The Horizontal Gaze Nystagmus Test
The Horizontal Gaze Nystagmus test
The Horizontal Gaze Nystagmus test is generally considered the most accurate of the three (3) Standardized Field Sobriety Tests and the only one not affected by age, size, or physical limitations. When consistently administered first, the National Highway Traffic Safety Administration research claims that this test prejudices some officers’ expectations and scoring of the other two (2) tests.
Nystagmus is defined as the involuntary jerking or twitching of the eye. There are more than forty (40) types of eye nystagmus, but the National Highway Traffic Safety Administration research claims that intoxicated drivers exhibit distinct eye movements. Specifically, the Horizontal Gaze Nystagmus test is intended to identify and measure three signs of intoxication. These include claims that:
(1) An intoxicated driver’s eyes experience nystagmus with less side to side eye movement than a sober driver’s eyes,
(2) The greater the alcohol impairment, the greater and more distinct the intoxicated driver’s eyes demonstrate nystagmus at the extreme gaze position, and
(3) An intoxicated driver can’t follow a slowly moving object smoothly with his eyes.
The crucial factor of measuring Horizontal Gaze Nystagmus is correctly “estimating” when the eye has reached a deviation angle of forty five (45) degrees.
The National Highway Traffic Safety Administration maintains that when a driver’s blood alcohol concentration is above .10, the twitching or jerking will begin before his eye has moved forty five (45) degrees to the side.
The Horizontal Gaze Nystagmus test requires the use of an object for the driver’s eyes to follow. This can be a fingertip, penlight, or pen. Whatever object is used, it must be held slightly above eye level and twelve (12) to fifteen (15) inches away from the nose. While administering the Horizontal Gaze Nystagmus test, the officer must:
(1) Instruct the driver that he is going to check his eyes, that he must keep his head still and follow the object only with his eyes, and that he must focus on the object until told to stop.
(2) Hold the object twelve to fifteen inches from the driver’s nose, slightly above eye level. The officer must move the object smoothly across the driver’s entire field of vision and check to see if the eyes are tracking together or whether one eye lags behind the other. If the eyes do not track together, it could be a sign of a medical disorder, injury, or blindness.
(3) Check to see that both pupils are the same size. If not, this could be evidence of a head injury.
(4) Start with the left eye and smoothly move the object as far to the right as the driver’s eyes can go in two seconds. He must then move the object similarly to the left to check the driver’s right eye.
(5) Starting with the left eye, check twice for all three clues in each eye.
(6) Check for the clues in this precise sequence: (a) lack of smooth pursuit, (b) nystagmus at maximum deviation, and (c) onset of nystagmus prior to forty five degrees.
(7) Move the object to the side until no white is showing at the side of the driver’s eye. Further, the officer must hold the position for four seconds.
(8) When checking for nystagmus onset angle, the officer must move the object at a speed that would take about four seconds to reach the edge of the driver’s shoulder while watching the eye for twitching and jerking.
(9) Keep faith with the four second speed of the object movement because if the object moves too fast, the officer could go past the point of onset or miss it altogether.
(10) Notice the driver’s eyes start to jerk or twitch before forty five degrees. Specifically the officer must check to see that some white is still showing on the side of the eye closest to the ear. If no white shows, this means either that the officer has taken the eye too far to the side (more than 45 degrees) or the person has unusual eyes that do not deviate very far.
The Horizontal Gaze Nystagmus test has three (3) scoring clues that are measured for each eye, for a maximum total of six (6) possible scoring clues. If four (4) or more clues are observed, the National Highway Traffic Safety Administration research concluded that the officer must classify the driver as having a blood alcohol concentration in excess of .10.
The scoring clues are as follows:
(1) Lack of smooth pursuit (the eyes bounce or jerk as they follow the object).
(2) Distinct nystagmus at maximum deviation (eye looking as far to the outside as possible) when held for four seconds. Some people’s eyes bounce or twitch at maximum deviation even when sober, but an intoxicated driver’s jerking is supposed to be “very pronounced, and easily observable.”
(3) Onset of nystagmus before the eye has moved forty five degrees.
The National Highway Traffic Safety Administration specifically does not support the position that the exact onset angle can be used to estimate a person’s specific blood alcohol concentration and considers this to be a misuse of the Horizontal Gaze Nystagmus test.