It determines the degree of alcohol and drug impairment by measuring the presence of nystagmus in the eye. However non-alcohol related nystagmus infrequently can occur in congenital, opticokinetic, and labyrinthine irritability cases. As a diagnostic tool, HGN is reliable, easy to use, and non-threatening. It overcomes physical handicaps, but does require officer training and practice as well as record keeping.