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How a Horizontal Gaze Nystagmus Test Helps Determine Impairment

Illinois drivers who are stopped for suspected DUI typically undergo a field sobriety test. In fact, these types of tests are used by law enforcement officials in every state. One of the portions of this three-part assessment of driver capabilities is what’s termed a horizontal gaze nystagmus (HGN) test. HGN refers to involuntary “jerky” eye movements that tend to become exaggerated while intoxicated.

Various National Highway Traffic Safety Administration studies suggest HGN testing is a reliable indicator of alcohol impairment. However, results from an HGN evaluation are only one piece of evidence from a field sobriety test that may be questioned when an attorney puts together a DUI defense for a client. The other two parts of the test involve one-leg-stand and walk-and-turn tests. Blood alcohol concentration (BAC) is also considered.

During an HGN test, a police officer usually turns a suspect away from the lights of the patrol car and oncoming traffic to avoid skewing results. The officer then instructs the driver to follow an object with their eyes. They’ll consider any jerky motions made as the suspect’s pupil follows the object toward their ear, whether or not there are irregular movements when the pupil is towards the edge of the eye and if the eye jerks before it is 45 degrees from the center of the face. Results may be affected by some medical conditions, certain illegal or prescription drugs and wearing contacts.

A DUI defense strategy may also involve attempts to throw out HGN findings and other test results. In some instances, it may be argued that standard guidelines established by the NHTSA weren’t followed when eye movements were checked. Additionally, a DUI lawyer might bring up medical or eye issues, such as a build up of inner ear fluid or eye strain, that could have affected HGN results.