Video Head Impulse Testing

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: The video head impulse test (vHIT) is a clinical assessment technique used to assess the function of the semicircular canals-the angular acceleration detectors-which initiate the vestibulo-ocular reflex (VOR). The HIT and vHIT are performed similarly (see assessment of VOR using HIT video LINK), although the bedside HIT is subjective and covert saccades (compensatory saccades occurring with head movements) cannot be appreciated. vHIT has the advantages of allowing the clinician to visualize 1) the corrective overt saccades (those corrective or refixation saccades that are the only sign of vestibular loss that can be appreciated on the bedside HIT), 2) covert saccades, and 3) measurements of gain and relative gain asymmetry (Figure 1). Gain is a general term used to describe the ratio of output to input in dynamic systems. VOR gain can be calculated in a variety of ways, but the most commonly used methods include: 1) ratio of the area under the eye velocity curve to the area under the head velocity curve (which is the method used to calculate gains in the Figures below), 2) ratio of the eye velocity to the head velocity at a certain time (usually peak velocity). A gain of 1.0 suggests that the output (eye movement) perfectly matches the input (head movement). 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: An introduction to video head impulse testing