Most people have undergone a Horizontal Gaze Nystagmus test at one time or another. Doctors use it to look for signs of neurological problems. Usually, the test administrator instructs the subject to follow a moving point, such as a fingertip, just by moving their eyes. If the pupils move involuntarily, the person probably has nystagmus.
Parker County peace officers also routinely administer this test during roadside DWI field sobriety tests. In fact, the HGN is one of only three scientifically-approved field sobriety tests. However, in this context, the HGN test is hardly dispositive.
Given the discussion below, a roadside HGN failure may give an officer probable cause to request a chemical sample. However, this test almost certainly does not prove guilt beyond a reasonable doubt. That's a much higher evidentiary standard.
What Causes HGN?
Alcohol impairment causes nystagmus. But it is not the only cause. Additionally, alcohol is not even the leading cause of nystagmus.
Birth defects and childhood brain injuries are the leading causes of nystagmus, which is also known as "lazy eye." Many times, the symptoms are so mild that people do not know they have nystagmus. As a result, when officer ask defendants if they have nystagmus, they may honestly answer "no" even if they have HGN.
Certain anti-seizure medications also cause nystagmus. In medication-induced nystagmus cases, the patient usually has no symptoms outside of a very stressful situation. A DWI arrest certainly qualifies as such.
In court, Weatherford criminal defense attorneys often ask officers if they ruled out other nystagmus causes before administering the test. The answer to that question is almost always "no." Therefore, given the scientific evidence, it's very hard for a jury to connect nystagmus and alcohol intoxication.
HGN Test Issues
In controlled laboratory conditions, the HGN test is about 80 percent accurate. But roadside HGN tests do not take place under controlled conditions.
First, as mentioned, the subject is usually very nervous. In a neurological test, if the subject is not relaxed, stress hormones may affect the outcome. Moreover, the roadside environment is uncontrolled. The sky is usually dark, bright squadcar lights flash in the background, and cars often whiz past at high speeds. All these variables may affect the results as well.
Additionally, the 80 percent accuracy figure is for nystagmus in general, and not alcohol-induced nystagmus in particular. If officers failed to eliminate other possible causes, there is no way to conclusively link the nystagmus and alcohol consumption.
The HGN test is arguably the least-conclusive field sobriety test. For a free consultation with an experienced criminal defense attorney in Fort Worth, contact Herreth Law. After-hours visits are available.