HGN Testing

Horizontal Gaze Nystagmus (HGN) is an involuntary jerking of the eye that occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his or her eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.08 or greater. HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.

It’s unfortunate that so many officers, who are trained by other officers, not physicians or other eye specialists, buy into this junk science.  Yes, they can see a person’s eyes jerking, but this is not an indicator that one is highly intoxicated.  It is important to note that even the National Highway Traffic Safety Administration (NHTSA) acknowledges that the HGN testing allows for the proper classification of only 77 percent of suspects.  That is, the HGN test will result in many false positives and cannot be considered a reliable indicator of intoxication.  Indeed, the HGN is not admissible in many state courts (although Texas allows it in most courts).  Police are permitted to use the test to establish probable cause to arrest a suspect.  People taking medication such as seizure and psychiatric medication may also “fail” the HGN test even though they are not intoxicated.  I’ve talked to board certified ophthalmologists who can’t believe police use HGN to determine intoxication!