A law enforcement officer pulls you over on suspicion of drunk driving and administers the standardized field sobriety tests. You have only had one drink, so you are not worried. She asks you to follow a pen with only your eyes as she moves it back and forth in front of your face. When she watches your eyes, her demeanor changes. Shortly afterward, you find yourself under arrest.
What made the officer so certain of your intoxication?
The horizontal gaze nystagmus test
If the officer performed the test correctly, she was watching your eyes for jerking movements as you followed the pen from side to side. She also watched to see if your eyes follow the pen smoothly. Alcohol can cause this involuntary motion, and in fact, most impaired individuals display this symptom.
Abnormal brain function
Medline Plus explains that the jerking motion the officer witnessed may have occurred because you have a health problem that originates in your brain, not your eyes. It begins in an area of your brain that controls eye movement. Even though the issue is evident to someone looking into your eyes, your brain does not register the movement, and it probably does not affect your vision.
Causes of nystagmus
You may have nystagmus because you take an antiseizure medicine. Dilantin is one of the most common medications that cause this condition. A head injury that affects that particular portion of the brain may also trigger nystagmus, as well as brain tumors, stroke or multiple sclerosis. Meniere disease, labyrinthitis or another inner ear disorder may cause the jerky eye movements. Even something as simple as a vitamin B12 deficiency could have led to your arrest. Some people have nystagmus from birth.
Back to the test
The officer should have asked you right away if you have any health conditions that could affect the outcome of the test. If she did not, then she made a procedural error. If you know you were not drunk, it may be worthwhile to ask your doctor to check you out for a nystagmus-causing condition. Tests for this may include a CT scan, an MRI or a test that records or measures eye movements.