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Horizontal Gaze Nystagmus

The Horizontal Gaze Nystagmus evaluation must be done properly to have validity. The effect is subtle and really only highly-trained, experienced officers perform the evaluation correctly.

There are three evaluations that have been standardized by the National Highway Traffic Safety Administration such that, if done properly, there exists a correlation between how those evaluations were performed by a subject and whether that person may be intoxicated. These evaluations are termed Horizontal Gaze Nystagmus, the Walk and Turn, and the One Leg Stand. The following is a brief synopsis of the Horizontal Gaze Nystagmus, or HGN, evaluation.

Horizontal Gaze Nystagmus (HGN) is an involuntary jerking of the eyes as a person gazes to the side. Officers and prosecutors like this test because a person is not aware their eyes may be displaying HGN, a person cannot control or stop it, and a person's tolerance for alcohol does not effect HGN. There are many causes of HGN, but among those causes are alcohol and drug ingestion. During the HGN evaluation, an officer will be observed to make several passes in front of a subject's face, asking the subject to keep their head still and follow a target object (the tip of their finger or a penlight) with their eyes and their eyes only. 

It is important to know that nystagmus, though real, is also very subtle. In practice, only experienced officers who've been well trained, and have done the evaluation for a long time, will be any good at determining if nystagmus is present. Police officers, even the best, are usually not ophthalmologists or experts in the physiology of the human eye. Yet at trial, they will be offered by the prosecution as someone skilled enough to perform what are very complex, delicate observations and, from those observations, make further important determinations. But to date, I am not aware of any camera that is in use which records the subject's eyes while performing the HGN evaluation so that, later, a judge or jury may look for themselves and decide whether they think nystagmus is present. 

When an officer performs the HGN evaluation on a DUI suspect, in order to obtain results that are meaningful, the procedure for instructing, and then observing, a subject must be followed strictly. Otherwise, the HGN evaluation is meaningless. For instance, if during a DUI investigation an officer tells the subject “You're not looking at my light,” or instructs the subject to stop moving their head, then that part of the evaluation has been compromised and must start over. Also, nystagmus can be induced by the officer. If the officer holds the target object too high such that the subject's chin is observed to move up, any observed nystagmus may be due to this added motion of the head versus simply alcohol.

There are many other human and external environmental factors that may influence and cause nystagmus which we will not go into here.

Assuming the officer has established that the HGN evaluation may be performed on a subject – no pre-existing conditions exist which would make the procedure invalid – the officer then proceeds to look for (1) Lack of Smooth Pursuit, (2) Distinct and Sustained Nystagmus at Maximum Deviation, and (3) the Onset of Nystagmus Prior to 45 Degrees in that order. To be a valid indicator of alcoholic intoxication, both the subject's right and left eyes must display the particular indicator of nystagmus. That is to say, if an officer observes Lack of Smooth Pursuit because of nystagmus in a subject's right eye, then they must also observe it in the subject's left eye if it is to be properly considered an indicator of alcoholic intoxication. If nystagmus is observed in one eye and not the other, either the officer made a mistake, or the subject has a medical issue (aside from intoxication) with their eyes. 

The HGN evaluations are cumulative. When performed properly, Lack of Smooth Pursuit nystagmus shows up when a person's Blood Alcohol Content (BAC) level is around 0.04, Distinct and Sustained Nystagmus at Maximum Deviation shows up at around a BAC of 0.08, and the Onset of Nystagmus Prior to 45 Degrees shows up at around 0.10 BAC. For example, a properly performed HGN evaluation on a subject with a relatively low blood alcohol level may indicate Lack of Smooth Pursuit, but no other indicators or clues of HGN. What is not valid would be for an officer to conclude they observed Distinct and Sustained Nystagmus at Maximum Deviation and/or the Onset of Nystagmus Prior to 45 Degrees, but not Lack of Smooth Pursuit.

When intoxication by alcohol is the culprit, the HGN clues come in pairs, for both right and left eyes, and the clues add up in order 1-2-3. 

The officer will also check a subject's eyes for Vertical Gaze Nystagmus (VGN). When present, VGN can indicate either a high BAC level for the subject, or that the subject is under the influence of other drugs. 

The above is a very cursory primer into what an officer is doing when they perform an HGN evaluation during a DUI investigation. Among many things not covered, also omitted are the actual mechanics of conducting the various parts of an HGN evaluation. These mechanics are important as there are many places for the officer to make mistakes, but without a through and sifting counter-investigation of HGN procedure, police errors will be held against the investigatee, not the investigator! Indeed, the prosecution may try to pin a large part of its DUI case against a defendant based on bad HGN evidence. And, unlike the other Standardized Field Sobriety Tests – the Walk and Turn and the One Leg Stand – a judge and jury can only see what the officer is doing as they perform the HGN evaluation.  They may not even be able to see that clearly! What definitely cannot be seen is what the officer says they observed, or thought they observed. It is critical for the defense of a DUI that the portion of the video showing the HGN evaluation is examined carefully, analyzed critically, and, truth be told, mercilessly picked apart when warranted. And, truth be told, it often is warranted.

Copyright © 2024 Alan J. Levine, Esq.

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