However, results also showed that it may be of benefit in easing motor symptoms and improving patients’ quality of life, although more studies are necessary.
The findings, “A double-blind, randomized controlled trial of duloxetine for pain inParkinson’s disease,” were published in the Journal of Neurological Sciences.
A neurodegenerative disorder, Parkinson’s is characterized by the gradual loss of dopaminergic neurons in the substantia nigra — a region of the brain responsible for movement control — together with brain inflammation.
Previous studies have shown that patients have a lower pain threshold, possibly due to imbalances in the levels of several neurotransmitters — chemicals that allow nerve cells to communicate — in the brain.
Duloxetine, marketed as Cymbalta and Irenka, is normally prescribed to treat depression and to ease pain in individuals with chronic pain disorders like fibromyalgia, or pain caused by nerve damage. The medication works by preventing certain neurotransmitters from being taken up by nerve cells in the brain, including those thought to be involved in pain processing in people with Parkinson’s.
“Reports on duloxetine’s effect on pain in patients with Parkinson’s disease, however, are rare. One study reported that levels of pain … were significantly lower after 6 weeks of treatment with duloxetine … among the 20 people who completed the study,” the researchers wrote.
Given this study’s small number of patients and its open-label design (lacking a placebo group for comparison), its findings are susceptible to bias.
A team of researchers in Japan, for this reason, carried out a randomized, double-blind, and placebo-controlled trial to evaluate duloxetine’s effectiveness at treating pain in people with Parkinson’s.
The trial (jRCTs061180028) enrolled 46 patients recruited at the Ehime University Hospital, and ran from May 2015 to September 2019. All patients were at the early progressive stages of Parkinson’s, and experienced mild to distressing pain.
Participants were randomly assigned to duloxetine or a placebo, both given orally. The process of randomization took into account patients’ age and pain intensity levels, which were assessed using the Visual Analogue Scale (VAS) for pain.
For the first two weeks, patients were given a duloxetine capsule (20 mg) or placebo capsule every day. Then, for eight weeks, they took two capsules of either duloxetine (total 40 mg dose) or a placebo daily. In the study’s last two weeks, participants went back to one 20 mg capsule of duloxetine or a placebo every day.
Other medications used by patients were maintained, unless “change was necessary”.
The study’s main goal was to assess changes in pain intensity levels from the study’s beginning (baseline measures) to week 10, using the VAS.
Exploratory outcomes included changes in the progression of motor symptoms (Unified Parkinson’s Disease Rating Scale Part III, UPDRS-III), depression (Beck’s Depression Inventory, BDI), and patients’ quality of life (Parkinson’s Disease Questionnaire-39, PDQ-39).
Nine of the 46 enrolled failed to complete the study, including seven — all in the duloxetine group — who discontinued due to adverse events.
No significant differences in pain intensity were found between people treated with duloxetine or those given a placebo through week 10.
However, duloxetine-treated patients experienced significant improvements in their UPDRS-III scores — which translates to a lessening of motor symptoms — and in three areas of the PDQ-39 (activities of daily living, emotional well-being, and communication) compared to those given a placebo.
“These results may indicate that duloxetine has beneficial effects on PD [Parkinson’s disease] patients in more ways than simply through its anti-depressant effects,” the researchers wrote.
“[W]e were unable to confirm previous reports suggesting a favorable effect of duloxetine on pain. Nevertheless, the results are informative, as they pertain to the second study of duloxetine for pain in Parkinson’s disease and the first … placebo-controlled clinical trial using duloxetine to Parkinson’s disease,” they added.
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