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Alcohol Gaze Nystagmus (agn)

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Definition - What does Alcohol Gaze Nystagmus (agn) mean?

Nystagmus or "an involuntary rapid movement of the eyeball, which may be horizontal, vertical, rotatory, or mixed" can be exaggerated for individuals who are severely intoxicated. Increased nystagmus can occur because alcohol slows the eyes ability to track objects smoothly, and the eyes will instead jerk or oscillate.

Although there are many types of nystagmus, police officers frequently use the horizontal gaze nystagmus or HGN test as part of their common battery of field sobriety tests. The procedure for the HGN test is outlined by the National Highway Traffic Safety Administration (NHTSA) in the DWI Detection and Standardized Field Sobriety Testing Student Manual.

For the HGN test the officer will evaluate the driver's pupils as they track an object. During the test the officer can make several observations which can help them determine if a driver is intoxicated or if they have ingested other types of drugs including inhalants, barbiturates, seizure medication or other depressants.

The horizontal gaze nystagmus test should only be completed after an officer has concluded the driver does not have any other type of injury or medical condition that may cause increased nystagmus. Common procedures for the HGN test include holding a stimulus (pen or small light) 12 - 15 inches from the driver's nose and moving it slowly from side to side. The movement should be smooth, and the tip of the stimulus should be held slightly above the driver's eyes.

Officers are looking for the following:

  • The driver is unable to smoothly track an object.
  • The eye jerks distinctly and has sustained nystagmus at maximum deviation.
  • The onset of the nystagmus is prior to 45 degrees.

The National Highway Traffic Safety Administration (NHTSA) has concluded that the reliability of the horizontal gaze nystagmus test is high, especially when it is used in combination with other divided attention tests.

Related Links

  • NHTSA information -- the National Highway Traffic Safety Administration's strategy on discouraging impaired driving

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