Horizontal Gaze Nystagmus

The horizontal gaze nystagmus is one of the three standardized field sobriety tests developed by the National Highway Traffic and Safety Administration to identify possible drunk drivers. Research has proven that this test is the most accurate of the three, with a 77% accuracy rating in detecting blood alcohol content levels of .10% or higher.

Nystagmus is the technical term for involuntary jerking or bouncing of the eyeball. It occurs when there is an irregularity in the inner ear system or a problem affecting the oculomotor control of the eye. A horizontal gaze nystagmus is a lateral or horizontal jerking movement when a person looks toward the side.

Nystagmus and horizontal gaze nystagmus can be traced to several different sources, including alcohol use and the consumption of other drugs that depress the central nervous system, such as inhalants or phencyclidine. These hinder the brain's ability to properly control the eye muscles.

The presence of such alcohol or drugs in the body's system results in jerking or bouncing movements in the eyeball. The higher a person's blood alcohol content level, the more obvious the nystagmus becomes.

In the horizontal gaze nystagmus test used by law enforcement officials, the officer positions an object, usually a pen or a finger, about one foot from the driver's face and then moves the object from one side to another while observing the driver's eye movements. During the observation, the officer tries to estimate the angle at which the jerking movements begin.

Jerking movements in the eye before the gaze reaches a 45-degree angle is indicative of a possible blood alcohol content level over .05%. The officer also checks the eye's stability when the gaze is as far to the side as it can go.

Even though this test has proved quite accurate and is widely used by law enforcement in assessing a person's blood alcohol content, many lawyers still try to challenge its credibility in court. In fact, many states to not allow the test to be used as evidence against a person in a court of law.

One of the main complaints is that the officers who administer the test are not medically trained. Therefore, their ability to asses the angle at which nystagmus begins is often skewed. Consequently, law enforcement officials tend to use the test in conjunction with other field tests, breathalyzer tests, or urine or blood tests to build a stronger case against a driver.