The Horizontal Gaze Nystagmus (HGN) Test in Missouri

Kimberly2 1 Author:

Kimberly J. Benjamin, Founder & Managing Attorney

What Is a Horizontal Gaze Nystagmus Test?

 If you have ever been pulled over for suspicion of DWI in Missouri, chances are you have heard of the Horizontal Gaze Nystagmus (HGN) test. This test—often referred to as an eye test or a “follow the pen” test—is often used by police officers to determine whether a person is under the influence.However, there are some situations where false positives may occur due to medical conditions or other factors, such as improper testing procedures or lighting issues during testing. If you are stopped on suspicion of DWI, it is essential that you understand how this test, as well as other field sobriety tests work before you submit to one. 

Field Sobriety Tests and a Horizontal Gaze Nystagmus Test

 The Horizontal Gaze Nystagmus test is one of the three field sobriety tests, according to the National Highway Traffic Safety Administration (NHTSA). The other two standardized field sobriety tests include Walk and Turn (WAT) and One Leg Stand (OLS).Each of these tests is designed to measure a specific reflex or response that some researchers believe is compromised after a person has been drinking. A field sobriety test is typically administered before a breath test. However, field sobriety tests are not always accurate, and unfortunately, your performance can be used as evidence needed to arrest you 

Horizontal Gaze Nystagmus Test Procedure

 Nystagmus is a medical term used to describe involuntary jerking or twitching of the eyes when an individual follows an object with their gaze. It is caused by the inability of the eyes to maintain smooth pursuit when tracking a moving object and can be seen when a person looks from side to side. The jerking motion usually increases in intensity as the angle between the line of sight and head tilt increases.The police officer administers the HGN test. When performing an HGN test, they will typically ask individuals to follow a moving object (most commonly, a finger or pen) with their eyes while keeping their heads still.They will move a pen back and forth and side to side in front of the driver’s face. The pen is usually positioned about one foot away from the driver’s face, and the officer observes the driver’s eye movements.During the test, the officer will look for three clues that indicate impairment: a distinct and sustained nystagmus at maximum deviation, lack of smooth pursuit, and the onset of nystagmus before 45 degrees.Drivers should also be asked whether they are wearing contact lenses and whether they suffer from a medical impairment. Law enforcement officers should also instruct drivers to remove eyeglasses because it’s easier for the officers to watch the driver’s eyes. 

Horizontal Gaze Nystagmus Reliability

 The reliability of HGN testing has been debated for many years, but it is believed that this test is relatively accurate if conducted correctly by trained professionals.It should be noted, however, that nystagmus can occur due to causes other than alcohol in certain individuals. Despite that, prosecutors often have arresting officers testify in a DWI court in Missouri about the clues and scoring of the HGN test since it is often used to support the assertion that you have been intoxicated.If you were driving under the influence and your blood alcohol concentration was over the legal alcohol limit in Missouri, make sure you contact an attorney as soon as possible.For more questions on the HGN tests in Missouri, contact our attorneys at Missouri DWI & Criminal Law Center and schedule a free consultation.

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