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What Can Pupil Response Reveal About Cannabis Use?

What Can Pupil Response Reveal About Cannabis Use?

To understand intoxication and impairment, CU neuro-ophthalmologist Prem Subramanian, MD, PhD, looks at how pupil response changes with cannabis consumption.

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Written by Kara Mason on April 18, 2024

When light hits the retina, the optic nerve carries a signal to the midbrain, where equal neural impulses are generated and sent to pupillary sphincter muscles, which cause the pupils to constrict.

How does cannabis use affect this process?

It depends, according to researchers from the University of Colorado School of Medicine who have developed a novel multi-step video processing procedure that breaks apart images of pupils from infrared videography and helps identify whether a person is intoxicated by cannabis.

In the future, it’s possible that a quick ocular evaluation could help make this identification possible in real time via a set of goggles. Cannabis intoxication can be more difficult to determine than other substances. A roadside sobriety test has been in use for alcohol intoxication since the 1980s, for example. Cannabis, however, affects the body differently and stays in the bloodstream long after intoxication wears off, making it more challenging to detect and requiring different testing.

“We've had trouble finding measures that will differentiate between acute intoxication and chronic usage,” explains Prem Subramanian, MD, PhD, professor of ophthalmology and Clifford R. and Janice N. Merrill Endowed Chair, who contributed to a new study that analyzes the outcomes from a light test administered with goggles utilizing infrared videography. “But pupil response seems to be a good indicator.”

Looking for reliable signs of intoxication

With a growing number of states legalizing cannabis, and related motor vehicle fatalities similarly on the rise, researchers say the basis of their work is in finding a dependable way to identify whether a person is at risk of a crash.

“Because we can't necessarily come up with a roadside test that tracks a person's reaction time in a reliable way, we're trying to use the measures of pupillary constriction and re-dilation to be a surrogate for their reaction time,” Subramanian says. “Intoxication with cannabis changes your pupillary reaction and slows your reaction time, but it also changes how your pupils respond to light and how they start to dilate again once the light is removed.”

Cannabis use also doesn’t lend itself to a test like a breathalyzer, as alcohol does.

Daily users of cannabis can have tetrahydrocannabinol (THC), the primary psychoactive agent, in their blood for days or weeks after consumption, making it an unreliable predictor of intoxication. A chronic user might show signs of having previously consumed the substance, but they wouldn’t exhibit the same pupil response as an acute user.

Pupils may serve as an important link to intoxication and be the key to responsible evaluation.

“Pupillary response may offer an avenue for detection that outperforms typical sobriety tests and THC concentrations,” the researchers say.

Developing a procedure and looking to the future

In the study, researchers created a procedure to see whether data on pupil size and the pupillary light reflex obtained from the infrared videography could determine the impact of acute cannabis smoking in occasional and daily cannabis use.

In a controlled setting, one set of participants were given the assessment via virtual reality (VR) goggles before and after smoking cannabis while the other group abstained from smoking. The researchers observed that pupil reactivity to light and re-dilation was reduced in the people who had just smoked.

“We were looking at the speed at which the pupil constricts and the amount that it constricts, and then how quickly it starts to dilate again once the light stimulus is removed,” Subramanian explains.

The results make a promising case for future use of such videography tests.

“I’d like to see others use this type of technology to take pupillary measurements now that we’ve demonstrated that it’s reasonable to do so,” Subramanian says.

In a real-world scenario, however, a roadside test that could confirm cannabis intoxication may still be far off.

“Ultimately, the hope is that there is a technology, like a VR goggle or similar device, that could measure pupillary reactions, rather than having a person trying to judge what’s normal or abnormal,” Subramanian says. “This could be a screening tool to identify those people where, perhaps, further intoxication testing is needed, but that’s also something we just don't have today.”

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Prem Subramanian, MD, PhD