What effect does onset of nystagmus prior to 45 degrees indicate during the HGN test?

During the Horizontal Gaze Nystagmus (HGN) test, the presence of nystagmus prior to the subject reaching a 45-degree angle is a significant indicator. Specifically, it suggests that the individual may be excessively intoxicated.

Nystagmus, which refers to involuntary eye movements, can be exacerbated by the consumption of alcohol and certain drugs. As the individual is instructed to follow a stimulus, the onset of nystagmus before achieving the 45-degree angle means that their body is not processing the stimulus effectively, which is consistent with the effects of high levels of intoxication.

In contrast, if nystagmus occurs after the 45-degree mark or not at all, it could imply normal coordination or good visual functioning. Therefore, recognizing the early onset of nystagmus points to potential impairment due to intoxication, which law enforcement utilizes as a critical component in assessing a driver's level of sobriety during field sobriety tests.