What is the Video Head Impulse Test?
Our nervous system is an intricate blend of functions & reflexes that combine together to form a network of nerve reactions. These nerve reactions & their various functions determine our body’s well-being, posture & balance among others.
For patients suffering from Vertigo, these nervous reactions are broken, or dysfunctional.
Vertigo patients suffer from a broken Vestibular-ocular reflex system (VOR), which is essentially a network determining eye movements that function to stabilize your gaze by countering movements of the head.
When a patient experiences loss of balance &/or disbalance in eye & ear functions, their Vertigo tests are recommended by doctors.
One such test is the Video Head Impulse Test (vhit test).
The Video Head Impulse Test is a form of videonystagmography test (vng test) that helps doctors determine if the patient is indeed suffering from a lack of vestibular coordination, a pre-eminent symptom of vertigo.
The Video Head Impulse test (vhit test) utilizes a new technology that uses a high-speed, lightweight video goggle to measure, either left or right, eye velocity & electronically record ‘saccades’, &/or any other abnormalities in patients with impaired VOR function.
The vHIT test provides a quick, objective measure of the Vestibular Ocular Reflex (VOR) in response to the movements of the head in the predefined range of natural, daily life movements.
The Video Head Impulse Test provides immediate information regarding side-specific vestibular functions. The vHIT test allows doctors to quickly & efficiently assess the patient & decide conclusively whether the dizziness is related to or is the outcome of, a vestibular disorder.
The test has been available in its various protocols for around for a long time now to help doctors decide effectively if the patient’s dizziness symptoms are a part of vertigo disorder.
The vng test, put in simple words, provides fast, reliable & bedside screening of the patient’s semicircular canal function (SCC) with zero or minimum equipment needed.
The vHIT test not only helps find catch up ‘saccades’ & other abnormalities in patients, but also helps draw up a three-dimensional analysis taking into account the three pairs of SCCs, & complements other traditional methods of testing vestibular responses.
The evolution of the videonystagmography test
Previously the Magnetic field-coil systems, also known as the ‘search coil techniques’, were used as the ‘gold standard’ of measuring & recording head impulse testing. These systems consisted of 2 or 3 magnets (monocular vs binocular recordings, respectively) that are worn in similar ways as contact lenses to enable measuring eye position in tandem with head position.
However, owing to the technical difficulties & the demands associated with magnetic coils, coupled with the fact that the method was expensive & time-consuming, & slippage plus patient discomfort made it unlikely for these to be used in everyday clinical practice.
Hamish Gavin MacDougall & his other doctor associates recorded video head impulse tests taken place at 250 Hz, using a camera mounted on a lightweight spectacle that recorded 250 frames per second. They concluded that the videonystagmography test was successful in detecting overt & catch-up saccades in all patient groups evaluated, & determined that the vHIT Test was on par with search coils in identifying peripheral vestibular defects.
The doctors collectively concluded that the vng vHIT test was easier to use than previous systems, including patients experiencing acute vestibular neuritis.
The doctors could witness that corrective saccades were easily detectable in patients with reduced horizontal canal VOR gain when delivering manually graded head impulses, following which they objectively analyzed the stimulus-response characteristics of the VOR. vHIT was unanimously noted to be easy and practical to apply.
How vHIT test works.
vHIT test paraphernalia is a portable system that facilitates quick, efficient & accurate measurement of VOR in response to head movement. The hardware consists of a lightweight cam, non-slip, dedicated goggles that enable quick changes in recording from right to left eye.
The test works on the introduction of an Inertial Measurement Unit (IMU). The IMU is a combined gyroscope & accelerator that measures the patient’s head movement while the camera captures & records eye movement, however, if the procedure uses a switchable mount camera, the movement of either eye can be recorded.
A USB connector is used as a computer interface, & the laser calibration used in the test is quick, simple & can be performed at any place.
The test also utilizes a high-speed camera for efficient eye tracking.
Immediately after the data is gathered, information regarding velocity regression & gain becomes available.
Doctors can then easily visualize spontaneous nystagmus & ‘catch-up saccades’ from the graphics.
vHIT Test protocol & interpretations
Once the goggles are securely placed on the patient, a quick, brief calibration is performed. The examiner instructs the patient to maintain eye focus on a specific object while they manipulate the patient’s head position quick & precise head movements.
The entire procedure takes less than 10 minutes to give immediate, site-specific results.
The cam performs repetitive calculations aiming to get an idea of the balance between the patient’s head & eye movements to determine whether the vestibular system is compromised.
These results are showcased in the form of ‘tracings’, that clearly show the presence of overt &/or covert saccades. The results are traced on a computerized graph, the slope of which determines the state of the patient’s vestibular system.
For patients who have a normal, well-functioning vestibular function, along with the left to right movement of the head, the eye should move in the opposite direction to maintain gaze on a fixed point in space.
However, for patients who are experiencing vestibular dysfunctions, the eye movements are often in tandem with the head requiring a corrective movement(one that helps the eye focus its gaze back on the object) called a ‘catch-up saccade’. The vHIT test analyzes this abnormal eye movement, analyzes it, & displays it on a digital graph, for the easy understanding of the doctors.
Compared to other tests to determine vestibular functions, like the Caloric test, the Video Head Impulse Test provides efficient, organized & accurate data regarding eye-head calibration.
However, the vHIT test is not meant to be used in the place of any of the tests currently being used to determine vestibular function or dysfunction among patients.