Understanding DWI Arrests in Missouri and the Horizontal Gaze Nystagmus Test

Horizontal Gaze Nystagmus (HGN) test is one of three standardized field sobriety tests (SFSTs) validated by the National Highway Traffic Safety Administration (NHTSA). It is also a test that police in Kirksville administer frequently during traffic stops when intoxicated driving is suspected. Though the NHTSA recognizes the HGN as a test that can indicate impairment, this test is not fool-proof. In fact, it has its disadvantages, which make admitting it as evidence problematic.

In Missouri, our DWI defense lawyer will investigate your case, including the types of and manner in which field sobriety tests like the HGN test were administered. At Nolan Law Firm LLC, we use our skills and resources to ensure you receive the best representation possible. Contact us at 660.956.4502 to schedule a consultation and learn more about how we will help you fight your DWI charge in Kirksville.

What is a Horizontal Gaze Nystagmus Test?

The Horizontal Gaze Nystagmus test is one of three standardized field sobriety tests used by the police in Missouri to help determine whether a driver is under the unlawful influence of alcohol or drugs.

How is the HGN Test Performed?

The officer conducting the HGN test should provide clear verbal instructions to the driver. The police officer should tell the driver to stand still, place hands to the side, and keep head still. Then they must be properly instructed to look at a stimulus, like a pen or another object, and follow it with both eyes while the officer moves the stimulus from right to left.

The officer assesses the driver's eyes while moving the object from side to side. HGN tests, however, are very technical in their application. Proper administration involves specific requirements on distance between the stimulus and the driver's nose (12 - 15 inches), timing and length of holds (hold the stimulus for at least 4 seconds at the maximum deviation), and how many times and ways the stimulus is passed back and forth.

The HGN test is meant to measure the involuntary jerking of the eye – known as nystagmus. A driver with a high blood alcohol concentration may exhibit involuntary jerking of an eye as the driver gazes toward the side while following the stimulus.

Three Major Clues of Intoxication

Police look for three major clues while administering the HGN test. Each eye is assessed for these three clues, so there are actually a total of six possible indicators of intoxication. If the officer determines four clues exist, that is supposed to indicate the driver's blood alcohol content (BAC) level is above 0.08 percent.

  1. Clue 1 involves the onset of nystagmus in either eye before 45 degrees.
  2. Clue 2 involves nystagmus in either eye when the eye gazes as far as possible to the right or left – in this case, there is a sustained and distinct jerking.
  3. Clue 3 occurs when the driver is unable to follow the stimulus – known as lack of smooth pursuit. 

Each clue requires specific motions or manners in which the stimulus is held or passed. Each clue also requires different timing. For example, the movement of the stimulus to determine the lack of smooth pursuit should be two seconds out and back to each eye while it is four seconds to move the stimulus from eye to the driver's shoulder to determine the onset of nystagmus prior to 45 degrees.

Ways to Challenge the HGN Test in Missouri

HGN tests can be challenged effectively by arguing against their reliability (these tests are highly subjective) or proving improper administration of the test (these tests require following strict and specific technical rules). Also, these tests can be challenged based on matters not associated with the test itself, but matters related to the driver or to the environment.

Common Challenges to the HGN Test

  • Unreliable based on police officer's subjective estimations and preconceived notions
  • Unreliable based on police officer's failure to administer the test properly
  • Unreliable based on external factors

Common External Causes of Failed HGN Tests

  • Bad weather
  • Administered at night in darkness or during the day with a glaring sun
  • Bad roads or other environmental issues
  • Patrol car lights flashing or other lighting issues
  • Driver's pre-existing health issues or medications, like ear disorders, eye disorders, head injury or brain damage, excessive amounts of caffeine, antihistamines, barbiturates, illness like the flu or vertigo

Keep in mind that there are more than 38 non-alcohol-related causes for nystagmus, and each of these can lead to a failed HGN test.

The HGN test is faulty. Your DWI defense attorney can may be able to highlight these weaknesses and create reasonable doubt in the prosecution's case against you. Our DWI defense lawyer will investigate and review the results of your HGN test and challenge it accordingly.

Contact Our DWI Defense Attorney in Kirksville Today

Field sobriety tests are a way police officers gather probable cause to arrest you for DWI charges. These tests, however, are rarely conducted in accordance with regulations and are faulty given their subjective nature.

At Nolan, Mulford & DeLeeuw, our DWI defense lawyer knows how to prepare and challenge field sobriety tests like the HGN test. To learn more about how we can help your DWI case, contact us by filling out the online form or calling us at 660.956.4502 to schedule a consultation.

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