The Video Head Impulse Test

Introduction

Many techniques are available for the assessment of the vestibular system, particularly the assessment of horizontal semicircular canal (SCC) function. The head impulse test (HIT) was first described by Halmagyi and Curthoys in 1988 and is the most widely used bedside test of SCC function. Since that time, a relatively limited number of vestibular research laboratories have used the magnetic field scleral search coil technique, the gold standard for recording eye movements, to record three-dimensional (3D) eye movement during the HIT to validate its use as a test of SCC function and to better understand the clinical utility of the test. The search coil technique is generally considered too invasive, expensive, and time-intensive for routine clinical use, and those limitations were the impetus for the development of two-dimensional (2D) eye-movement recording via high-speed video cameras. The initial studies using high-speed digital video cameras to record eye movement during the HIT (now referred to as the video head impulse test or vHIT) were published by several independent research laboratories, with the critical experiments demonstrating comparable results for simultaneous video and scleral search coil recordings for head impulses in horizontal and vertical planes in normal controls and in selected patients with well-defined vestibular losses. Since those initial studies, at least four vHIT devices have been developed, and the routine clinical use of the vHIT has expanded rapidly. The purpose of this chapter is to provide an overview of the HIT with an emphasis on the vHIT. The reader is also directed to Halmagyi et al. for a comprehensive review of this topic.

Background of the Head Impulse Test

The angular vestibulo-ocular reflex (VOR) ensures gaze stability during head rotations by generating eye movements that are equal and opposite to head rotation. The gain of the VOR (eye velocity divided by head velocity) for natural head movements, therefore, approaches unity in healthy individuals. The VOR has three main anatomic components: (1) the SCCs and the superior vestibular nerve and inferior vestibular nerve afferents in the peripheral vestibular system, (2) the vestibular and ocular motor nuclei in the brainstem, and (3) the extraocular muscles. The SCCs are positioned in three nearly orthogonal planes within the head, allowing for the detection of head rotation in 3D space. The SCCs function as angular accelerometers in a push-pull fashion with two coplanar canals on each side of the head working together, i.e., the left and right horizontal SCCs, the right anterior and left posterior SCCs or “RALP,” and the left anterior and right posterior SCCs or “LARP.” For example, during rightward head rotation in the horizontal plane, the discharge rate of the right horizontal SCC afferents increases and, at the same time, the discharge rate of the left horizontal SCC afferents decreases relative to the resting discharge rate. The difference in output between the right and left horizontal SCCs drives the leftward compensatory eye movement of the VOR so that the eyes remain still in space during head rotation and enable stable vision. The observation or measurement of eye movement, therefore, can aid in the detection and localization of vestibular pathology because of the relationship between the function of vestibular sensory receptors in the inner ear and the compensatory eye movements produced by the VOR. The majority of bedside and laboratory tests of vestibular function involve the observation or measurement of horizontal eye movements (i.e., horizontal VOR) produced by stimuli that activate the horizontal SCCs and the superior vestibular nerve.

The HIT is used to assess dynamic function of the SCCs and was initially described as a bedside test used to measure the function of each horizontal SCC. The HIT is based on two principles or laws (Ewald’s laws) in vestibular physiology: (1) eye movements evoked by stimulation of a single SCC occur in the plane of that canal, and (2) excitatory responses have a larger dynamic range than inhibitory responses. Specifically, because the three SCC pairs (horizontal, RALP, and LARP) are nearly orthogonal to each other, a head impulse delivered in the plane of one pair will stimulate mainly that pair and not the other two SCC pairs. In addition, the VOR during a canal-plane impulse toward a particular SCC is driven largely by that SCC and not by its coplanar counterpart because of the asymmetric response (excitatory > inhibitory) of primary vestibular afferents (see Fig. 8.1 ). The HIT, therefore, can assess the function of each SCC separately and, in a patient with unilateral vestibular hypofunction, the gain of the VOR during ipsilesional head impulses (i.e., head rotation toward the side of the vestibular loss) will be lower than the gain during contralesional head impulses.

