A nystagmus DUI defense challenges the horizontal gaze nystagmus test on four fronts: the test’s known error rate, medical conditions and medications that cause the same eye movements, environmental conditions at the stop that produce false clues, and officer deviations from NHTSA’s standardized protocol. Any one of these can weaken the evidence. Layered together, they can dismantle it.
Why the Eye Test Isn’t as Reliable as Officers Say
Officers routinely testify that the HGN test is the most scientifically validated field sobriety test. That is true, and it still leaves plenty of room to attack. NHTSA’s San Diego validation study found that a four-clue threshold correctly classified about 88 percent of subjects at or above 0.08 BAC. Earlier research put the test’s standalone accuracy at 77 percent when the threshold was 0.10 BAC.
Flip those numbers around. A 12 percent error rate at the four-clue level means roughly one in eight drivers who display four clues are sober or under the limit and get called impaired anyway. And those figures come from controlled research with trained technicians. Real roadside stops introduce variables the studies never measured, and that gap is where most successful defenses live.
Medical Conditions That Produce the Same Eye Movements
Many health conditions cause involuntary eye jerking that looks exactly like alcohol-induced nystagmus. No amount of officer training can tell them apart during a roadside stop.
Inner ear disorders are the most common cause. Benign positional vertigo and labyrinthitis disrupt the vestibular system, which is the same neural pathway alcohol affects. The brain receives faulty balance signals and the eyes twitch in response. Neurological conditions including multiple sclerosis and brain lesions can interfere with the nerves controlling eye movement, producing nystagmus that has nothing to do with drinking.
Some people are born with it. Congenital nystagmus, also called infantile nystagmus syndrome, causes constant or intermittent eye movements from birth, and many people who have the condition are unaware until a traffic stop makes it an issue. Severe astigmatism and chronic eye strain can make smooth tracking difficult. Even ordinary fatigue and sleep deprivation can trigger brief episodes of lateral eye movement an officer may score as impairment.
If you have any documented condition affecting your eyes or vestibular system, an independent evaluation from a neuro-ophthalmologist or oculographer can establish that your nystagmus exists regardless of alcohol consumption. That kind of documentation is devastating to a prosecution leaning on HGN evidence.
Medications That Trigger Nystagmus
Prescription medications are an overlooked factor in HGN cases. Several widely prescribed drug classes list nystagmus as a known side effect, and a person taking them at therapeutic doses will show eye jerking indistinguishable from the alcohol-induced version.
Anti-seizure medications are the biggest category. Carbamazepine, phenytoin, valproic acid, and lamotrigine are prescribed to millions of Americans for epilepsy, nerve pain, and mood disorders. The FDA-approved labeling for carbamazepine lists nystagmus explicitly as an adverse nervous system reaction. Lithium, commonly prescribed for bipolar disorder, is another documented cause. These drugs do not impair driving at proper doses, yet they reliably produce the exact eye movements the HGN test is designed to detect.
The implication is simple. If you were taking one of these medications at the time of the stop, the officer’s observation of nystagmus tells the jury nothing about whether you had been drinking. A pharmacy record showing an active prescription, paired with medical literature on the drug’s side effects, can neutralize the HGN evidence entirely.
Environmental Conditions at the Scene
Even a healthy, sober driver can produce false clues when the setting interferes with the test. The most common culprit is optokinetic nystagmus, where the eyes involuntarily track rapidly moving visual stimuli in the environment. Passing headlights on a busy highway trigger this response almost reflexively. So do the flashing red and blue lights on the patrol car behind you, and officers rarely reposition their vehicles before running the test.
High-contrast shadows, flickering streetlights, and oncoming headlights all make focusing on the officer’s stimulus harder. Wind, rain, and cold cause tearing and involuntary squinting that officers may misread as tracking difficulty. NHTSA’s own training materials acknowledge that the test should be given in a setting with minimal visual distraction, which roadside stops rarely offer.
Documenting these conditions matters. If dashcam or bodycam footage shows patrol lights flashing directly behind the driver during the test, or heavy traffic passing within feet of where the test was conducted, those conditions give the jury a concrete, non-alcohol explanation for whatever the officer recorded.
Protocol Violations Captured on Camera
NHTSA designed the HGN test as a standardized procedure. The published accuracy rates only hold when every step is followed exactly as trained. When officers deviate, the scientific basis for the test collapses. This is often the most effective defense strategy because the deviations are captured on video.
