Background: Law enforcement officers routinely conduct psychophysical tests to determine if an impaired driver may be intoxicated or in need of medical assistance. Testing includes assessment of eye movements, using the Horizontal Gaze Nystagmus (HGN) and Vertical Gaze Nystagmus (VGN) tests, which are conducted at roadside by patrol officers. These tests previously have been validated when the subject is placed in a standing posture with head upright. However, certain conditions require that the subject be tested while seated or supine. Under these conditions, Positional Alcohol Nystagmus (PAN) could be induced and mistaken for HGN or VGN.
Methods: The study was conducted at law enforcement training academy alcohol workshops in the Pacific Northwest. Ninety-six volunteer drinkers were tested when sober and three times after drinking alcohol by 40 volunteer officers experienced in administering the tests. Blood alcohol concentration (BAC) was measured objectively with a calibrated breath analysis instrument each time a subject was tested.
Results: The number of eye movement signs observed during the HGN test at any posture increases with increasing BAC. The presence of VGN at any test posture occurs only in the presence of signs of HGN and only at high levels of impairment. PAN was most often observed at BACs of 0.08% and higher, but was never confused with the observation of HGN or VGN, regardless of test posture.
Conclusions: The HGN test administered in the standing, seated, and supine postures is able to discriminate impairment at criterion BACs of 0.08% and 0.10%. The VGN test can identify high levels of impairment at any test posture. Therefore, these tests can be used by an officer to determine if a driver is impaired, regardless of whether the driver is standing, seated, or supine.