Horizontal Gaze Nystagmus (HGN) refers to an involuntary jerking occurring as the eyes gaze toward the side. In administering the HGN test, the officer has the suspect follow the motion of a small stimulus with the eyes only. Nystagmus is a term used to describe a bouncing eye motion. Although there are several types of nystagmus, alcohol causes two types alcohol gaze nystagmus, which includes HGN, and positional alcohol nystagmus. The officer must administer the test in a way that ensures the suspect’s eyes can be clearly seen, either in a well-lit area or by using a flashlight to illuminate the suspect’s face. The suspect should not face toward the blinking lights of a police car or passing cars, which may cause optokinetic nystagmus.
The officer gives the suspect the following instructions
- I am going to check your eyes
- Keep your head still and follow the stimulus with your eyes only
- Keep following the stimulus with your eyes until I tell you to stop.
An officer is typically looking for three clues per eyes that would show enhanced nystagmus and therefore indicate a chance that the blood alcohol concentration is greater than 0.08. There is a maximum score of six clues, or three per eye. Four or more clues indicate a BAC of greater than 0.08. Validation applies only when the tests are administered in the prescribed standardized manner. If any one of the standardized field sobriety tests elements is changed, the validity is compromised.
The Lack of Smooth Pursuit
As the eye moves from side to side, does it move smoothly or does it jerk noticeably? The eyes
can be observed to jerk or bounce as they follow a smoothly moving stimulus, such as a
penlight. The eyes of an unimpaired person will follow smoothly, i.e., windshield wipers
moving across a wet windshield.
Distinct and Sustained Nystagmus at Maximum Deviation
When the eye moves as far to the side as possible and is kept at that position for several
seconds, does it jerk distinctly? Distinct and sustained nystagmus will be evident when the eye is held at maximum deviation for a minimum of four seconds. People exhibit slight jerking of the eye at maximum deviation, even when unimpaired, but this will not be evident or sustained for more than a few seconds. When impaired by alcohol, the jerking will be larger, more pronounced, sustained for more than four seconds, and easily observable.
Onset of Nystagmus Prior to 45 Degrees
As the eye moves toward the side, does it start to jerk prior to a 45-degree angle? The point at which the eye is first seen jerking. If the jerking begins prior to 45 degrees it is evident that the person has BAC above 0.08. The higher the degree of impairment, the sooner the nsytagmus will be observable.
There are a number of factors, however, that are often mistakenly interpreted as signs of intoxications. Examples include
- Physical Disabilities
- Mental Disabilities
- Distractions During Testing
- Poor Balance and Coordination
- Sleep Deprivation
- Weather Conditions
- Road Conditions
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