The Horizontal Gaze Nystagmus (HGN) test is a field sobriety test used by law enforcement to determine whether or not a suspected driver is under the influence of alcohol. The test involves an officer asking a driver to follow some stimulus, usually a pen, with their eyes to the left and right. The officer will then determine when the eye begins involuntarily jerking, or exhibiting nystagmus. While all eyes experience this involuntary twitch when looking at a significant angle, those with higher blood alcohol levels begin twitching sooner. According to the National Highway Transportation Safety Administration, HGN tests are 77% reliable in determining whether or not a driver’s BAC is above .10%.
How does an officer determine nystagmus?
A HGN test cannot confirm or deny the amount of alcohol that a suspected driver may have in their system, but indicates to the police officer that there may be some level of impairment present.
When testing HGN, an officer will check for the following questions:
- Are the eyes moving smoothly from one side to the other or noticeably jerking?
- Are the eyes jerking when they have moved as far as possible to one side?
- Does the eye begin jerking before it has reached a 45-degree angle?
There are a few factors that can affect the outcome of an HGN test. A suspect that is wearing eyeglasses may make it more difficult for the officer to see the angle at which the suspect’s eyes begin to twitch. Additionally, if the HGN test is not administered in a well-lit area, the officer may not be able to tell when nystagmus occurs. Suspects should not be facing headlights or the lights of a police vehicle since this can cause nystagmus without the help of alcohol. If any of these apply in a suspected driver’s
case, the results of the HGN test may be inadmissible in court.
There are other defenses to HGN tests, including the ability of the officer to determine a 45-degree angle, the presence of prescription drugs in the body of the accused, the presence of a head injury, and more. Working with a DUI defense attorney will be the best bet in clarifying the cause of nystagmus and whether the results of this test can be permitted in court.