The concept of head impulse testing (HIT) was introduced in 1988 as a bedside measure of semicircular canal (SCC) paresis (Halmagyi & Curthoys, 1988). The test evolved into an objective assessment of the angular vestibulo-ocular reflex (VOR) of the six SCCs: horizontal, anterior/superior, and posterior. It is now referred to as the video head impulse test (VHIT). The VHIT is considered a true vestibular test due to the high acceleration (upwards of 2000–4,000°/s2) head movements which do not allow for contributions from the oculomotor system to elicit corrective eye movements (Halmagyi et al., 2017). The VHIT may be completed in addition to caloric irrigations or isolation when caloric irrigations cannot be performed. Examples may include anatomical variants (e.g., atresia or tympanic membrane perforations) or patient tolerance of the procedure. Caloric irrigations are a low frequency response of the VOR, whereas VHIT covers natural and active high frequency VOR range.
The VOR stabilizes visual images within the fovea of the eyes during head movements, creating equal and opposite eye movements to head movements (see Chapter 1). When the VOR is intact, the eye velocity to head velocity ratio is close to 1.0, or calculated as a VOR gain response close to 1.0. Gain is typically calculated between points of 60 milliseconds after initiation of head translation to the point of zero head movement (MacDougall & McGarvie, 2013; Weber, Aw, Todd, McGarvie, Curthoys, et al., 2008). If the VOR is impaired, the neural input to elicit compensatory eye movements is no longer proportional to head velocity resulting in decreased VOR gain response (<1) and retinal slippage, in which the image of interest slips off the fovea within the retina. A corrective saccade is then initiated to reposition the eyes back to the target of interest.
During VHIT, the patient wears lightweight goggles with an accelerometer to measure head velocity and a high-speed infrared camera to capture eye movements. With quick, unpredictable head movements (impulses), the equipment displays a head movement curve in the direction of the head impulse and simultaneously displays from the camera an eye movement curve (pupil tracing). The eye movement tracing should appear equal in amplitude and velocity, but in the opposite direction as the head tracing (Figure 7–1).
The gain can be calculated by measuring the ratio of the area under the eye velocity curve to the area under the head velocity curve or by measuring the instantaneous gain of eye velocity/head velocity (1:1 ratio in healthy individuals), depending on manufacturer calculations. In cases of vestibular dysfunction, there is a reduction in VOR gain (i.e., pupil movement is lower in amplitude and velocity to eliciting head movement) and the presence of corrective saccades (CS; Figure 7–2). The measured gain response may be <1.0 on the side(s) with the impaired VOR function. In head impulse testing, there is potential for the patient to generate two types of CS:
Figure 7–1. Normal VHIT results in a healthy individual. Eye tracings (dashed curve) are equal and opposite to the head tracings (black curve) with no evidence of corrective saccades. Gain is within the normal range (1.0).
• Overt—a voluntary saccade that is generated after the head has stopped moving
• Covert—an involuntary saccade that is generated during the head impulse (while the head is still in motion)
Figure 7–2. Abnormal video head impulse (VHIT) in a patient with vestibular loss. Eye tracings (dash curve) are not moving equally in amplitude and velocity to head tracings (black curve) with evidence of corrective saccades. Gain is below the normal range (0.5).
VHIT can be performed in a well-lit room, rather than in a dark environment, as there is no threat of the visual system reducing the VOR response (Curthoys et al., 2016). This is not the case with other low frequency measures of the VOR such caloric irrigations, where is it necessary to prohibit the visual system from contributing to retinal image stabilization.
The following steps are recommended for optimal patent setup:
1. Eye makeup should be removed before testing.
2. Seat the patient in a sturdy chair positioned 1 to 1.5 meters from a visual target (typically 1 cm in size).
Note: VOR gain will increase with decreased distance from the visual target in healthy individuals without vestibular loss (Judge, Rodriguez, Barin, & Janky, 2018).
3. Ensure that the visual target is eye level for the patient and that the patient can see the target. Patients will not be able to wear glasses during testing, but contact lenses are acceptable.
Note: It is not a problem for patients to be tested without the use of corrective lenses. There is little effect of loss of visual acuity on VOR outcome parameters (Judge et al., 2018).
4. Place the infrared goggles on the patient and adjust the camera to focus on the pupil. Instruct the patient on goggle placement over their eyes and the need for a tight fit.
Note: The goggle strap should be tight to minimize goggle slippage during testing; however, it is important to make sure that the goggles are not pushing on the eye socket, causing temporary double vision.
5. Most VHIT systems have one camera that may be fixed over the right eye or with a ball and socket camera that can be repositioned from right to left eye. Some systems allow for binocular viewing.
Note: When using monocular recordings, it is common to see a right vs. left eye asymmetry (i.e., higher gains for the eye being recorded; Weber, Aw, Todd, McGarvie, Pratap et al., 2008).
6. Once the goggles are properly positioned on the patient, ensure that the eye(s) are centered horizontally, and the pupil is positioned within the region of interest (ROI; Figure 7–3).
7. Next, perform the calibration procedure. Calibration typically involves a saccade task (i.e., moving the eyes quickly to follow a laser project light or pattern of lights). Ask the patient to hold their head still while performing calibration. Make sure that the crosshairs (if applicable) are within the pupil and/or ROI is stable during the test. It may be difficult to keep crosshairs/pupil stable in some patients with restricted pupils.
8. If performing vertical canal impulses, some systems have an additional calibration step called head calibration. This step will calibrate the head position before beginning the head impulses. The examiner should stand behind the patient, place their hands on the patient’s jaw or head (away from the goggle straps) and gently move the patient’s head in the yaw plane and then pitch plane (about five sinusoidal oscillations in each direction).
Figure 7–3. Image of the proper positioning of the eye (centered horizontally) with crosshairs stable within the pupil.