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Nystagmus: “The Eye Test”


You may have heard of the "eye test" in DUI investigations.  This is the nystagmus field sobriety test or, more accurately (there are 47 different kinds of nystagmus),  the horizontal gaze nystagmus test.  It is one of the most commonly used field sobriety tests, as it is one of three which make up the federally-approved "standardized battery" of tests, or SFSTs.

The test is essentially a measurement of the movement of the eye.  Simply stated, "nystagmus" refers to a distinctive involuntary jerking of the eyes; horizontal gaze nystagmus is a pendular (back and forth) movement.  This type of nystagmus is commonly measured by the officer in three three different ways, each time using an object such as a pencil, penlight or finger placed a foot or so in front of the suspect’s nose and moving it slowly to the left and then to the right.

The first part of the test is to determine the angle of onset of nystagmus — that is, the angle at which the moving eye begins the jerking motion.  The suspect looks straight ahead and, without moving his head, moves his eyes slowly to the right or left.  The officer is supposedly able to detect when the nystagmus begins and is supposedly able to estimate the angle from straight ahead at the point where it begins.  If the onset is prior to 45 degrees, in theory, the blood alcohol level will be over .05%.

The second part of the test is to note whether the jerking becomes more "distinct" when the eye is moved to the lateral extreme — that is, when there is no longer any white of the eye visible at the outside of the eye.

The third part is to determine whether there is a lack of smooth pursuit: rather than following a moving object smoothly from the beginning, the eye jumps or "tugs".

Under federal standards, the officer is supposed to use an objective scoring criteria for each of the three tests, and the total score determines whether the supect passed or failed. 

In reality, few officers understand the test, administer it correctly, or use objective scoring.  Many simply report that they "detected the presence of nystagmus", and subjectively count that as a failure.  It is, however, the characteristics of nystagmus, not the simple presence,  which is relevant to determining possible impairment.  And, unfortunately, many things cause nystagmus and some of us have it under normal conditions.

Further problems with using the nystagmus test in DUI investigations have been summarized by a noted expert in the area, Dr. L. F. Dell’Osso, Professor of Neurology at Case Western Reserve University School of Medicine and Director of the Ocular Motor Neurophysiology Laboratory at the Veteran’s Administration Medical Center in Cleveland:

Using nystagmus as an indicator of alcohol intoxication is an unfortunate choice, since many normal individuals have physiologic end-point nystagmus…Without a neuro-opthalmologist or someone knowledgeable about sophisticated methods of eye movement recordings, it is difficult to determine whether the nystagmus is pathologic. It is unreasonable that such difficult judgments have been placed in the hands of minimally trained officers. Dell’Osso, "Nystagmus, Saccadic Intrusions, Oscillations and Oscillopsia", 147 Current Neuro-Opthalmology 147.

See also an interesting article by Umeda and Sakata entitled "Alcohol and the Oculomotor System", 87 Annals of Otology Rhinology 69, wherein scientists concluded that gaze nystagmus was one of the least sensitive eye measurements of alcohol intoxication. The nystagmus which officers are trained to believe indicates intoxication is naturally present in some individuals without the presence of alcohol. It can also be caused by many other factors, as the Supreme Court of Kansas has noted after a review of the scientific literature:

Nystagmus can be caused by problems in an individual’s inner ear…. Physiological problems such as certain kinds of diseases can result in gaze nystagmus….Furthermore, conditions such as hypertension, motion sickness, sunstroke, eyestrain, eye muscle fatigue, glaucoma, and changes in atmospheric pressure may result in nystagmus. The consumption of common substances such as caffeine, nicotine, or aspirin also lead to nystagmus almost identical to that caused by alcohol consumption. State v. Witte, 836 P.2d 1110.

Obviously, drinking coffee and smoking cigarettes are not unusual. And note that most DUI arrests occur late at night — just when eyestrain and eye muscle fatigue are most expected.

As indicated in previously, there are three parts of the nystagmus test. But it is the angle of onset segment that is most critical — primarily because the “distinct” nystagmus and “smooth pursuit” in the other two tests are fairly subjective, while an angle has a certain mathematical nicety to it.

However, the officer’s ability to estimate this angle is critical. The nystagmus test is premised upon a formula that requires the angle of onset to be subtracted from 50 to obtain a very rough estimation of blood-alcohol concentration (BAC). An angle of 45 degrees from center, for example, may indicate a possible .05% blood-alcohol concentration; anything before that — for example, 43 degrees indicating .07% — results in a “failure”. Clearly, if the officer is mistaken in his “guesstimate” by only 5 degrees, a true 47-degree (.03% BAC) “pass” becomes an observed 42-degree (.08% BAC) “fail”.

So how does the officer measure the angle of onset with precision?

He doesn’t. At best, he is giving a very rough estimate. Recognizing the importance of the officer’s skill in estimating angles of onset, the National Highway Traffic Safety Administration recommends that officers use an angle-measuring template and practice with four or five subjects: “Check yourself monthly with the device to be sure that your accuracy has been sustained.”

The simple fact is that no officer actually does this; the last time he used a protractor to estimate angles was in the police academy. The most common method actually used is to assume that the 45-degree angle from the eye intersects the held object at the suspect’s shoulder: if nystagmus is observed before the pencil or finger reaches a line projecting straight out from the edge of the shoulder, the suspect “fails”. Very simple. Of course, the fatal flaw to this method (other than now requiring two estimations) is that we all have different shoulder widths.

In an interesting crime lab study reported in 25 Journal of the Forensic Society 476, 12 police officers measured the angle of onset of nystagmus in 129 actual cases where DUI suspects had been arrested but had not yet been tested by blood, breath, or urine. The officers used a special protractor to help them accurately measure the anlge of onset. Result? Even with the aid of a protractor, they consistently overestimated the angle at low BAC levels and underestimated it at high BACs. The researchers from the police crime lab concluded that nystagmus cannot be used to accurately predict blood alcohol concentration.

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