If a subject displays nystagmus at 45 degrees, what is implied about their level of impairment?

When a subject exhibits nystagmus at a 45-degree angle during the Horizontal Gaze Nystagmus (HGN) test, it indicates a significant level of impairment. Specifically, nystagmus becomes noticeable at this angle when a person has a Blood Alcohol Concentration (BAC) typically at or above 0.08%. This threshold is significant because it aligns with the legal definition of impairment for drivers in many jurisdictions, including Arizona.

The mechanism behind this is that high BAC levels can severely affect the central nervous system and disrupt the vestibular system, which governs balance and eye movements. Therefore, a subject displaying nystagmus at 45 degrees is likely under the influence to such an extent that it impairs their ability to safely operate a vehicle, thus suggesting a likely impairment with a BAC exceeding the legal limit.