The Horizontal Gaze Nystagmus "Test" Explained
The suspect must be instructed to look straight ahead, keeping the head still while following and focusing on the stimulus with the eyes until told to stop. The stimulus must be twelve to fifteen inches in front of the suspect's eyes for ease of focus. The officer is trained to receive an acknowledgement from the suspect that the stimulus is at a comfortable distance from the suspect's eyes and to document same.
Even though this test is not a vision test, per se, eyeglasses are to be removed in order for the officer to make a more accurate determination of the final total points. If the suspect can not see the stimulus after removing the eyeglasses they must be allowed to perform it with them on. According to the manual, hard contact lenses are to be removed so as to avoid dislodging when the eyes are out at maximum deviation or to prevent damage to the eyes.
The manual says not to administer the test if the suspect is wearing hard contacts. A person with a glass eye or only vision in one eye can not be given this test. An evaluation of just one eye and a doubling of the score, assuming that the other eye will render the same results is both erroneous and improper. If the suspect has what is known as the lazy eye condition, the officer is trained to test one eye while the other eye is covered by the suspects hand, then to switch same. A person who is color blind is not validated for this test as they will probably have a pathological Nystagmus which is normal, and natural for that person.
Three to four percent of the general population will exhibit a pathological Nystagmus. This can be caused by some type of neurological disorder, brain damage, epilepsy or pathological disorder which the suspect is born with or of unknown etiology. A large disparity between the right and left eye can clue the officer into this problem. At an accident scene, if the suspect sustains a concussion, this may bring on a pathological Nystagmus thereby invalidating this test.
Although very few test conditions affect gaze Nystagmus, there are certain administrative procedures that must be followed. As previously mentioned, the stimulus must be placed twelve to fifteen inches in front of the suspect's eyes. The stimulus should be held above eye level, so that the eyes are wide open when they look directly at it. Due to narrowness of certain individuals eyes it becomes more difficult to make a fair evaluation of the Nystagmus unless the eyes are wide open.
If the officer believes that the Nystagmus might be there, it can not be scored, as the benefit of the doubt must be given to the person that is being tested. The officer is also trained to administer this test with the suspect looking into a quiet background, away from the police cruisers overhead. This is to avoid the probability of evaluating an induced condition known as opto-kinetic Nystagmus. This is brought on when a person focuses on several objects at one time or objects that are moving away. The quiet background that is recommended is away from the view of passing motorists. This opto-kinetic Nystagmus is a defense mechanism of the body in order to keep the eyes from tiring. There are numerous visual or other distractions that may impede this test. Certain environmental factors such as wind, dust, etc. may interfere with the performance of the Nystagmus test.
As explained earlier, Nystagmus is the involuntary jerking of the eyes. In intoxication, three signs will often be observed. Each eye must be checked separately. The first clue that is looked for is known as smooth pursuit.
First Clue: Lack of Smooth Pursuit
What the officer is trained to look for is the suspect's inability to pursue a moving stimulus smoothly while focusing that stimulus being moved horizontally, from side to side. If the suspect moves his head to the side at any time, the score may be invalid regardless of which clue we are looking for. An example of what is being looked for is like a marble rolling across a smooth pane of glass. This will be a very smooth pursuit. If the suspect is under the influence, the eyes will bounce or jerk in similar fashion as that same marble, but now rolled across a piece of sandpaper. The officer is instructed to check the left eye first by moving the object to the officer's right. The object must be moved smoothly, at a speed of about two seconds to bring the suspect's eye as far to the side as it can go. Any choppy or shaky hand movements or movement that is too fast by the officer may induce a Nystagmus in the suspect's eyes and invalidate the scoring and test. The officer is instructed to make two or more passes in front of the eye to be absolutely certain that what they are seeing is Nystagmus and a valid clue. If this clue is scored as Nystagmus the suspect is assessed one point. If the suspect has this clue emanating in one eye, it is no guarantee that it will be exhibited in the other eye. This should be the easiest clue to see.
