Alcohol and Nystagmus

Alcohol slows our responses. When a person is impaired by alcohol, the nerves in the eyes, however, are stimulated and do not behave as normal. While the slower responses means that a person has difficulty following an object with the eyes, and a severely intoxicated person may have to actually move the entire head to attempt to track an object with the eyes, nystagmus is exaggerated. The eyes will twitch or jerk at less than the 45-degree-angle response in a sober person.

There are 47 types of nystagmus, but alcohol is involved with just two types: alcohol gaze nystagmus (AGN) and positional alcohol nystagmus (PAN). The standardized field sobriety test for horizontal gaze nystagmus tests a person for alcohol gaze nystagmus.

In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight horizontally with his eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center.

If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.10 or greater or the driver may have ingested any of a number of drugs such as seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.

NHTSA research indicates that this test allows proper classification of approximately 77 percent of suspects.