The common failures are specific and measurable:
- Holding the stimulus too close or too far from the driver’s face instead of the required 12 to 15 inches, which changes how the eyes track and can create clues that would not exist at the correct distance.
- Moving the stimulus faster than two seconds per pass during the smooth pursuit check, or holding at maximum deviation for less than four seconds.
- Holding the stimulus at the wrong height. It should sit slightly above eye level; too high or too low forces the driver to tilt their head and changes the angle at which nystagmus appears.
- Skipping the preliminary checks for equal pupil size, resting nystagmus, and equal tracking. Without these, the officer never screened for medical causes before attributing the result to alcohol.
- Checking each eye only once instead of the required minimum of two passes per clue.
Bodycam and dashcam footage is the single most valuable tool for identifying these violations. The camera captures hand movements, pass speed, and environmental conditions simultaneously. Defense attorneys routinely slow footage frame by frame to measure whether timing matches NHTSA requirements. When it doesn’t, the argument for suppression or reduced weight becomes much stronger.
When the Chemical Test Contradicts the HGN
One of the most powerful nystagmus defenses arises when a breath or blood test comes back below 0.08 BAC despite the officer reporting four or more HGN clues. This directly contradicts the premise that four clues reliably indicate a BAC at or above the legal limit, and it puts the driver squarely in the roughly 12 percent of cases the test gets wrong.
The absence of a chemical test can also work in your favor. If the arrest rested primarily on HGN observations and you were never given a breath or blood test, the prosecution is leaning on a screening tool with a known error rate as its central evidence. Jurors tend to find this less convincing than numbers from a chemical analysis.
Challenging Admissibility Before Trial
Even a properly administered HGN test still has to survive a legal challenge before the jury hears about it. Most states and all federal courts evaluate scientific evidence under the Daubert standard, which asks the judge to examine peer review, known error rate, maintained standards, and acceptance in the scientific community. A smaller group of states still uses the older Frye standard, which looks only at whether the technique is generally accepted in the relevant scientific community.
The result is a patchwork. Several states have ruled that HGN results are not admissible as scientific proof of intoxication at all. Others allow HGN only as circumstantial evidence of impairment or support for probable cause, and prohibit officers from using it to suggest a specific blood alcohol level. The restrictions reflect persistent judicial skepticism about whether a roadside eye examination meets the scientific reliability threshold required for criminal prosecution.
A judge evaluating admissibility will also scrutinize the officer’s training. NHTSA provides a standardized field sobriety curriculum, and officers who cannot show completion of both initial training and periodic refresher courses may be found unqualified to interpret results. If training has lapsed or the officer cannot articulate the scientific basis for the test, the defense has grounds to argue the testimony should be excluded.
Can You Refuse the Eye Test?
Field sobriety tests, including the HGN test, are voluntary. You are not legally required to perform them. This is a critical distinction from chemical tests of breath, blood, or urine, which are governed by implied consent laws. Refusing a chemical test after arrest triggers automatic license suspension in virtually every state and can carry additional penalties. Refusing to follow an officer’s finger with your eyes carries no such consequence.
The practical reality is more complicated. Declining will not end the encounter. The officer can still arrest you based on other observations like the smell of alcohol, slurred speech, or erratic driving. In some jurisdictions, your refusal to perform field sobriety tests may be mentioned at trial, though the rules vary widely. Some states prohibit the prosecution from using FST refusal as evidence of guilt; others allow it as circumstantial evidence of consciousness of guilt.
From an evidence standpoint, refusing the HGN test means one less piece of evidence the prosecution can present. You are declining a voluntary, imperfect screening tool that exists primarily to give the officer probable cause to arrest you.
What to Gather Now
A strong nystagmus defense rarely rests on one argument. The best cases layer them: the officer rushed the timing, patrol lights were flashing during the test, and the driver takes carbamazepine for a seizure disorder. Any single factor might not be enough. Together they create real doubt about whether the HGN results mean what the prosecution says they mean.
Start by requesting every piece of available footage. Bodycam and dashcam recordings are often the most objective record of how the test was actually administered versus how the officer described it in the arrest report. Pull your medical records and pharmacy history if you have any condition or medication that could cause nystagmus. If the case justifies the expense, an independent examination by a neuro-ophthalmologist can document whether you have baseline nystagmus that exists regardless of alcohol consumption.
The HGN test is the most scientifically grounded of the three standardized field sobriety tests, which is why prosecutors lean on it so heavily. Most grounded is not the same as infallible, and the gap between a controlled laboratory and a dark highway shoulder at 1 a.m. is wide enough to defend in.