If you were stopped by the police, especially at night, and the officer had any suspicion that you might have been drinking, he will have conducted a horizontal gaze Nystagmus test. According to FielSobrietyTests, there are hundreds of known causes of nystagmus. Some people even have nystagmus for no known reason.

Horizontal gaze nystagmus is one type of nystagmus that police use as a field sobriety test. It involves following a moving object or an object held off to one side. The National Highway Traffic Safety Administration has studied and standardized the HGN test, which is incorporated into the training police officers receive on field sobriety tests.

Properly administered, the test begins by checking the eyes for equal pupil size and equal tracking. This step can reveal some of the medical conditions that caused the nystagmus, though many officers omit this step. There are three clues or signs that officers are trained to look for. The first is a lack of smooth pursuit. This is a jerkiness while the eyes are following a moving object. It should take four seconds for the stimulus to get from one side to the other.

Next, the officer checks to see if there is distinct and sustained jerking when the eyes were held as far to the side as they can look. This is called nystagmus. At maximum deviation, the stimulus should remain at the side for at least four seconds. Finally, the officer checks to see if the nystagmus persists at 45 degrees or earlier. Measuring a 45-degree angle without any measuring device can be difficult, and many officers do this incorrectly. Once again, the stimulus should remain stationary for at least four seconds.