Horizontal Gaze Nystagmus (HGN) is an eye test.
Nystagmus is defined as an involuntary jerking of the eyes as the eyes move from the center position off to the side. Alcohol and some drugs cause HGN. If administered properly, HGN is 77% determinative of the subject having a blood alcohol count (BAC) over .10 percent.
In order to get an accurate result, HGN must be performed properly. In order to rule out the possibility that HGN has been caused by a medical condition, the eyes of the subject must be observed in a resting position. The officer should be facing the subject, with both officer and subject standing. The eyes must first be checked for equal pupil size, resting nystagmus, and equal tracking. Equal tracking refers to the ability of the eyes to follow an object together. If the pupils are noticeably unequal in size, or if the eyes do not track together, there is a chance that the nystagmus is the result of injury or a medical condition.
After it is determined that there is no injury or medical condition, the officer will begin the HGN test. The officer and the subject should both be standing and facing each other. If the subject is wearing eyeglasses, the subject should remove them. The officer should give the subject clear directions. These directions are: 1. “I am going to check your eyes” 2. “Keep your head still and follow this stimulus with your eyes only” 3. “Keep following the stimulus with your eyes until I tell you to stop”
The stimulus that the officer uses may be a pencil, pen, or slim flashlight, called a penlight. The stimulus must be held 12-15 inches from the subject’s nose and held slightly above eye level. There are 3 different “clues” that the officer is looking for.
Clue One: The Lack of Smooth Pursuit
The stimulus begins in the center position and then is moved to the subject’s left, as far as the subject’s eyes can go to the side, so that no white is showing in the corner of the eye. This position is held for 2 seconds and then the stimulus is brought to the subject’s right, as far as the subject’s eyes can go to the side. This position is also held for 2 seconds. This procedure is then repeated. The officer is looking to see whether the eyes move smoothly or if they jerk or bounce. An example of the eyes moving smoothly is like a marble moving across a pane of glass. An example of jerking or bouncing is a marble moving across a piece of sanding paper, in that it does not move in a straight line.
Clue Two: Distinct and Sustained Nystagmus at Maximum Deviation
The stimulus will begin in the center position and then move to the subject’s left as far as the subject’s eyes can go. This position is held for 4 seconds and then the stimulus is brought to the subject’s right as far as the subject’s eyes can go to that side and held for 4 seconds. This procedure is then repeated. The officer is looking to see if the eye has distinct and sustained jerking when the eye is at maximum deviation (off to the side).
Clue Three: Onset of Nystagmus Prior to 45 Degrees
The stimulus begins in the center position and then moved to the subject’s left, approximately the edge of the subject’s shoulders, and held for 4 seconds. This is then repeated, to the subject’s right. This procedure is then repeated. The officer is looking for a distinct jerking. If the officer sees jerking, the officer is supposed to hold the stimulus in place and confirm that the jerking is present. In order to determine that the stimulus is indeed brought to just before 45 degrees takes a great amount of skill and practice on the part of the law enforcement officer.
The officer will note all clues exhibited at each stage, which includes both the right and the left eye. If the officer determines that the subject exhibited 4 or more clues during the HGN test, then there is a 77% probability that the subject has a BAC above .10 percent.
Caution
If the officer holds the subject’s eyes at maximum deviation for more than 30 seconds, the subject’s eyes will become fatigued and will reflect nystagmus. This is referred to as Fatigue Nystagmus. By: DARREN KAVINOKY