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Potential issues with a horizontal gaze nystagmus test

On Behalf of | Feb 18, 2023 | DWI & Alcohol |

The horizontal gaze nystagmus (HGN) test is a common field sobriety test used by Texas law enforcement to detect impairment caused by alcohol or drugs. This test is designed to measure the involuntary movement of the eyes, which can be a sign of impairment.

During an HGN test, the officer will ask the individual to follow an object, such as a pen or flashlight, with their eyes as the officer movies it in front of the driver. The officer will observe the individual’s eyes for signs of nystagmus, which is a rapid, involuntary movement of the eyes that occurs when the eyes move from side to side.

Causes of nystagmus

Nystagmus can be caused by a variety of factors, including alcohol consumption, drug use, fatigue and certain medical conditions. When an individual is impaired, the nystagmus can be more pronounced and occur at smaller angles of gaze.

The HGN test is based on the premise that when an individual is impaired their eyes will not be able to smoothly track the moving object, and nystagmus will be more pronounced. The test is designed to be a quick and easy way for law enforcement to determine whether an individual is potentially guilty of drunk driving and should be subjected to further testing.

Problems with the HGN test

The HGN test is not foolproof. Several factors can cause nystagmus, and not all of them are related to impairment. For example, certain medical conditions and medications can cause nystagmus, even when an individual is not impaired.

Furthermore, the HGN test must be administered correctly to be accurate. The officer must hold the object at the correct distance from the individual’s eyes, and the individual must be instructed to follow the object with their eyes only, without moving their head.

Despite its limitations, the HGN test is still a widely used tool for law enforcement. It is often used in conjunction with other field sobriety tests, such as the walk-and-turn test and the one-leg stand test, to build a case for impairment.

The HGN test is a valuable tool for law enforcement in detecting impairment caused by alcohol or drugs. However, it is crucial to understand its limitations and to remember that it is just one part of a larger picture in determining impairment.