Application of videography during head impulse test in children: A possible replacement for the caloric test?

Objective: In the vestibular system, many pathologies, particularly in children, decrease the canalar reactivity in such a way that high and low frequencies are simultaneously affected. Since Curthoys and Halmagyi (1) described the Head Impulse Test (HIT) in 1988 we know that we can check each semicircular canal individually with passive high frequency stimulation. Our aim is to quantify the VOR gain as measured during the HIT, in order to increase the sensitivity in such a way that we could, at least in some cases, replace the caloric test. Study design: To evaluate the sensitivity of the HIT combined with videography, we have measured the horizontal VOR gain in a population of 36 acoustic neuroma patients, and compared the results with unilateral weakness (UW) as measured with the Jongkees formula in the caloric test. A patient sitting in front of a camera is instructed to gaze continuously at a steady target. Their face is filmed at 25 FPS (frames per second) with a CCD (charge-coupled device) black and white camera located 1 metre in front. An infrared light source illuminates the face and creates a corneal reflection. Head rotation angle is calculated from variations in the grey scale image of the head, while gaze direction deflection is deduced from the coordinates of the corneal reflection referenced to pupillar centre. Results: Our results show good proportionality between UW(%) in the caloric test and the VOR gain weakness measured as a percentage in the HIT. However, we have found one case in which the caloric test indicated a significant UW of 38% while HIT remained in the normal range. Conclusion: The Head Impulse Test and caloric tests are different, but they are complementary. Using a camera, and automatic image processing, we are able to increase the sensitivity of the HIT in such a way that we are able to detect any VOR weakness of more than 40%.