Field Sobriety Tests and Lateral Gaze-Evoked Nystagmus: An Ophthalmologist’s Perspective
Perspective
Steven Rauchman, MD ✉️. DWI Journal. Volume 2. Issue 1. November 3, 2025. DOI: 10.61874/dwij/acjd4236
Published under a Creative Commons license CC BY-NC-ND 4.0.
Abstract
Horizontal Gaze Nystagmus (HGN) is a type of involuntary eye movement often used by law enforcement officers during roadside sobriety tests to assess alcohol impairment. This article presents an ophthalmologist’s perspective on the scientific and clinical limitations of this practice. It explains that nystagmus can result from many causes unrelated to alcohol, including fatigue, medications, chronic illnesses, neurological conditions, and even natural variations in healthy individuals.
Despite its routine use, HGN testing at the roadside relies on subjective observation rather than accurate measurement. Officers are not trained medical professionals and do not have the tools necessary to accurately assess eye movement, measure angles of onset, or rule out alternative medical explanations. Environmental factors, such as flashing lights and moving vehicles, can further interfere with test results.
Given these concerns, this article questions the reliability of HGN as evidence in DUI cases and proposes a practical solution: requiring clear video recordings of the test to allow expert medical review. While impaired driving remains a serious public safety issue, enforcement tools must be both scientifically valid and legally sound. A more accurate, objective approach would help protect individual rights while preserving the integrity of DUI enforcement.
Introduction
Driving under the influence (DUI) of alcohol accounts for 10% of all arrests in the USA (Gjerde & Jamt, 2025). A significant percentage of traffic fatalities involve intoxicated drivers (National Highway Traffic Safety Administration, 2022). The consumption of alcohol is clearly implicated in most DUIs, making this issue of considerable social, legal, and medical significance. According to the 2022 National Survey on Drug Use and Health, 29.5 million people aged 12 and older had Alcohol Use Disorder, demonstrating that alcohol abuse is a significant social problem (National Institute on Alcohol Abuse and Alcoholism (NIAAA, 2025).
Burns provides a comprehensive summary of field sobriety research, documenting that police officers’ examinations of suspected alcohol-impaired drivers include the administration of the Standard Field Sobriety Tests (SFSTs) (Burns, 2003). These tests include Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn test, and the One-Leg-Stand test (Burns, 2003).
While blood and breath analyses of alcohol levels in drivers are crucial in DUI cases, this perspective focuses on the value and limitations of Horizontal Gaze-Evoked Nystagmus (HGEN) and related eye findings from an ophthalmologic perspective. Although the Walk-and-Turn and One-Leg-Stand tests are noted, the primary emphasis is on how interpretation intersects with ocular physiology, measurement challenges, and confounders relevant to HGN. Legal standards are acknowledged but not analyzed in depth.
SFST background and the Colorado validation Work
Burns references the accuracy of DUI testing by law enforcement in earlier studies by Burns and Moskowitz and by Tharp, Burns, and Moskowitz (Burns, 2003; Burns & Moskowitz, 1977; Tharp, Burns, & Moskowitz, 1981). The most exhaustive field test, often treated as representative, is Burns and Anderson, funded by the Colorado Office of Transportation Safety and associated with NHTSA initiatives (Burns & Anderson, 1995). These studies underpin current guidance but contain assumptions and field constraints that warrant scrutiny from a neuro-ophthalmic perspective. Diagnosing gaze-evoked nystagmus in the field is challenging and, in clinical practice, is performed under standardized conditions by trained eye-care professionals.
There is a high correlation between individuals who fail a field sobriety test and those arrested for elevated blood or breath alcohol levels, but an important question remains: What percentage of drivers pulled over for routine moving violations already have elevated BAC? If that percentage is high, the field sobriety test may add little or no incremental information. The validation literature does not directly address this base-rate problem (Burns & Anderson, 1995; Burns & Moskowitz, 1977; Tharp et al., 1981).
