Can a low concentration, topical capsaicin relieve neuropathic pain in SCI?
What we’re talking about isn’t so much a sauce as it is a cream—a skin cream, to be exact. Dr. Lukas Linde, a postdoctoral fellow in UBC’s Department of Anesthesiology, Pharmacology, and Therapeutics and a member of Dr. John Kramer’s ICORD lab, was recently awarded a Craig H. Neilsen Foundation grant for a project titled Less is more: Low concentration topical capsaicin to relieve neuropathic pain in SCI. The project will expand on research by UBC master’s student Hannah Goodings, and will explore the effectiveness of her recently-developed low concentration capsaicin intervention to manage a specific type of SCI neuropathic pain—segmental (or peripheral) pain in the arms, at the level of injury. This likely isn’t the first time you’ve heard about using capsaicin (the active or “hot” ingredient in peppers) to treat neuropathic pain. High concentration (eight percent) topical capsaicin patches have been successfully used to treat peripheral neuropathic pain for some time.
High concentration capsaicin vs. low concentration
Our skin has sensory neurons called nociceptors. One particular type, TRPV1, sends sensations of heat and pain to the brain. SCI often causes TRPV1 receptors at the level of injury to misfire or misreport burning sensations to the brain. Applying high concentration capsaicin seems to deactivate or “burn out” these receptors so that they lose their ability to send pain signals to the brain.
There are several problems with high concentration topical capsaicin. It produces incredible burning pain on the skin where it’s applied (which kind of defeats the purpose), so it has to be mixed with lidocaine to numb that burning. It can only be administered under physician supervision, and isn’t approved for use in Canada. Most important, however, is that while it does “burn out” rogue TRPV1 receptors, studies have shown that it does not prevent later development of central sensitization—more specifically, secondary hyperalgesia, which is heighted pain sensitivity in skin areas adjacent to the treatment area.
In other words, high concentration capsaicin seems to deactivate the intended TRPV1 receptors, but those receptors are still involved in later development of pain sensations from neighbouring regions. Enter Hannah Goodings and her work with low concentration capsaicin. “Hannah’s MSc project explored the mechanisms of long duration, low concentration topical capsaicin uses in young healthy adults,” says Linde. “The key finding from her study was that, over time and with repeated use, low concentration topical capsaicin not only defunctionalized the pain receptors where it was applied, but it further prevented the development of sensitization.
This last part is the exciting part, as we may actually be interrupting some of the underlying processes that lead to the development of the hyperalgesia (increased pain) and allodynia (pain from non-painful stimuli) observed in neuropathic pain. This is an important distinction, as we are suggesting that, while both high and low concentration can knockout pain receptors in the region they are applied, only low concentration applied for long durations appears to also prevent the development of central sensitization like pain from neighbouring regions.”
About the ICORD study
Linde’s study, titled, Neuropathic pain relief after spinal cord injury – a study using heat (capsaicin) cream will try to replicate these findings in people with SCI who have segmental pain in their arms. He is recruiting 40 candidates, who will be randomized to receive either a regular, over-the-counter, low concentration capsaicin cream or a placebo cream. Participants will apply creams twice daily for 15 consecutive days in the area of pain. They will undergo complete pain assessments prior to starting treatment, immediately following treatment, and 15 days after treatment concludes.
“Ideally, we will see two things,” he says. “First, reduced pain ratings in the regions we apply topical capsaicin. Second, reduced secondary hyperalgesia (secondary pain) when we subsequently apply a heat pain model to induce a temporary state of central sensitization. For us, this would demonstrate that topical capsaicin works not only to “burn out” pain receptors in the periphery, but they also help mitigate the future development of pain in neighbouring regions.”
Interested in participating?
Results won’t be known until 2022, but if benefits are confirmed, Linde says he will work to promote adoption of the treatment into regular practice. “Right now, low concentration topical capsaicin is readily available over the counter,” he says. “It’s often an ingredient included in various topical pain treatments found in drug stores. It’s likely that people are already using topical capsaicin—and that it is providing some pain relief. Given good results, we will be able to help speak to mechanisms of how topical capsaicin is working, and be in a position to better provide recommendations on how to apply and use it to relieve pain—what region, for how long, how to assess signs it is working, etcetera.”
He invites readers interested in participating, or who want more information about the project, to contact him directly via email at firstname.lastname@example.org.
This article was originally published in the Fall 2021 issue of The Spin and featured two other studies on neuropathic pain relief. Read more here:
- Cognitive Multisensory Rehabilitation: Restoring body awareness for neuropathic pain relief
- 2D and 3D virtual reality for neuropathic pain relief: The Power of Illusion for Neuropathic Pain