Nystagmus is a medical term referring to an abnormal and involuntary jerking of the eyeballs. A horizontal gaze nystagmus test is used as a field sobriety test by law enforcement when evaluating someone suspected of driving while drunk. While it’s true that alcoholic beverage consumption can cause nystagmus, so can many medical conditions and medications, including:

The degree of observed involuntary jerking movements and lack of equal pupil size is at least partly subjective and open to interpretation by someone not professionally trained to do so. In fact, a horizontal gaze nystagmus test is only about 77 percent accurate. 

The Horizontal Gaze Nystagmus (HGN) Test and Maximum Deviation

If you’re stopped by law enforcement for suspected drunk driving, the horizontal gaze nystagmus (HGN) test will be performed as follows: 

The officer will tell you to look straight ahead and follow a small object, such as a pen, held at your nose level with equal tracking. The officer will tell you to follow the pen’s movements without moving your head. 

The officer is looking for jerking movements caused by alcohol impairment. A lack of smooth pursuit and equal tracking as the suspect’s eyes move implies that they may be inebriated. You will be told to move your eyeballs at a horizontal gaze as far to the side as they will go.

This is called nystagmus at maximum deviation. If distinct nystagmus is observed four or more times during the test, it’s enough evidence of intoxication and probable cause for a DUI arrest. 

Failing the Horizontal Gaze Nystagmus Test

It’s not surprising to fail a field sobriety test, even if someone is not impaired. The police officer could even be at fault if they:

In the case of recent alcoholic beverage consumption, distinct and sustained nystagmus may persist, and oftentimes jerking continues for many hours after the last drink, well after the person is no longer under the influence of the substance and not a danger on the road. 

This is only one example of a false positive horizontal gaze nystagmus test. Even worse, some people have a degree of natural nystagmus. Resting nystagmus may be present in individuals who have had a recent head injury.

Myths about Drunk Driving and Standardized Field Sobriety Tests

How to Choose the Best DUI Attorney

If you’ve been arrested for DUI, especially if it involves a failed horizontal gaze nystagmus test, you should seek legal counsel immediately. A skilled DUI attorney can fight the horizontal gaze nystagmus test results based on their unreliability.

You will need an aggressive defense to protect your license and perhaps even your freedom if an officer claims that you weren’t capable of following instructions and there was a lack of smooth pursuit of eye tracking during the nystagmus test. 

When choosing an attorney to defend you on a DUI charge, these are some qualifications to look for:

Choosing the Right Defense Is Important

Most of the time, your case outcome will depend on your lawyer’s skill and experience, how well or poorly you performed on your horizontal gaze nystagmus or field sobriety tests, and the scientific evidence of nystagmus at maximum deviation presented by the officer during the arrest.