Critical Appraisal of NHTSA’s Horizontal Gaze Nystagmus: The Science and the Law
This NHTSA and National Traffic Law Center guide is widely cited as a compendium of prior research and recommendations on HGN (National Traffic Law Center, 2021). Notably, its acknowledgments list a range of contributors but no ophthalmologists, despite reliance on eye-movement assessment. The introduction asserts that “the consumption of alcohol or other drugs hinders the ability to correctly control eye muscles. The resulting abnormal eye movements are readily identifiable by properly educated and trained observers such as law enforcement officers”. While alcohol can cause horizontal gaze-evoked nystagmus, determining the presence and type of nystagmus is a complex medical assessment that lies beyond the scope of roadside training.
The guide characterizes HGN as “three separate assessments of independent eye movements: smooth pursuit, nystagmus at maximum lateral gaze, and onset of nystagmus prior to full deviation” (National Traffic Law Center, 2021). As defined in neuro-ophthalmic texts, however, evaluating gaze-evoked nystagmus requires, at minimum, confirmation of fixation and visual acuity, plus differentiation from numerous physiologic and pathologic causes (American Academy of Ophthalmology, 2022 to 2023; Thurtell, 2013). The Science section simultaneously concedes that vestibular-related eye movements are common but “are not assessed by law enforcement,” and that “neither fixation ability nor visual acuity are assessed” (National Traffic Law Center, 2021). Those omissions undermine reliability because they are foundational steps in any clinical evaluation (American Academy of Ophthalmology, 2022 to 2023; Thurtell, 2013).
The same section notes that distinguishing eye-movement types often requires specialized recording instruments, tools rarely available at the roadside, which further limits accuracy (National Traffic Law Center, 2021). In addition, terms such as Alcohol Gaze Nystagmus (AGN) and Positional Alcohol Nystagmus (PAN) are not standard in ophthalmic literature and imply alcohol-specific patterns that are not recognized clinically (National Traffic Law Center, 2021). Finally, although the guide states the HGN test is “very easy to administer,” even specialists find careful assessment of HGEN technically demanding. Protocol elements such as moving the target at about 10 degrees per second and judging onset before about 45 degrees presuppose timing and angle-measurement capabilities that officers do not have (National Traffic Law Center, 2021).
A related policy inconsistency is notable. State DMVs uniformly screen visual acuity for licensure but do not screen for nystagmus. If nystagmus evaluation were essential to driving safety, a licensing-stage program would presumably exist. The government cannot simultaneously deem visual acuity essential at licensure while treating roadside nystagmus assessment as both easy and decisive, yet omit it from licensing standards.
Definitions and clinical context: GEN, HGN, and OKN
Gaze-evoked nystagmus (GEN): rhythmic drift of the eyes back toward primary position with corrective saccades when holding an eccentric gaze. Low-amplitude endpoint nystagmus can be physiologic at extreme gaze. Sustained, large, or asymmetric forms are typically pathologic and should prompt evaluation (American Academy of Ophthalmology, 2022 to 2023; Thurtell, 2013).
Horizontal Gaze Nystagmus (HGN or HGEN): the lateral tracking sign emphasized during SFST. Some protocols associate angle-of-onset thresholds with alcohol effects (National Traffic Law Center, 2021).
Optokinetic nystagmus (OKN): a normal response to moving visual scenes, such as passing headlights, which can mimic HGN during roadside testing (American Academy of Ophthalmology, 2022 to 2023).
Given the diversity of nystagmus types and overlap in appearance, even trained ophthalmologists may find differentiation challenging without standardized conditions and instrumentation (American Academy of Ophthalmology, 2022 to 2023; Thurtell, 2013).
