Wednesday, May 20, 2009

The eyes have it

Arresting officers in DWI cases love to get on the stand and announce that your client exhibited all six "clues" in the horizontal gaze nystagmus (HGN) test and, as a result, it was a foregone conclusion that your client was intoxicated.

But wait just a second - even though this "test" is couched in scientific terms, the follow-the-pen show on the video is far from being either scientific or valid.

Nystagmus is defined by NHTSA as "an involuntary jerking of the eyes." NHTSA states that alcohol causes horizontal gaze nystagmus.

The American Academy of Opthamology defines the condition as "is an unintentional jittery movement of the eyes" that "usually involves both eyes and is usually exaggerated by looking in a particular direction."

So, for your police officer trained to diagnose eye conditions by another police officer, nystagmus is an indicator of intoxication. To a trained medical practitioner, however, nystagmus is a medical condition.

When an officer detects "nystagmus" someone is likely to be placed under arrest and hand-cuffed. When a trained medical practitioner detects nystagmus, on the other hand, he is going to conduct a thorough evaluation and will more than likely call in another specialist to examine the patient.

When an officer conducts the pen-and-eye test in the field, there are no controls for the effects of weather, lighting or traffic conditions. The officer must estimate the time for each pass of the pen and the distance the pen travels to the side. When a trained medical practitioner conducts an eye test to determine if nystagmus is present, the test is performed in a controlled environment with medical instruments by a doctor who went to medical school.

Finally, when an officer detects "nystagmus," he is looking for evidence to confirm his suspicion that the motorist is intoxicated. When a trained medical practitioner detects nystagmus, his job is to eliminate all possible causes until he can diagnose the underlying cause.

The scientific literature indicates a myriad of conditions responsible for nystagmus - from epilepsy to Parkinson's disease and from muscular dysfunction to head trauma. But, according to the police officer's diagnosis, the only cause for nystagmus of a driver being tested at the side of the road is alcohol consumption.


Feisty said...

Thanks for the post. I learned something interesting about Nystagmus the other day: according to the NHTSA, it's a "robust" indicator of intoxication, and 4 clues are sufficient for arrest, even though the NHTSA EXPECTS to see nystagmus at BAC levels as low as .03. They even had a few false positives, in which study participants showed 4 clues of nystagmus at BAC levels as low as .016. A link to the study is here:

I also recently posted on this issue a here:

Hopefully you can use this study in court.

Paul B. Kennedy said...

Thank you for your comment. I will check out both the NHTSA study and your blog post.