Horizontal Gaze Nystagmus (HGN) Tests in San Antonio DWI Cases
Driving while intoxicated (DWI) in San Antonio often involves more than just breathalyzers. Police across Bexar County commonly use the Horizontal Gaze Nystagmus (HGN) test as a primary tool to claim impairment. While presented as scientific evidence, the HGN test frequently fails due to improper administration, environmental factors, and medical conditions. Understanding HGN and its limitations is crucial for defending your rights after a San Antonio DWI arrest.
What Is the Horizontal Gaze Nystagmus (HGN) Test?
The HGN test observes involuntary jerking of the eyes as they follow a moving object. Officers in San Antonio, including SAPD and Bexar County Sheriff’s deputies, use it to estimate alcohol impairment. While NHTSA guidelines set precise procedures, field conditions and human error often lead to inaccurate results.
How the test is performed:
- A stimulus, like a pen or flashlight, is held at eye level.
- Each eye is checked separately for jerking while tracking the object.
- Officers score the extent and angle of the eye movement to assess impairment.
Why HGN Tests Often Fail in San Antonio
1. Officer Training and Procedure Errors
Even minor deviations from NHTSA protocols can invalidate the test. Common mistakes in San Antonio include:
- Moving the stimulus too fast or slow.
- Failing to properly instruct the driver.
- Skipping steps for each eye independently.
2. Environmental Distractions
Traffic, flashing police lights, uneven road surfaces, or stop locations along I-10 or Loop 410 can disrupt a driver’s ability to follow the stimulus, leading to false positives.
3. Medical Conditions
Eye disorders, neurological conditions, vertigo, or fatigue may mimic nystagmus. Drivers on prescription medications like antidepressants, anti-anxiety meds, or antihistamines often display eye movements unrelated to alcohol.
4. Subjective Scoring
The test relies entirely on the officer’s judgment. This subjectivity introduces a significant margin for error, making HGN results unreliable in court.
HGN Evidence in San Antonio Courts
Texas courts allow HGN evidence, but defense attorneys routinely challenge its scientific validity. In Bexar County, judges have excluded HGN results when officers failed to follow proper procedures. Top DWI lawyers review bodycam and dashcam footage, officer notes, and timing to identify errors that can undermine prosecution claims.
Chart: Factors That Lead to False HGN Results in San Antonio
| Factor | Effect on Results | Local Example |
| Officer procedural errors | Missed steps, improper scoring | SAPD officer rushes the 90-second test |
| Environmental distractions | Disrupted eye tracking | Stop near Downtown with flashing lights |
| Medical issues | Mimics alcohol-related nystagmus | Vertigo or eye disease |
| Prescription medication | Mimics impairment | Xanax, antidepressants, antihistamines |
| Subjective officer judgment | Inconsistent evaluation | Officer interprets minor eye movements as impairment |
Defense Strategies for HGN-Based DWI Charges in San Antonio
An experienced DWI lawyer can:
- Identify improper test administration and timing errors.
- Introduce medical records showing conditions that mimic nystagmus.
- Challenge subjective officer scoring and credibility.
- Present expert testimony to dispute the scientific reliability of HGN results.
Effective defense can result in evidence suppression, reduced charges, or case dismissal.
Frequently Asked Questions About HGN and DWI in San Antonio
Can I be arrested based solely on an HGN test?
Yes, officers may rely heavily on HGN, but courts prefer additional evidence like breath tests or field sobriety tests.
Is HGN scientific proof of intoxication?
No. HGN only indicates potential impairment and can be affected by numerous factors unrelated to alcohol.
Do San Antonio judges accept HGN evidence?
Judges may allow it, but evidence can be challenged successfully in many cases.
Can medical conditions or prescription medications help my defense?
Yes. Documentation of neurological disorders, eye conditions, or medications can explain abnormal eye movements and undermine HGN reliability.
Local Trends: HGN Use in Bexar County
Data shows that SAPD and Bexar County officers increasingly rely on HGN during roadside DWI stops. While intended to support prosecutions, improper testing often leads to contested results. Courts frequently exclude HGN evidence when attorneys highlight officer errors or unreliable conditions.
Why You Need a San Antonio DWI Lawyer Experienced in HGN Cases
Ried Pecina Trial Lawyers specialize in challenging HGN-based DWI charges. We examine officer procedures, environmental conditions, and medical evidence to protect your rights. Our expertise in Bexar County courts helps us reduce or dismiss charges, safeguarding your freedom, license, and future.
Call Ried Pecina Trial Lawyers Today
If you faced a DWI arrest in San Antonio involving an HGN test, call Ried Pecina Trial Lawyers at (210) 893-0000 for a free consultation. Our Best San Antonio DWI lawyers understand the pitfalls of HGN tests and fight aggressively to defend your case.