Measurement Limits At The Roadside: Angle Of Onset And Stimulus Control
HGN protocols put weight on estimating the angle of onset and controlling the speed of the moving target. Training materials describe moving the target at roughly 10 degrees per second and noting the onset prior to approximately 45 degrees (National Traffic Law Center, 2021). Without instruments to time motion or measure angles, these are estimates, not measurements. That invites variability and bias. Objective capture, such as video-oculography, allows later expert review and reduces subjectivity (Ritter, Bertolini, Straumann, & Bögli, 2020).
Environmental Confounds In Nighttime Stops
Roadside guidance instructs officers to avoid facing flashing police lights or passing traffic when conducting HGN or GEN because OKN can be induced by moving luminance patterns and is not alcohol-specific (American Academy of Ophthalmology, 2022 to 2023; National Traffic Law Center, 2021). In nighttime traffic environments, the likelihood of OKN contaminating observations increases. Tests should be conducted in the absence of such stimuli to minimize misclassification.
Prevalence And Population Considerations
If HGN or a visually similar GEN were rare in the general population, it might serve as a useful screening tool. If not, specificity erodes. The Leicestershire survey includes visually impaired cohorts (Sarvananthan et al., 2009). These are presumably not drivers. The estimate of 24 per 10,000 for the frequency of nystagmus is without foundation. The survey even notes that the population of Leicestershire is 925,000 people. There was no effort to systematically evaluate this population. Laboratory work demonstrates physiologic GEN at small gaze angles in many healthy observers. In particular, Whyte and colleagues reported physiologic GEN in 21% at 10 degrees and 34% at 20 degrees using binocular infrared video-oculography (Whyte, Petrock, & Rosenberg, 2010). Ritter and colleagues found that most healthy individuals display GEN and rebound nystagmus at modest eccentricities. In one dataset, 71% exhibited GEN at 30 degrees (Ritter et al., 2020). These eccentricities lie squarely within those probed during roadside HGN, which implies a substantial false-positive risk when onset angles are judged subjectively (Ritter et al., 2020; Whyte et al., 2010).
Medical Confounders Commonly Encountered in the Forensic Settings
Chronic alcoholism: may produce cerebellar degeneration and associated nystagmus independent of acute intoxication (Lau, Campos, Gai, et al., 2024). Distinguishing acute alcohol-related HGN from chronic alcoholism-related nystagmus is challenging. Not all individuals with alcoholism have recently consumed alcohol when stopped.
Medications: benzodiazepines are prevalent and can affect oculomotor control (Maust, Lin, & Blow, 2019).
Fatigue: can impair oculomotor control and alter eye-movement stability (Connell, Thompson, Turuwhenua, Srzich, & Gant, 2017).
Vestibular disorders: NHANES 2001 to 2004 data estimate objective vestibular dysfunction in about 35% of U.S. adults aged 40 and older. This corresponds to about 69 million people and indicates that vestibular contributions to eye movements are common in drivers (Agrawal, Carey, Della Santina, Schubert, & Minor, 2009).
Stimulants and exposures: the NHTSA guide itself acknowledges caffeine can induce nystagmus in those with vestibular problems (National Traffic Law Center, 2021).
Head trauma: post-crash evaluations must consider trauma-related nystagmus when accidents precede testing (Rauchman et al., 2023).
These factors are plausibly encountered among DUI arrestees and can confound HGN interpretation.
Practical Qualifications and Training Limits
Some legal resources imply that limited officer training suffices to diagnose nystagmus. In clinical practice, evaluation begins with fixation and visual acuity, often followed by a targeted examination including a vestibular assessment to classify nystagmus (American Academy of Ophthalmology, 2022 to 2023; Thurtell, 2013). Such assessment typically requires specialized training and controlled conditions. One could imagine expanded training and instrumentation for roadside use, yet in practice, this is rarely feasible at the curb and risks conflating screening with diagnosis (American Academy of Ophthalmology, 2022 to 2023; National Traffic Law Center, 2021; Thurtell, 2013).
