Horizontal Gaze Nystagmus
The Horizontal Gaze Nystagmus (HGN) is commonly known as the “eye test.” The name of the test can be divided into three parts: 1) Horizontal – the eye travels from side to side. 2) Gaze – to look steadily or intently at something and 3) Nystagmus – the rhythmic back and forth oscillation of the eyeball that occurs when the eyeball that occurs when there is a disturbance of the inner ear system or the oculomotor of the eye.
Nystagmus is an involuntary motion, meaning the person exhibiting nystagmus cannot control or is even aware of it.
The procedure for the officer to check the possible impaired driver for Nystagmus is outlined below. It should be noted that some officers perform this test while the driver is still in his vehicle, but this test is invalid because the driver’s head is by definition not straight.
1) Check for eyeglasses
2) Verbal instructions to place his hands on his cheeks and to follow his pen or other item without moving his head from side to side.
3) The officer places his pen directly in front of the driver’s eyes and approximately 12-15 inches away.
4) The officer should watch the eyes for movement.
5) In some cases, there will be no Smooth Pursuit of the eyes.
6) In some cases, there will be distinct and sustained nystagmus at maximum deviation (holding the pen so the driver sees it as far right or left as possible).
7) In some cases, the officers will observe nystagmus prior to 45 degrees on either side (when 90 degrees is looking straight forward.
8) The officer will then total the cues.
The officer should look for three cues of nystagmus in each eye (total of six cues).
1) The eye cannot follow a moving object smoothly.
2) Nystagmus (or jerking) is distinct and sustained when the eye is held at maximum deviation for a minimum of four seconds.
3) Then angle of onset of nystagmus is prior to 45 degrees.
Based on the original research, if the officer observes four or more cues, it is likely that the driver’s Blood alcohol content is above .08.
There are a number of causes of nystagmus that has nothing to do with alcohol. A complete list can be obtained from the literature but they include influenza, vertigo, epilepsy, measles, muscular dystrophy, brain hemorrhage and the use of PCP. Additionally, conditions such as hypertension, motion sickness, sunstroke, eyestrain and glaucoma may result in gaze nystagmus.
Positive cues for nystagmus alone may result in evidence for the case but the driver (and officer) should realize that there are many other reasons for nystagmus.
Youtube Video for HGN Test: