Horizontal Gaze Nystagmus Test

One of the first tests, the police will administer is the Horizontal Gaze Nystagmus Test (HGN). The horizontal gaze nystagmus relates to the relationship between drinking alcohol and the onset of nystagmus in the human eye that has been well documented by researchers. Nystagmus is a natural phenomenon related to the involuntary jerking of the eyes. Alcohol does not cause nystagmus, but may exaggerate or magnify the involuntary jerking of the eyes. Police officers who have trained correctly conduct the HGN in the following manner:

1. Begin by asking “Are you wearing contact lenses”. Make a note whether or not the suspect wears contact lenses before starting the test.

2. If the potential suspect is wearing eyeglasses, have the suspect remove them.

3. The officer should give the suspect the following instructions from a position of safety:

o “I’m going to check your eyes.”

o “Keep your head still and follow the stimulus with your eyes only.”

o “Keep focusing on the stimulus until I tell you to stop.”

4. The police officer should then position the stimulus, usually a pen, approximately 12 to 15 inches from the suspect’s nose and slightly above the suspect’s eyes.

5. The officer should then check the suspect’s eyes for the ability to track the stimulus. According to National Highway Safety Transportation Administration (NHTSA), if the eyes don’t track together it could indicate a possible medical disorder, injury or blindness.

6. The police officer should next check to see that both pupils are equal in size. If the pupils are not equal in size, this may indicate a head injury not related to alcohol.

7. The police officer should next check the suspect’s left eye while moving the stimulus to his right. The officer should move the stimulus smoothly at a speed that requires about two seconds to bring the suspect’s eyes as far to the side as they can go. While the police officer moves the stimulus, he is to look at the suspect’s eyes and determine whether the eye is able to track the object smoothly. Then the police officer should move the stimulus all the way to the left, back across the suspect’s face, checking to see if the suspect’s right eye is able to track the stimulus smoothly. Movement of the stimulus should take approximately two seconds out and two seconds back for each eye. The police officer should then repeat the procedure for the other eye.

8. Next, the officer should check both eyes for lack of smooth pursuit, and also examine the suspect’s eyes for distinct nystagmus at maximum deviation beginning with the suspect’s left eye. The police officer should simply move the stimulus to the suspect’s left side until the eye has gone as far to the side as possible. Usually, no white will be showing in the corner of the eye at maximum deviation. The police officer should use the stimulus to hold the suspect’s eye at that position for about four seconds and observe the eye for distinct nystagmus. The police officer should then move the stimulus all the way across the suspect’s face to check the right eye, holding that position for approximately four seconds and then repeating the procedure.

9. After law enforcement checks the suspect’s eyes at maximum deviation, he or she should then check for onset nystagmus prior to 45 degrees by starting to move the stimulus to the right (the suspect’s left eye) at a speed that would take about four seconds for the stimulus to reach the edge of the suspect’s shoulder. The police officer should watch the eye carefully for any sign of jerking. If the police officer observes jerking, they should stop the test and verify that the jerking continues. The police officer should then move the stimulus to the left (the suspect’s right eye) at a speed that would take about four seconds for the stimulus to reach the edge of the suspect’s shoulder. Again, the law enforcement officer should watch the suspect’s eye carefully for any sign of jerking. If the police officer sees jerkiness of the eyes, he should stop and verify that the jerking continues in the procedure.


Did you fail The HGN?

The police officer will look for the following three clues on the HGN test to determine whether the suspect’s blood alcohol content is above a .10. The following three clues are as follows:

· the eye cannot follow an object smoothly

· nystagmus is distinct when the eye is at maximum deviation

· the angle of onset of nystagmus is prior to 45 degrees

According to NHTSA, if the police officer observes 4 or more clues total for both eyes it is likely that the suspect’s blood alcohol content is above .10.

Hiring an experienced criminal defense attorney is imperative as there are many causes of exaggerated nystagmus other than being impaired by alcohol. For example, some other possible causes of nystagmus are inner ear problem, flu, infection, measles, syphilis, brain hemorrhage, epilepsy, glaucoma, toxins, arm muscle imbalance, exposure to solvents such as paint, prescription drugs, excessive amounts of caffeine or nicotine, motion sickness, eye muscle fatigue, and vertigo among others. An experienced criminal defense attorney can determine if the police officer properly conducted the test and whether or not the cause of the suspect’s nystagmus was related to alcohol or perhaps related to one of the other conditions stated above.

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