Eye examinations are particularly interesting because they do not actually say much about impairment at all. What they do say a lot about is the condition of a person’s eyeball and whether that person may have suffered head injuries, has or is suffering a stroke or a seizure, or whether a person may have neurological conditions. Of course, a police officer is in no position to determine any of this.
So read on to find out the three types of eye examinations that are used in the DRE Evaluation.
Ah, another example of the DRE evaluation being used to look for symptoms that are present with alcohol in order to detect drug impairment. The Horizontal Gaze Nystagmus test was originally designed by NHTSA for the purposes of looking for alcohol impairment. In fact, the statistics relied on by NHTSA claim that this test is 88% reliable at identifying alcohol-impaired drivers if four or more clues are present. Clues, by the way, are the parts of the test the officer is designed to look for in the case.
The problem with the HGN is that, by and large, it is not expected to be present with drug impairment. For example, only CNS depressants, inhalants, and PCP will produce HGN. And bear in mind too that CNS depressants include alcohol. I have to wonder why a DRE test is required at all in cases of people who are impaired by PCP. I mean, have you ever seen what goes down with PCP use? A DRE evaluation is definitely not necessary.
The HGN is administered by an officer with a pen or a pen like object, known as a stimulus. Essentially, the stimulus is passed in front of the eyes and held at certain points so that officers can assess whether there is any involuntary jerking of the eyeball. But there are a number of limitations on its use. And when administered incorrectly, which apparently occurs around 90% of the time, the test does not produce reliable information. The stimulus must be a certain distance from the person’s eyes not too far or too close, level with their eyes not too high or too low, moved not too slowly and not too quickly, and moved to the just right spot for measuring whether there is distinct and sustained nystagmus at maximum deviation, not too far or not far enough.
It’s like Goldilocks and the Three Bears, but for your eyeballs.
But nystagmus is not just a symptom of drug use. It is also a symptom of all sorts of other medical conditions, head injuries, and can even be hereditary. The presence or absence of nystagmus, with so many other explanations simply does not reliably indicate or even hint at impairment, much less impairment by a drug.
Vertical Gaze Nystagmus
This one is not much better. In fact, it’s basically the Horizontal Gaze Nystagmus but in the opposite direction. Instead of the officer engaging in moving the stimulus back and forth, the stimulus is moved up and down. Except, medical experts aren’t entirely certain why vertical gaze nystagmus appears when it does appear. And the answer is not always impairment by drugs.
As with the HGN, the VGN doesn’t really lend much information to the DRE officer about what drugs might be the cause of what they are seeing. It is said to be present at high concentrations of CNS depressants and inhalants, and again present with PCP. Pro tip: if you’ve got a person on PCP to the stage where they are now willingly taking the VGN evaluation step, they’re not on PCP. So together the VGN and HGN just really seem to act as confirmation bias of one another.
Given the absence of solid medical explanation for why VGN may appear in some people, it is dangerous if not foolhardy to effectively guess that it means a person has taken a high dose of CNS depressants, inhalants, or is on PCP simply because their eyeballs move around in a weird way.
Lack of Convergence
This one is really simple. So simple, in fact, that you probably determined whether you had issues with lack of convergence on the playground in elementary school. It is merely the ability to cross your eyes. Apparently, according to the infinite scientific wisdom of NHTSA, a person who is using PCP, inhalants, CNS depressants, or cannabis will not be able to cross their eyes.
Lack of convergence is, interestingly, the only eye movement-related cannabis symptom that is included in the DRE matrix for the assessment of the results. Which leads me to think that the NHTSA DRE developers were taking their opinions about eye movement and cannabis from this slightly paranoid Grasscity forum question.
And, if you’re sensing a trend, these lack of convergence tests are also consistent with known medical conditions that do not have known causes or explanations. Maybe it is smoking too much cannabis, but I highly doubt it.
One of the concerning aspects about the eye tests — and you’ll note heavy reliance on the eyeballs overall in the DRE program — is that, as my older readers will appreciate, your eye function deteriorates as you age. And so does your ability to stare at a book or TV screen or cross your eyes or follow a stimulus with smooth pursuit. But aging is not drug impaired driving, and yet the DRE appears designed to make it out as though it is.
Next week we will cover the divided attention psychophysical tests. Sounds super fancy, but I promise they are anything but. Check back then.