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Variances in the Horizontal Gaze Nystagmus Test: Impacting the Validity of DWI Assessments

Getting pulled over on suspicion of driving while intoxicated (DWI) can be a nerve-wracking experience, especially when subjected to field sobriety tests like the Horizontal Gaze Nystagmus (HGN) test. Administered to gauge the presence of nystagmus—an involuntary jerking of the eyes—this test serves as one of the go-to tools for law enforcement to evaluate impairment. However, the HGN test is not foolproof. Various factors can influence its outcome, and these variances can greatly affect its validity.

At Zendeh Del & Associates, PLLC, our team of legal experts led by Jonathan Zendeh Del, a DWI Super Lawyer, is equipped to challenge the validity of HGN tests and other evidence. With over 100,000 lawyers in Texas and only 16 designated as DWI Super Lawyers, our expertise in this domain is unparalleled. In this blog post, we will delve deep into the variances in the HGN test that can impact its accuracy.

What is the HGN Test?

Before understanding the variances, let's briefly discuss what the HGN test entails. In this test, a police officer asks the suspect to follow a moving object—usually a flashlight or pen—with their eyes while keeping their head still. The officer then observes the movement of the eyes for indications of impairment, including the angle at which the eye begins to jerk and whether the jerking is distinct.

Medical Conditions

Neurological Disorders

Conditions like multiple sclerosis or brain lesions can cause nystagmus, irrespective of alcohol consumption. Being unaware of these conditions can lead officers to wrongly interpret the results.

Eye Problems

Individuals with lazy eyes, astigmatism, or other eye issues may exhibit nystagmus naturally. Such medical issues can thus create false positives in the test.

Environmental Factors


Too much glare or insufficient light can distort the officer's view of the eye, affecting the test results.


A busy street with flashing lights, moving vehicles, or other distractions can make it difficult for the test subject to focus, affecting the accuracy of the HGN test.

Test Administration

Officer Training

The HGN test demands precise administration. Lack of proper training can result in flawed execution and unreliable results. Officers need to follow NHTSA (National Highway Traffic Safety Administration) guidelines rigorously for the test to hold water in court.

Sequential Testing

When administered alongside other field sobriety tests like the Walk-and-Turn or One-Leg Stand, the sequence in which the tests are conducted can affect fatigue and concentration, thereby influencing HGN outcomes.

Other Influencing Substances

Prescription Medication

Certain medications induce nystagmus as a side effect. Even over-the-counter antihistamines or sedatives can impact the test.

Caffeine, Nicotine, and Aspartame

Stimulants like caffeine and substances like nicotine and aspartame can also trigger nystagmus in some individuals, casting further doubt on the test's reliability.

Legal Implications

Understanding these variances is crucial when challenging a DWI arrest in court. If any of these factors are applicable, they can be used as a potent defense strategy to question the validity of the HGN test and, by extension, the entire arrest. At Zendeh Del & Associates, PLLC, we specialize in dissecting such details to build a robust defense for our clients.


While the HGN test is a common tool used by law enforcement to gauge impairment, it is fraught with potential inaccuracies. From medical conditions and environmental factors to the way the test is administered, numerous variances can affect its validity.

If you're facing a DWI charge and believe that the HGN test results are questionable, it is imperative to consult an experienced legal team well-versed in DWI defense strategies. As one of the best DWI lawyers in the state, Jonathan Zendeh Del and his team at Zendeh Del & Associates, PLLC, are equipped to provide you with the legal representation you need. Our lines are open 24/7 at (409) 740-1111. Don't let a flawed HGN test dictate your future; call us today to discuss your case.