The police officer asked you to follow a pen with your eyes. You did. Minutes later, you were in handcuffs. The Horizontal Gaze Nystagmus (HGN) test is a standard field sobriety test used in Virginia DUI stops, but "standard" doesn't mean infallible. A positive HGN result isn't the end of the road. An experienced Chesapeake DUI defense attorney can evaluate the way the HGN test was administered and who conducted it.
What Is the HGN Test and Why Do Officers Use It?
The HGN test is one of three standardized field sobriety tests used in the NHTSA SFST program. Police officers use it alongside the walk-and-turn and one-leg-stand tests to assess possible impairment during a DUI stop.
What Does Nystagmus Mean?
Nystagmus is the involuntary jerking of the eye. Alcohol consumption tends to amplify this response, particularly when the eyes track a moving object toward the periphery of vision.
During the HGN test, an officer moves a stimulus (typically a pen or small flashlight) horizontally in front of the driver's face and watches for specific signs of nystagmus at different points. Officers look for three clues in each eye. The presence of four or more total clues is considered a sign associated with alcohol impairment under NHTSA scoring guidelines.
Certain drugs, not just alcohol, can also produce HGN. Officers may also check for Vertical Gaze Nystagmus, though VGN is not one of the three standardized SFSTs.
What Can HGN Results Establish?
NHTSA-backed research has found HGN to be one of the stronger roadside screening tools when properly administered, but it is not a precise measure of BAC. More importantly, HGN is used primarily to establish probable cause for arrest, not to determine BAC with precision.
Virginia courts may allow officers to testify about field sobriety tests as evidence of impairment, though judges determine how much weight that evidence should receive. A positive HGN result does not prove guilt, and it does not tell the court exactly how much a driver had to drink.
What Can Go Wrong With an HGN Test?
The reliability of a HGN field sobriety test depends on proper training, correct administration, and an accurate accounting of the driver's medical history. Several factors can produce a false positive result.
Medical Conditions and Medications
Nystagmus isn't exclusive to alcohol impairment. Certain medications, neurological disorders, inner-ear problems, and other medical issues can produce eye movements unrelated to alcohol. Conditions and substances that may cause nystagmus independent of alcohol include:
- Inner ear disorders. Conditions like labyrinthitis or benign paroxysmal positional vertigo (BPPV) directly affect eye movement and can mimic HGN cues.
- Prescription medications. Anticonvulsants, sedatives, and some antidepressants are known to produce nystagmus as a side effect.
- Neurological conditions. Multiple sclerosis, traumatic brain injury, and stroke history can all affect how the eyes track a moving object.
- Natural nystagmus. A small percentage of people have baseline nystagmus that exists regardless of any substance use.
Officer Training and Test Administration
NHTSA provides standardized training for officers to administer field sobriety tests, and courts often look at that training when evaluating the credibility of the results. Suppose an officer holds the stimulus too close or moves it too quickly. Either error can compromise the results. These aren't minor technicalities. They go directly to whether the test produced valid data. Roadside lighting, traffic, and other field conditions can make the officer's observations harder to interpret.
How a Defense Attorney Evaluates HGN Evidence
When HGN results are part of a DUI case, experienced DUI defense attorneys won't simply accept them at face value. James E. Short's background as a former Assistant Commonwealth's Attorney gives him a clear view of what the prosecution needs to prove and where that evidence tends to fall short. A defense review of HGN results typically examines:
- Officer certification. Was the officer properly trained to administer the standardized HGN test?
- Protocol compliance. Was the test conducted at the correct distance and pace, in the correct sequence?
- Field conditions. Did roadside lighting, traffic, and other environmental factors affect the reliability of the officer's observations?
- Medical history inquiry. Did the officer screen for signs of medical impairment before administering the test?
Taking the Next Step After a DUI Charge in Chesapeake
A DUI conviction in Virginia carries serious consequences. A first offense includes a mandatory minimum $250 fine and a one-year license revocation. For a first offense, Virginia law imposes an added mandatory minimum five-day jail term for a BAC of at least 0.15 but not more than 0.20, and 10 days if the BAC is above 0.20. A criminal record can affect employment, particularly for jobs involving driving or background checks.
HGN results feel significant in the moment, but they are not the final word. Whether the test was properly administered, whether a medical condition explains what the officer observed, or whether the officer had the required training are all questions worth asking.