This is an essential test in the protocol.
It can be carried out:
- Horizontally: the practitioner rapidly moves the patient’s head from right to left and back again approximately 10 times (Baloh and Honrubia). Eye movements are examined once the head is back in its initial position. Any nystagmus triggered under these conditions is pathological in nature, and indicates asymmetry in the vestibulo-ocular circuits. It beats towards the stronger side.
Haines et al. have suggested that this type of “revealed” nystagmus could reflect an asymmetry of the velocity storage mechanism in the vestibulo-ocular pathways. Rotation towards the healthy side would result in higher velocity storage than on the deficit side. Furthermore, this hypothesis is in keeping with the Ewald’s second law.
- Vertically: the test is carried out under the same conditions except the head movement goes in an up-down direction. However, under such conditions, the observed nystagmus does not have any unequivocal semiological value. In fact, a vertical nystagmus can be observed in subjects who are not suffering from any pathology of the receptors or the vestibular pathways (Baloh and Honrubia).