After clicking / tapping on the lateral test mode, the screen is expanded to include the pulse direction diagram (top right). The diagrams for recording the head and eye velocity and their tabular comparison, pulse counter and the info area appear in the screen.

Lateral pulse direction diagram

Technical information on naming and assigning the head axes can be found at Head motion axis assignment.

The direction of execution of the head impulses is shown in the impulse direction diagram. The head orientation areas show the optimum head position for the lateral head impulse test:

With the body and head in a straight position, please set the zero point of the head orientation by clicking / tapping on the head.

The horizontal semicircular canals tested with the lateral vKIT are tilted backwards by approx. 20-30° in relation to the horizontal plane. For this reason, it is advantageous to tilt the subject's head forwards by 20-30° (pitch positive) to carry out the test. If the head is in the corresponding area, the target areas in the impulse-direction diagram green shown.

Lateral head impulses are short (~10°) jerky head rotations to the right and left.

Implementation

Performing the lateral head impulse test

Preparation

Head orientation

Please like top the subject's head position as described above.

Gaze fixation target

To perform the lateral vKIT, the subject needs a gaze fixation target that is centrally located in their field of vision at a distance of 1m. Please position the gaze fixation target so that it can be easily recognized by the subject even when the head is tilted 20-30° forward.

Pupil centering

Please ensure that the pupil is centered in the video image in the current orientation of the patient's head. Repeat the pupil centering if necessary.

Implementation

During the test, please ensure that the pupil can be easily recognized. If necessary, regularly ask the test person to open their eyes wide.

The test can only be performed if the pupil can be easily recognized by the system!

Good pupil recognition is characterized by the fact that the cross drawn in the pupil is stable.

The system automatically recognizes head impulses to the right and left in the set lateral plane.
Rock the patient's head slightly to the right and left to loosen it.
Keep an eye on the patient's eye on the screen and instruct the patient to open the eye if their eyelashes or eyelid begin to cover the pupil.
Perform a head impulse when the pupil is not covered. To do this, make a short (~10°) jerky movement of the head to the right or left side. Important: After the impulse has been carried out, the head remains in the end position and is then slowly turned back to the starting position.
Please refer to the info area after performing the head impulse. This first displays the data recording:
The data analysis is then displayed immediately:
Once the data has been analyzed, the success or failure of the head impulse is displayed in the info area: Furthermore: the head pulse counter increases the time course of the head and eye velocity is displayed the gain value is updated.
Repeat further head impulses each time after the data analysis of the previous pulse is completed.

The following video shows the sequence of a video head impulse test examination with 7 impulses per side.

Interpretation of the measurement

The video head impulse test is used to assess the function of the vestibulo-ocular reflex (VOR).

A functioning VOR causes an eye movement that is directed against a head movement. The compensation of the head movement by an opposing eye movement is the basis for sharp vision.

In the above illustration, a head impulse was performed to the right, the red curve shows the speed of the head in °/s over a period of 600ms. The gray curve shows the speed of the pupil. As the test subject's eyes were fixed on a fixed gaze target, the eye was moved almost synchronously in the opposite direction to the head by the VOR.

The two curves are compared after 40ms, 60ms and 80ms and via the integral of 0...100ms. The quotient of the head and eye velocity is calculated for the aforementioned times/sections and is indicated in a table as "Gain".

Ideally, the gain value is 1.

In subjects with reduced VOR function, this compensatory movement is not possible or worse. The eye initially moves with the head or significantly slower than the head movement. Only after a slight delay will the subject then make a rapid eye movement to bring the target back into view. This eye movement is known as a catch-up saccade.

There are two types of catch-up saccades:

Open (overt) saccades
These occur after the head has already come to a standstill again. These saccades are also visible to the naked eye.
Covered saccades
In some patients, saccades already occur during head movement and are not visible to the naked eye.

The gain value is reduced by the delayed / reduced speed of the eye at the beginning of the head movement. In the literature, a gain of < 0.7 (1) or 0.6 (7) is considered a failure of the affected semicircular canal.

In addition to a reduction in the gain value, catch-up saccades should be clearly visible.

References

Corrections during the measurement

Various errors can occur during the measurement of any head impulse test, such as the reflexive closing of the eye by the test person:

Incorrect measurements can be easily removed by selecting the relevant point in the counter with a click / tap:

The selected curve is then highlighted in relation to the others.

Click / tap on the counter point again to deselect it.
You can "click through" all points to select a previous curve.
Click / tap on the trash can icon in the info area to delete the highlighted curve.