In head impulse test, patient is looking at earth fixed target while passive and unpredictable movement quickly turns his head. In subjects with preserved VOR function the vision stays at target regarding eye movement at same velocity but opposite direction. While in patients with deficient VOR function eyes don’t move, or eyes velocity lag head velocity and after finishing quick head turn, eyes make corrective saccadic movement in order to regain the target. So, video head impulse test measures eye and head velocity and calculates the gain and also detects the corrective saccades if present.
On contrary, during suppression head impulse test, patient is looking at head fixed target. During unpredictable and passive quick head turn in subjects with unaffected VOR, eyes moves at same velocity but different direction. In order to regain target which moved with head, eyes have to make anticompensatory saccadic movement in head direction. While in patients with absent VOR eyes moves with head and because there’s nothing to distract them from looking at head fixed target, after the head turn there’s no need for any corrective saccade. If anything of VOR is preserved it will drive eyes to move in opposite direction of head and make at least small anticompensatory saccade. This idea makes this test indicator of any VOR residual function. Also during suppression head impulse test the camera records eye and head movement and equipment calculate the gain, anticompensatory saccades emerge after the head movement making it easier to calculate the gain without saccades during head movement as can be the case in video head impulse test where covert saccades emerge during head movement.
|HIMP unaffected side||HIMP affected side|
|SHIMP unaffected side||SHIMP affected side|
To recap.: in suppression head impulse test, normal subjects are the one who have saccades after head turn. And this anticompensatory saccades are the one who mark functioning VOR. While reduction of anticompensatory saccades and their absence are the sign of deficient or absent VOR.