What Is the HGN Test?
The Horizontal Gaze Nystagmus (HGN) test is one of the three standardized field sobriety tests (SFSTs) used by law enforcement officers in Leon Valley and across Texas during DWI investigations. The test involves observing the involuntary jerking of a person’s eyes as they follow a slowly moving object, typically a pen or flashlight, from side to side.
Nystagmus becomes more pronounced when a person is impaired by alcohol or certain drugs. Officers are trained to look for specific “clues” in eye movement that may suggest intoxication. However, this test is not foolproof—and it’s often challenged in court by experienced DWI attorneys.
How Police in Leon Valley Use the HGN Test in DWI Stops
When a driver is pulled over in Leon Valley for suspected DWI, the officer may request a field sobriety test. The HGN is usually conducted first. The officer will ask the driver to stand still and follow an object with their eyes while keeping their head still.
Texas police are trained to look for three clues in each eye:
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Lack of smooth pursuit
– The suspect’s eyes fail to follow a moving object smoothly. Instead of gliding fluidly, the eyes jerk or bounce as they track the stimulus, which can be a sign of intoxication.
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Distinct and sustained nystagmus at maximum deviation
– When the eye is held at the far edge of its movement (maximum deviation) for at least four seconds, visible and consistent involuntary jerking of the eye is observed. This is considered a strong indicator of impairment.
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Onset of nystagmus before 45 degrees
– As the eye moves to the side, jerking begins before it reaches a 45-degree angle from center. Early onset of nystagmus is one of the key clues officers use to estimate a high blood alcohol concentration.
If four or more clues are observed, the officer may determine that there’s probable cause for a DWI arrest. But this doesn’t automatically mean you’ll be convicted.
Scientific Controversy Around HGN Testing
While the HGN test is recognized by the National Highway Traffic Safety Administration (NHTSA), it is not universally accepted in court as conclusive proof of intoxication. Several factors can cause false positives:
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Neurological disorders
– Conditions such as multiple sclerosis, epilepsy, or Parkinson’s disease can cause involuntary eye movements that mimic intoxication.
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Head injuries
– Recent or past trauma to the head may affect the brain’s ability to control eye movement, leading to false positives during an HGN test.
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Eye conditions
– Disorders like astigmatism, nystagmus (unrelated to intoxication), or strabismus can interfere with accurate test results.
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Fatigue
– Extreme tiredness can impair coordination and eye control, causing jerky eye movements similar to those seen in intoxicated individuals.
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Certain prescription medications
– Drugs such as antidepressants, anti-anxiety medications, antihistamines, or seizure medications may affect the central nervous system and eye response, potentially skewing HGN test outcomes.
An experienced Leon Valley DWI defense attorney can challenge the validity of the HGN results by questioning the officer’s training, environmental conditions, or your medical history.
Is the HGN Test Admissible in Bexar County Courts?
Yes—but with limitations. In Bexar County, prosecutors often use HGN evidence during DWI trials. However, under Texas law, HGN test results must meet certain admissibility standards:
- The test must be administered correctly per NHTSA guidelines.
- The officer must be properly certified to conduct the HGN test.
- The results must be interpreted accurately.
Defense attorneys often file motions to suppress HGN evidence if the test was improperly administered or if the officer lacks sufficient training.
Defending Against HGN Evidence in Leon Valley DWI Cases
At Ried Pecina Trial Lawyers, we regularly challenge the use of HGN evidence in Leon Valley DWI cases. Our attorneys understand that many variables can influence the test and that mistakes in administration are common. Some common defense strategies include:
- Arguing that the officer lacked proper certification or training
- Demonstrating environmental distractions (e.g., flashing lights, uneven terrain)
- Pointing to medical conditions that mimic intoxication
- Cross-examining the officer’s field notes and video evidence
Why You Shouldn’t Assume You’re Guilty After an HGN Test
Too many drivers in Leon Valley assume they’ve failed the HGN test and give up. Don’t. This test is just one piece of evidence, and it can often be discredited with expert testimony, medical records, or even dash cam footage.
Call a Leon Valley DWI Lawyer for HGN Defense
If you’ve been arrested for DWI after failing an HGN test in Leon Valley, TX, it’s critical to speak with a lawyer who understands how to challenge field sobriety tests.
Call Ried Pecina Trial Lawyers at (210) 893-0000 for a free consultation. Let us protect your record, your license, and your future.