Bedside Head Impulse Test

The bedside HIT is the most widely used bedside test of SCC function and has largely been used to assess the function of horizontal SCCs. To perform the bedside HIT, the clinician sits in front of the patient, holds the patient’s head, and instructs the patient to keep staring at an earth-fixed target (e.g., the clinician’s nose). The clinician turns the patient’s head abruptly and unpredictably to the left or right, through a small angle (10–20 degrees). Patients with normal VOR function will be able to maintain their gaze on the target during head rotation to either side (i.e., the head rotation produces a short-latency compensatory eye movement [the VOR] that is equal and opposite of the head rotation). In contrast, patients with unilateral vestibular loss (UVL) will be unable to maintain their gaze on the target during ipsilesional head rotation. Instead, the eyes will move with the head (because of the reduction in VOR gain) and are taken off target so that at the end of the head rotation the patient must make a voluntary corrective saccade back to the target. The corrective or “catch-up” saccade is visible to the clinician and is, therefore, called an overt saccade. The observation of an overt saccade is an indirect sign of horizontal SCC hypofunction on the side toward which the head was rotated. The bedside HIT depends on the timing and size of the corrective saccades and on the ability of the clinician to accurately observe the corrective saccades. Corrective saccades are observed at the end of both rightward and leftward head impulses in patients with bilateral vestibular loss (BVL).

Advantages of the bedside HIT include the ability to detect UVL and BVL, no equipment cost, short test time, portability, and the ability to assess horizontal SCC function at frequencies of head rotation that are representative of head movements that occur during activities of daily living. In addition, the bedside HIT is less likely to elicit the vertigo and occasional nausea associated with caloric stimulation.

There are, however, a number of significant limitations related to the bedside HIT. Specifically, the bedside HIT is a subjective test, and there is no objective measure of the corrective saccades or VOR gain; the outcome of the test is based on the clinician’s subjective visual observation of the presence or absence of overt saccades; and the interpretation of the bedside HIT depends on the experience of the clinician. Furthermore, during the bedside HIT, the magnitude of the head acceleration is unknown and likely varies within a single clinician/patient and between individual clinicians/patients. Corrective saccades that occur during the head rotation are called covert saccades, and these cannot be observed by the clinician.

The false-negative rate of the bedside HIT in patients with peripheral vestibular disorders has been estimated at 14% based on the rate of occurrence of isolated covert saccades detected with the vHIT. The clinical application of the bedside HIT has been limited to the evaluation of horizontal SCC function and has not been used routinely for the assessment of vertical SCCs.

Magnetic Field Scleral Search Coil Head Impulse Test

The magnetic field scleral search coil technique has been used to record 3D eye movement during the HIT, and has demonstrated that covert saccades occur during the HIT in patients with UVL and BVL and that the presence of covert saccades can produce false-negative results for the bedside HIT even in patients with total UVLs. The 3D search coil technique has also been used to measure the function of individual vertical SCCs using head impulses delivered in the vertical plane. Importantly, it was shown that 3D and 2D (horizontal and vertical eye movement) scleral search coil techniques were equally accurate in detecting isolated hypofunction in horizontal and vertical SCCs, indicating that 2D methods (i.e., video pupil tracking) are capable of assessing all six SCCs independently.

Video Head Impulse Test

Based on the bedside HIT first described by Halmagyi and Curthoys, the vHIT is a relatively new clinical test of dynamic SCC function that uses high-speed digital video camera(s) to record eye movement during and immediately after head impulses in horizontal and vertical planes. The stimulus for the vHIT is the same stimulus used for the bedside HIT and consists of manual, passive (clinician moves patient’s head), unpredictable, brisk head rotations with peak angular velocity of ∼100 to ∼400 degrees/second and a peak angular acceleration of ∼1000 to ∼4000 degrees/second. The vHIT instrumentation consists of high-speed (∼250 frames/second) monocular or binocular digital infrared video camera(s), a laptop computer, and software. The video camera uses pupil detection methods to record 2D eye movements. The vHIT detects and records abnormal eye movements (i.e., overt and covert saccades) and provides measures of VOR gain. Depending on the vHIT device, the camera is either embedded in head-worn goggles or mounted on a tripod facing the patient. Head movement is recorded by an inertial measurement unit (triaxial linear accelerometer and gyroscopes) mounted on the head-worn goggles or by the change in the angle of head position during the head impulse as recorded by an external camera. Notably, prototypes of at least two commercially available vHIT devices have been validated with comparable results obtained for simultaneous video and magnetic field scleral search coil recordings for head impulses in horizontal and vertical planes in normal controls and in selected patients with well-defined vestibular losses.