Second Clue: Distinct Jerkiness at Maximum Deviation
After they have checked the first eye for the smooth pursuit clue, they must check the same eye for what is called distinct jerkiness at maximum deviation. This is accomplished by simply moving the object to the side until the eye has gone as far to the side as possible. At maximum deviation, no sclera or white will be showing in the corner of the eyeball. The officer must hold the eyeball at that position for two or three seconds, and observe the eyeball for distinct jerkiness. The jerkiness must be both distinct and obvious.
If the officer can not make this distinction from a slight nystagmus, the benefit of the doubt must be given the suspect. The officer may make the mistake of not bringing the eyes out to side as far as they can go or too rapidly return the stimulus and incorrectly score this part of the test. The criteria of no white showing in the corner of the eye must be met. A Nystagmus may be incorrectly mistaken for physiological Nystagmus if the officer does not hold the stimulus out to the side for two or three seconds.
In approximately 50% of the suspects they encounter, a physiological Nystagmus will be apparent when the eye initially arrives at maximum deviation. This is due to a person not normally following an object all the way to the side with their eyes, but turning their head to view that object. Consequently, a certain degree of uncomfortableness is experienced, causing a slight twitching of the eyes at this location. If the officer brings the eyes out to the side and then immediately brings them back, he may be scoring this physiological Nystagmus inappropriately.
Everyone has a physiological Nystagmus, but it is not visible to the naked eye. The reason it exists is to exercise the eye muscles, lubricate them, and prevent atrophy. Remember the jerkiness must be distinct and obvious to be scored a point.
Final Clue: Angle of Onset
This is the most difficult to evaluate. If a suspect exhibits this clue in one eye, the probability increases that all the other clues will be seen as well. This does not work in reverse though. If a suspect has any of the first two clues, it is no guarantee that the third clue will be there. Because the 45-degree angle is a key factor in assessing a suspect's degree of alcohol influence, it is important to know how to estimate that angle.
The person is told to follow the stimulus until they are looking down the 45-degree diagonal. This obviously is used as practice to recognize the angle. A rule of thumb that is being taught is in order to estimate the 45-degree angle, the stimulus must be halfway between the suspect's ear and nose on the side being tested or just outside the shoulder area.
The estimation of this angle is critical, since studies have shown that as the alcohol increases the angle will decrease. Although this may be the case, this angle should not be used to estimate a specific amount of alcohol in the bloodstream. In order to properly score this part of the test, the officer must move the object to the 45-degree angle of gaze, taking about four seconds. As the eye follows the object, the jerking is looked for. If Nystagmus is observed, the stimulus is stopped and the officer must make an observation that it is continuing at this point. If it continues, then the officer must make sure that there is still white showing in the corner of the eye and the angle must be estimated to be prior than 45-degrees. If it does not, the object must continue to be moved until the jerking does occur and continues or until the 45-degree angle is reached. If no white of the eye is showing, the eye has either been taken too far to the right, which would be maximum deviation that is being evaluated, or the person has unusual eyes that will not deviate very far to the side. The criterion of onset before 45-degrees only can be used if some white can be seen at the outside of the eye. Too often the officer incorrectly estimates the angle or scores this with no white showing in the corner of the eye or both.
Ask yourself this: Can you effectively pick a 45 degree angle without having actual lines to use as a reference point?
This test is deemed the most reliable test in determining probable cause to believe someone is under the influence of an alcoholic beverage. This depends on the proper administration, proper scoring, and proper training is adhered to. Even if this be the case it is still only 77% reliable. This test should not be administered if the suspect is in a supine position. It can be administered to them if they are sitting or standing as long as they are erect.
At an accident scene, with a suspect lying down and this test performed, it would be invalid due to positional alcohol Nystagmus being the cause of the Nystagmus encountered. This is a vestibular Nystagmus that is evident when the amount of alcohol in the vestibule system is in unequal proportions to the amount of alcohol in the bloodstream and reacts to gravity such as the changing position of the head.
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