Horizontal Gaze Nystagmus Test
The horizontal gaze nystagmus test is one of three standardized field sobriety tests that officers typically ask DWI suspects to perform.
What Is Horizontal Gaze Nystagmus
Horizontal Gaze Nystagmus or HGN is a side to side movement or bouncing of the eyeball. It is an involuntary motion that the person cannot control. A person with HGN is unaware of it because the bouncing of the eye does not affect the person’s vision.
How the Test Is Supposed to Be Administered
As with all SFSTs, the officer conducting the test must follow the protocol established by the National Highway Traffic Safety Administration (NHTSA) or the validity of the test results is compromised. The suspect follows a stimulus (a finger, pen, or pen light) with the eyes. The officer observes smooth pursuit, nystagmus prior to a 45 degree angle, and nystagmus at maximum deviation.
NHTSA Test Protocol
The officer is instructed to give the test as follows:
Give pre-test instructions to suspect.
- Have suspect remove glasses if any.
- Tell suspect to:
- put feet together and place hands at the sides of the body.
- keep the head still during the test.
- look at the stimulus.
- follow the movement of the stimulus with the eyes only.
- continue looking at the stimulus until told that the test is over.
Commence the Test.
- 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 start with the suspect’s left eye.
- Move the stimulus so that 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 so that 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 so that 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: (1) Lack of smooth pursuit. (2) Distinct nystagmus at maximum deviation. (3) 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.
Interferences and Errors
Environmental conditions may interfere with the test. These include wind or dust irritating the subject’s eyes, or visual or other distractions such as rotating or strobe lights from the patrol car or passing traffic. In addition, nystagmus may have causes other than alcohol. Head injuries, brain tumors, disorders of the inner ear and other medical conditions may be responsible. Moreover, significant numbers of normal individuals have naturally occurring nystagmus. Fatigue nystagmus may be occur if the subject’s eyes are kept at maximum deviation for more than 30 seconds. Various medications, as well as caffeine, nicotine, or aspirin may cause nystagmus.