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One of the tests a police officer can use to determine whether you’ve driven drunk is known as the “horizontal gaze nystagmus (HGN) test.” Few people outside the medical profession may have heard of nystagmus, much less know what it has to do with a DUI. Here’s what you need to know about the test, including HGN and nystagmus definitions, facts about “field sobriety tests,” and what to know about failing them.

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Nystagmus Definition

Nystagmus is essentially uncontrolled eye movement. It can occur in three directions:

  • Horizontal nystagmus (side to side)
  • Vertical nystagmus (up and down)
  • Rotary nystagmus (circular motion)

It can manifest in one of two patterns: As pendular nystagmus, where the eyes move back and forth; or jerk nystagmus, where the eyes drift in one direction and jerk in the opposite direction.

Nystagmus, by definition, can be congenital or acquired through a disease or alcohol or drug use. Congenital nystagmus can’t be treated, but acquired nystagmus can be treated by treating the condition that causes it. Surgery or glasses may help in rare cases, but, as you likely know, not in cases where the cause is drugs or alcohol.

Field Sobriety Test Definition

A “field sobriety test (FST)” is a physical activity-based test that helps police officers estimate a driver’s level of impairment. Unlike a breathalyzer test, it doesn’t determine blood alcohol content, but instead serves as further evidence of impairment in arrests and later criminal cases.

Field sobriety tests can be standardized or non-standardized. Standardized FSTs have been studied by the National Highway Traffic Safety Administration (NHTSA) and determined to provide the most accurate estimates of intoxication levels. There are three standardized tests:

  1. Walk and turn test
  2. One leg stand test
  3. Horizontal gaze nystagmus test

Any other test, such as counting backwards or putting your finger to your nose, is non-standardized and therefore given less value as evidence. You may be surprised to know a vertical gaze nystagmus test is a non-standardized FST.

Though these tests are known for detecting drunk drivers, you may be given an FST, including a horizontal gaze nystagmus test, for drugs as well.

HGN Test Definition

Combining the FST and nystagmus definitions, an HGN test helps a police officer determine–approximately–how intoxicated you may be. The officer slowly moves a “stimulus” (a finger, pen, or pencil) back and forth in front of your eyes, and watches how your pupils track the movement. There are three clues which total one point per eye if observed, for six total possible points:

  1. Lack of smooth pursuit. Where the eyes jerk or bounce as they follow the stimulus, indicating nystagmus
  2. Distinct and sustained nystagmus at maximum deviation. Where the eyes jerk or bounce in a pronounced manner when the stimulus is held all the way to the left or right for more than four seconds
  3. Onset of nystagmus prior to 45 degrees. Where the nystagmus begins early in the test, such as when the stimulus is in front of the driver’s shoulder

Remember from the nystagmus definition provided earlier: The officer is looking for a pronounced jerking motion or back and forth movement. The more points a driver gets during the HGN test, the higher their BAC is estimated to be.

More About HGN Tests And DUI Charges

The following information can give you an idea of what to expect if you fail an HGN sobriety test. Only an attorney will be able to usefully apply the information to the facts of your case. DUIs can have serious, long-term consequences, so think twice before representing yourself.

How To Administer The Test

The test should be given in accordance with NHTSA guidelines. The officer should give the following instructions, not necessarily in this order:

  1. Remove glasses if you are wearing them
  2. Stand with feet together and hands at your side
  3. Don’t move your head during the test
  4. Keep your gaze on the stimulus (a finger, pen, or pencil) for the entire test
  5. Follow the stimulus with your eyes

The officer should ask if you understand the instructions. They should also perform at least 14 “passes,” (movement of the stimulus from one side to the other) because the eyes are checked twice, individually, for each clue. The stimulus should be 12 to 15 inches from the driver’s face. Based on the horizontal vs. nystagmus definition, the officer should perform the test from side to side, not up and down.

Where And Why The Test Is Used

The test can be used in any jurisdiction–most states recognize it as scientific evidence when administered properly. It is considered the most accurate FST. In addition to accuracy, reasons for its use include the ease with which it can be administered (you can be standing, sitting, or even lying down), and the strong scientific connection between alcohol and drug use and eye movement.

What HGN Results Mean For Your DUI Case

The more points you score during the test, the more likely the officer will arrest you for drunk driving or ask you to submit to a chemical BAC test. Failing an HGN test does not ensure a conviction, but it can be used as evidence against you, and is considered stronger evidence than other FSTs.

How To Defend Against HGN Results

There is some good news: DUI lawyers know what to look for in terms of defending against HGN results. These are potential defenses to negative HGN test results which may or may not be accepted by a judge:

  • The test was administered facing the flashing lights from the police car
  • Inclement weather skewed the results
  • Nystagmus, by definition, can be caused by any number of medical conditions such as brain damage
  • The nystagmus was caused by caffeine or aspirin
  • The test was not administered following NHTSA guidelines
  • The officer was not qualified to administer the test
  • The test is not 100 percent accurate
  • The nystagmus was not pronounced or sustained, and could have been from normal fatigue
  • In cases where the test results are considered “expert” testimony, the state did not follow proper protocol, which includes notifying the defense and turning over the report ahead of time

Refusing A Field Sobriety Test

Refusal of an FST can be used as evidence against you in a DUI case, but does not indicate a certain level of intoxication or impairment on its own. The consequences of refusing an FST are not as harsh as refusing a chemical BAC test, which violates implied consent laws in every state.

Keep in mind that alcohol tends to make the drinker feel more capable than they really are. Unless you are certain you’re under the legal limit, taking an FST could give the police officer stronger evidence of impairment against you.

Map Out Your HGN Defense With A DUI Lawyer

You’ll need more than FST and nystagmus definitions to mount a defense to a DUI charge. Let an experienced DUI lawyer steer you in the right direction.

Legal Disclaimer: This article contains general legal information, but does not constitute professional legal advice for your particular situation and should not be interpreted as creating an attorney-client relationship. If you have legal questions, you should seek the advice of an attorney licensed in your jurisdiction.

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