The VOR (vestibulo-ocular reflex) gain is most helpful for diagnosing ototoxicity and other bilateral vestibulopathies. There are several bedside methods of diagnosing a reduced VOR.
The opthalmoscope test is particularly simple. Here, while viewing the patient's eyes through their distance spectacles and while they are attempting to see something in the distance, the examiner grently moves the patient's head from side to side at about 1 cycle per second, while observing the back of the eye. Ordinarily one simply locates the optic disk.
In normal persons, as the head is moved from side to side, the disk counter-rotates, so that the disk appears to be still even though the head is in motion. This shows that the VOR is working at the frequency being tested (usually about 1 hz).
In persons with a reduced or hyperactive VOR, the disk moves with the head, causing an appearence of disk motion to the examiner.
|Image of ophthalmoscope, from https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cm&part=A3460|