Horizontal gaze nystagmus test
DUI standardized field sobriety tests
There are three field sobriety tests that have been “standardized” by the National Highway Traffic Safety Administration:
Definition of nystagmus
Nystagmus is the rhythmic back and forth oscillation of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye.
All people have slight oscillation of the eyes normally. There are two major types of eye movements: pendular and jerk. Horizontal Gaze Nystagmus (HGN) is a type of jerk nystagmus with the saccadic movement toward the direction of the gaze.
HGN is an involuntary motion, meaning the person exhibiting the nystagmus cannot control it. Furthermore, the subject exhibiting nystagmus is unaware that it is happening because the bouncing of the eye does not affect the subject’s vision.
Administration of the test
The suspect follows a stimulus with the eyes. The officer observes smooth pursuit, nystagmus prior to a 45 degree angle, and nystagmus at maximum deviation.
The officer is instructed to give the test as follows:
- Have suspects remove their glasses if they are wearing them.
- Tell suspects to put their feet together and place their hands at their sides.
- Tell suspects to keep their head still during the test.
- Tell suspects to look at the stimulus.
- Tell suspects to follow the movement of the stimulus with their eyes only.
- Tell suspects to continue looking at the stimulus until they are told that the test is over.
- Position the stimulus approximately 12 to 15 inches from the nose in and slightly above eye level to commence the test.
- Check for equal tracking of the eyes.
- Check for equal pupil size.
- Check the eyes for lack of smooth pursuit. Always starting with the suspect’s left eye. Move the stimulus at a speed where it takes 2 seconds to bring the eye as far to the side as it can go and 2 seconds back. Perform twice in each eye.
- Check the eyes for distinct nystagmus at maximum deviation. Start with the left eye. Move the stimulus at a speed where it takes 2 seconds to bring the eye as far to the side as it can go. Hold in the maximum position for at least 4 seconds. Perform twice.
- Check the eyes for the onset of nystagmus prior to 45 degrees. Start with the left eye. Move the stimulus at a speed where it takes 4 seconds to reach the edge of the suspect’s shoulder. When nystagmus is observed, stop and verify that the jerking continues.
- Total the clues.
- Check for Vertical Nystagmus. Position the stimulus horizontally 12-15 inches in front of the suspect’s nose, slightly above eye level. Instruct the suspect to hold the head still and follow the object with the eyes only. Raise the stimulus until the suspect’s eyes are elevated as far as possible. Hold for 4 seconds and watch for evidence of jerking.
Scoring the test
The Horizontal Gaze Nystagmus test has three clues of impairment:
- Lack of smooth pursuit.
- Distinct nystagmus at maximum deviation.
- Onset of nystagmus prior to 45 degrees.
The test is administered with two passes, so a total of six clues are possible. A minimum of four clues are needed to determine if the suspect’s BAC level is above 0.10 percent.
Possible interferences or errors
The officer is warned that there are conditions that may interfere with the test. Such conditions include wind or dust irritating the subject’s eyes, or visual or other distractions impeding the tests. Examples include rotating or strobe lights or traffic passing in close proximity.
In addition, the Officer is warned that nystagmus may be due to causes other than alcohol, and that fatigue nystagmus may be present if the subject’s eyes are kept at maximum deviation for more than 30 seconds.
Also, various medications, as well as Caffeine, nicotine, or aspirin may cause nystagmus.