Horizontal Gaze Nystagmus

Horizontal Gaze Nystagmus (HGN) refers to an involuntary jerking of the eye that occurs as the eye moves from side to side. This reflexive movement is controlled by the central nervous system and can be exaggerated by alcohol, certain drugs, and neurological conditions. Law enforcement officers frequently use the HGN test as part of field sobriety testing to detect impairment during DUI stops. The test is based on the premise that intoxicated individuals will exhibit more pronounced jerking at lower angles of eye movement. The National Highway Traffic Safety Administration (NHTSA) recognizes HGN testing as one of the three standardized field sobriety tests (SFSTs) commonly administered by police.

• How HGN Occurs – Nystagmus is a natural response to movement, but excessive nystagmus can indicate impairment. Alcohol affects the brain’s ability to control eye movement, leading to exaggerated jerking when the eye moves horizontally. This reaction becomes more pronounced as a person consumes more alcohol.
• Neurological Basis – The brainstem and cerebellum regulate eye movement, and disruptions caused by alcohol, drugs, or medical conditions can make involuntary jerking more noticeable. Some prescription medications, such as anti-seizure drugs or muscle relaxants, may also cause abnormal nystagmus.
• Factors That Influence HGN – While alcohol is a common cause, fatigue, inner ear disorders, and head injuries can also lead to abnormal eye movement. Officers are trained to differentiate between alcohol-induced nystagmus and nystagmus caused by medical conditions, but false positives can still occur.

How Law Enforcement Uses the HGN Test
Police officers use the HGN test as a tool to assess potential impairment, but the accuracy of this test depends on how it is administered. The NHTSA guidelines require officers to follow specific steps to ensure the test is performed correctly. However, improper administration or external factors can affect results, leading to unfair DUI charges. The American Optometric Association has noted that while the test is widely accepted, it should not be used as the sole indicator of intoxication.

• Test Administration – Officers ask the suspect to follow a stimulus, such as a pen or finger, while watching for involuntary jerking. Three main clues are assessed: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus before reaching 45 degrees.
• Accuracy and Reliability – The HGN test is considered one of the most reliable field sobriety tests, with a claimed accuracy rate of approximately 77% in detecting alcohol impairment. However, mistakes in administration or environmental distractions can reduce its reliability.
• External Factors That Affect Results – Bright lights, flashing police car lights, and pre-existing medical conditions such as vestibular disorders or head trauma can influence eye movement. These factors may lead to false positives, which could result in wrongful DUI arrests.

Challenging HGN Test Results in Court
Defendants facing DUI charges based on HGN test results can challenge the validity of the test in court. Because HGN testing relies on subjective observations, defense attorneys often question whether the test was properly administered and whether alternative explanations exist for the observed nystagmus. The Innocence Project has highlighted cases where improper forensic evidence, including flawed field sobriety tests, contributed to wrongful convictions.

• Officer Training and Certification – The effectiveness of HGN testing depends on the officer’s training and adherence to NHTSA guidelines. If an officer lacks proper certification or deviates from testing protocols, the results may be deemed unreliable in court.
• Alternative Explanations for Nystagmus – Attorneys often present evidence that factors other than alcohol could have caused the nystagmus. Medical conditions, fatigue, and anxiety can all lead to involuntary eye movement that mimics the effects of intoxication.
• Cross-Examining the Arresting Officer – During DUI trials, the officer’s administration of the test is often scrutinized. If the officer failed to perform the test correctly or introduced bias in their observations, the defense may argue that the HGN test results should be dismissed as evidence.

Medical Conditions and Prescription Drugs That Cause Nystagmus
Not all cases of nystagmus are related to alcohol or drug impairment. Various medical conditions and prescription medications can lead to abnormal eye movements. It is crucial for individuals accused of DUI to provide medical documentation that may explain their symptoms. 

• Inner Ear Disorders – Conditions such as Meniere’s disease, vertigo, and labyrinthitis can cause abnormal eye movements, which may be mistaken for intoxication during an HGN test.
• Neurological Disorders – Brain injuries, strokes, and conditions such as multiple sclerosis affect the brain’s ability to control eye movement, sometimes resulting in nystagmus.
• Medications That Induce Nystagmus – Prescription drugs, including anti-seizure medications, antidepressants, and certain muscle relaxants, can lead to involuntary jerking of the eyes. A person taking these medications may fail an HGN test despite being completely sober.

Legal Defenses Against HGN-Based DUI Charges
If you are facing DUI charges based on an HGN test, you have legal options to challenge the evidence. Defense attorneys can use expert witnesses, medical records, and procedural errors to contest the validity of the test results. Courts may suppress HGN evidence if officers failed to follow proper protocols or if other explanations for nystagmus exist.

• Presenting Medical Evidence – If a defendant has a history of neurological disorders or an ear condition, medical records can be used to explain why their HGN test may have appeared abnormal.
• Challenging Officer Testimony – An attorney may cross-examine the officer to determine whether they followed proper procedures or introduced bias when interpreting the test results.
• Introducing Expert Testimony – Medical experts or optometrists can testify that HGN is not a definitive indicator of intoxication, particularly when alternative explanations exist.

Lawyers Corner Can Help
If you have been charged with a DUI based on an HGN test, understanding your rights is essential. The HGN test is not foolproof, and many factors can lead to inaccurate results. Lawyers Corner provides legal insights and resources to help individuals defend themselves against DUI charges. Our team can connect you with experienced attorneys who can review your case and determine the best defense strategy. Contact us today through our contact page for expert legal assistance.