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The presence of distinct and sustained nystagmus during the Horizontal Gaze Nystagmus (HGN) test is an indicator of alcohol impairment. Nystagmus refers to the involuntary movement of the eyes, and in the context of the HGN test, it specifically assesses how the eyes track moving objects. When an individual is under the influence of alcohol, this eye movement can become more pronounced, showing distinct and sustained patterns in response to horizontal gaze.

In law enforcement context, HGN is commonly used during field sobriety tests to help officers determine whether a driver might be impaired by alcohol. This measurement is crucial because it helps in establishing probable cause for further investigation or for an arrest. The HGN's reliability in assessing impairment from alcohol, as opposed to other influences like visual acuity or motor skills, makes it a pivotal tool for officers in evaluating a suspect's level of impairment.