TUESDAY, Feb. 22, 2005 (HealthDay News) -- Doctors might someday use cocaine to diagnose Parkinson's disease in a rather unconventional way: via drops to the eye.
By comparing the amount of pupil dilation caused by an eye drop used in ophthalmology offices to dilation from a cocaine eye drop, researchers in Japan said they could accurately identify people with Parkinson's.
At present, there is no specific diagnostic test for Parkinson's disease, which is caused by the gradual death of brain cells producing the neurotransmitter dopamine. Symptoms of this motor system disorder include tremor, trembling, rigidity or stiffness, slowness of movement and imbalance.
"Unfortunately, there is no clear way to diagnose Parkinson's," said Dr. Spriridon Papapetropoulos, an assistant professor of neurology at the University of Miami School of Medicine.
"As we speak, the diagnosis remains clinical," which means the physician must use various criteria to figure it out. Some tests are available, including a smell test, but they are unreliable and rarely used, he said.
The current findings are far from definitive, however.
"This eye drop test is a potential diagnostic tool for Parkinson's disease," said Dr. Shun Shimohama, senior author of a research letter published in the Feb. 23 issue of the Journal of the American Medical Association. However, he pointed out that autonomic neuropathy -- damage to the nerves that regulate the involuntary part of the nervous system -- may affect the results. Also, the size of the study was small, he added.
"We need to have the correct diagnosis, and if there is a simple test that can give us the correct diagnosis we might as well use it," Papapetropoulos said. But, he had reservations.
"This research letter was a very interesting research idea, and it's potentially useful," Papapetropoulos said. "However, this is just a research letter with very limited information about the clinical characteristics of the patients involved, about the methods that they used, about patient selection. And the scientific community should do a larger clinical study on this topic."
"We must collect more data for practical use," added Shimohama, of the department of neurology at Kyoto University Graduate School of Medicine in Kyoto, Japan.
The trial involved 38 patients with Parkinson's Disease, 20 controls and 10 individuals with multiple system atrophy (MSA), a neurodegenerative disease with various symptoms involving movement, blood pressure and more.
Researchers recorded a baseline pupil diameter for all participants using an infrared videocamera in fixed daylight brightness.
Each participant was then giving phenylephrine solution (commonly used in eye exams and before and after eye surgery) in both eyes. Their pupil diameter was recorded 60 minutes later.
After a minimum of 72 hours, the same amount of a five-percent cocaine solution was placed in each participant's eyes, with the pupil diameter recorded one hour later.
The researchers then calculated the difference between dilation caused by phenylephrine and that caused by the cocaine.
There was no statistically significant difference in phenylephrine dilation between people with Parkinson's and people with MSA. However, cocaine-induced dilation was significantly less in the Parkinson's group than in the other two groups, with little difference between the controls and those with MSA. And the difference between phenylephrine-induced and cocaine-induced dilation was greater in the Parkinson's group than in the control or MSA groups.
What is the biology behind this finding?
According to Shimohama, cocaine blocks the uptake of the neurotransmitter norepinephrine. The build-up of norepinephrine at the nerve receptors causes dilation. Less cocaine-induced dilation means that sympathetic nerve terminals have been lost, he explained.
The side effects of the procedure appear to be minimal. "Phenylephrine and cocaine eye drop tests can cause conjunctivitis rarely," Shimohama said. "Some patients felt slight brightness after the eye drop test for about 30 to 60 minutes."
But if something in the illicit drug can help Parkinson's, then something in a Parkinson's drug might lead to a treatment for those addicted to cocaine, another study suggests.
In an odd reversal, scientists reporting in Feb. 23 issue of Neuroscience have found that a substance similar to a drug used to treat Parkinson's disease blocks the stimulating effects of cocaine and might one day become a therapy for addiction.
When mice were given a substance similar to benztropine, a drug used to treat Parkinson's symptoms, then injected with cocaine, they did not manifest any of the hyperactive behavior typically associated with cocaine.
More information
The National Institute of Neurological Disorders and Stroke has more on Parkinson's.
