What is the primary indicator of impairment when evaluating horizontal gaze nystagmus?

The primary indicator of impairment when evaluating horizontal gaze nystagmus is the distinction of nystagmus at maximum deviation. In the context of the Drug Recognition Expert (DRE) evaluation, horizontal gaze nystagmus involves the involuntary jerking of the eyes as they look to the side. When observing this phenomenon, specifically at maximum deviation, it indicates the extent of impairment due to substances affecting the central nervous system, such as alcohol or certain drugs.

The appearance of nystagmus at this point is critical, as it suggests that the individual has a significant level of impairment. The presence of nystagmus at maximum deviation points to the body's inability to maintain normal eye function, which is directly linked to impairments typically associated with intoxication. This evaluation technique is key in assessing whether an individual is under the influence, making it a crucial part of the DRE’s assessment process.

The other options may indicate various conditions or symptoms, but they do not directly correlate to the measurement of impairment in the same way that the observation of nystagmus does. Headaches and difficulty standing still can arise from many causes that are not related to drug use, while an increased heart rate may not necessarily indicate impairment and can occur for numerous