Gaze Nystagmus

Nystagmus is an involuntary jerking of the eyeball. There are a variety of different types of nystagmus. Gaze nystagmus can be made apparent by evaluating a driver's eye as it moves from left to right.

Law enforcement officers can use the HGN or horizontal gaze nystagmus test to determine if a driver is intoxicated. This is done by evaluating the lateral jerking of an eye as it moves from side to side. The nystagmus may also be exacerbated by depressants, inhalants or phencyclidine. The greater the impairment the more pronounced the jerking or the nystagmus of the eye.

HGN tests have been shown to be a reliable indication of intoxication but have been less reliable at indicating the blood alcohol concentration of a driver. The National Highway Traffic Safety Administration (NHTSA) has developed very specific testing procedures and evaluation processes for law enforcement officers to increase the accuracy of the HGN test.