Independence of Tests within SFST
Combined battery scoring assumes independence across tasks. Vertigo or imbalance that accompanies nystagmus may degrade performance on Walk-and-Turn or One-Leg-Stand, which entangle outcomes and compromise interpretation when tests are treated as independent contributors to impairment.
Predictive Value And Base Rates: What Would A Validating Study Require?
It is not enough to note that alcohol can cause HGEN. To demonstrate incremental value, we must know the pretest probability of illegal BAC among drivers stopped for routine moving violations. A properly controlled design would breath-test all such drivers, regardless of HGN, and then determine whether roadside HGN adds discrimination beyond that base rate. If the pretest base rate is already high, HGN contributes little predictive value. If it is low, any added value must be demonstrated empirically. A medical test is not meaningful if it is used merely to manufacture probable cause, which is a legal standard rather than a physiologic one.
When HGN is Present but BAC is Negative: Duty of Care
Drivers who display nystagmus yet have negative breath or chemical tests may have non-alcohol causes, such as vestibular disease, medication effects, or central nervous system pathology. In clinical practice, such findings would prompt neuro-ophthalmic and vestibular evaluation and, when indicated, neuroimaging. Current roadside use provides no mechanism to inform or refer such individuals. Establishing a path for notification and medical follow-up would better align enforcement with public health.
Conclusion
From an ophthalmologist’s perspective, the roadside interpretation of HGN is constrained by four issues. First, officers estimate the angle of onset without instruments (National Traffic Law Center, 2021; Ritter et al., 2020). Second, environmental OKN can contaminate observations (American Academy of Ophthalmology, 2022 to 2023; National Traffic Law Center, 2021). Third, non-alcohol physiologic GEN at small angles is common in healthy people (Ritter et al., 2020; Sarvananthan et al., 2009; Whyte et al., 2010). Fourth, medical confounders such as chronic alcoholism, medications, fatigue, and trauma are prevalent (Agrawal et al., 2009; Connell et al., 2017; Lau et al., 2024; Maust et al., 2019; Rauchman et al., 2023). As a result, horizontal gaze-evoked nystagmus cannot be reliably assessed in drivers pulled over for suspected DUIs. Simply stated, this is a complex medical evaluation that is beyond the scope of law enforcement at the curbside.
In addition, there are many causes of horizontal gaze nystagmus that occur independent of alcohol consumption, and several of these conditions are common in the driving population. That makes any uninstrumented nystagmus observation a substantial confounding variable (Agrawal et al., 2009; Ritter et al., 2020; Whyte et al., 2010). Related vestibular symptoms, such as vertigo, which can influence eye-movement findings, cannot be adequately assessed by officers during a roadside stop (American Academy of Ophthalmology, 2022 to 2023; National Traffic Law Center, 2021). Notably, state DMVs check visual acuity to establish minimum licensing requirements. These same agencies do not require or track nystagmus testing, which underscores the absence of a public-policy framework for its roadside use.
DUIs remain a major public-safety and political concern, and there is strong public demand to keep impaired drivers off the road. In that context, this paper has presented scientific evidence that questions the value of HGN in DUI arrests. As a pragmatic compromise, short of ideal laboratory-grade instrumentation, I recommend making a clear, front-facing video of the subject’s eyes and the officer’s performance of the test a standard protocol. When body-camera footage has sufficient resolution, it should be made available to all parties so that qualified experts can review the recording and provide an objective opinion (Ritter et al., 2020). This approach is not perfect, for reasons discussed above, but it would improve transparency, reduce subjectivity, and better align the process with clinical standards.
Ultimately, this is not purely a scientific question. It is also a policy question that implicates public safety. Compromise is preferable to a deadlocked debate. Law enforcement, government agencies, prosecutors, and medical experts can work toward a workable consensus that prioritizes safety while grounding decisions in physiology, measurement science, and fair procedure.
Disclosures
The author is an expert consultant in DUI-related legal cases.
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