses are varied so that the patient cannot predict them.
Enough valid impulses are collected for each direction, usually about ten to twenty.
The software helps to reject impulses with too little head velocity; slow impulses weaken the assessment.
The vertical canals lie in the same plane as paired partners on opposite sides.
The LARP plane contains the left anterior and right posterior canals; for this plane the head is turned about forty five degrees right.
The RALP plane contains the right anterior and left posterior canals, and the head is turned about forty five degrees left.
In this position, a chin-down impulse stimulates the anterior canal, and a chin-up impulse stimulates the posterior canal.
Meanwhile, the patient keeps looking at the target straight ahead.
Vertical canal testing is technically harder, so it requires experience and care.
The vestibular nerve has two main divisions, superior and inferior.
The superior division serves the lateral and anterior canals and the utricle.
The inferior division serves the posterior canal and most of the saccule.
So testing the six canals separately helps to identify which nerve division is affected.
Because the caloric test assesses only the lateral canal, it cannot show, for example, a problem affecting only the posterior canal.
Goggle slippage on the head is the most common artifact and can make gain falsely high or falsely low.
Slippage often shows itself as gain well above one and as wavy, irregular eye velocity traces.
Blinking, and the upper eyelid covering the pupil especially on downward impulses, can spoil the recording.
Predictable timing can lead the patient to move the eyes in advance and produce misleading saccades.
A patient who does not keep looking at the target, or a tense neck, also lowers recording quality.
Interpreting the results without removing artifact-affected impulses can lead to the wrong conclusion.
A clinical tip: before starting, fit the goggles tightly and calibrate carefully.
After recording, do not look only at the gain number on the screen; review the head and eye velocity traces one by one.
If normal gain comes with repeated, consistent saccades, the finding may be meaningful.
Conversely, if you see gain far above one, think of goggle slippage first.
In superior vestibular neuritis, gain usually falls in the lateral and anterior canals on the affected side.
In the rarer inferior involvement, only the posterior canal may be affected.
In bilateral vestibular loss, low gain and saccades are seen in the canals on both sides.
In Meniere's disease, the caloric response may be reduced while vHIT often stays normal.
In a patient with acute, continuous dizziness, a normal head impulse test may suggest a central cause and needs urgent medical assessment.
These patterns are examples; interpretation is made together with the history and other tests.
The caloric test and vHIT do not replace each other; they complement each other.
The caloric test assesses low-frequency VOR function, while vHIT assesses high-frequency VOR function.
The caloric test examines only the lateral canal, while vHIT can examine all six canals.
So the results of the two tests do not always point in the same direction.
We will cover the clinical meaning of this dissociation in detail in a later lesson.
VOR gain is the ratio of eye velocity to head velocity, and its ideal value is close to one.
Lateral canal gain is normally above about 0.8; for the vertical canals the cutoff is usually lower.
Covert saccades occur during the head movement, overt saccades after the head has stopped.
The six canals are tested separately in the lateral, LARP and RALP planes.
Test yourself: what is the corrective saccade called that occurs during head movement and cannot be seen by the naked eye?
The correct answer is B: a covert saccade.
Covert saccades can only be detected on video recording; this is one of the most important contributions of vHIT.
In the next lesson we will learn the VEMP tests that assess the otolith organs: cVEMP and oVEMP.
Subscribe to the channel so you never miss a new lesson. See you next time!

Keywords

Click a keyword to search all lessons for that term.

Prerequisites

LESSON 154 The head impulse test and HINTS

Comments

Questions and comments appear after approval.

No comments yet. Be the first to comment.