Natural pain-inhibiting mechanism may worsen due to neuropathic pain


Lay summary by Arman Mohseni

Edited by Rona Herzog, Crystal Han

This is a lay summary of the original research paper by Dr. John Kramer, Dr. Martin Gagné, Dr. Isabelle Côté, Dr. Mélanie Boulet, Dr. Catherine Jutzeler, Dr. Catherine Mercier. Find the original research paper here.

Neuropathic pain is defined as pain in regions of the body which are supplied by neurons which have been damaged as a result of injury. This pain can be found in regions which may have a complete loss of feeling, or regions with a partial loss of sensation. To read more about neuropathic pain, see this web illustration.

Neuropathic pain is a common problem following spinal cord injury, but the causes are not fully understood. One possible explanation is the poor functioning pain-inhibiting mechanisms in the body, which may result in exacerbation of the neuropathic pain.

What was the purpose of the study?

The purpose of this study was to determine the relationship between neuropathic pain and natural pain-inhibiting mechanisms. The mechanisms are measured by a procedure known as conditioned pain modulation (CPM). This was achieved by following participants with recent spinal cord injuries for roughly three months over the course of their rehabilitation. During this longitudinal study, researchers measured the natural pain-inhibiting mechanisms in participants who experienced neuropathic pain and those who did not, and compared the two over time.

How was the study done?

35 participants were involved in the study, all with SCI. They did not have a history of neuropathic pain before their SCI, but some did report neuropathic pain after spinal cord injury. The effectiveness of the natural pain inhibiting mechanisms was measured using conditioned pain modulation.

  • Conditioned pain modulation follows the principle that “pain should inhibit pain.” This means that an individual should perceive a painful stimulus as being of lower intensity when another painful stimulus is acting on a distant location of their body.
  • Initially, a temperature-controlled probe was placed on the arm, and a temperature causing a moderate pain is applied for 2 minutes. The participant is asked to rate the perceived pain. This is called the test pain stimulus.
  • Afterwards, the participant was asked to place their hand in extremely cold water, to induce pain. This is called the conditioning stimulus, and should trigger natural pain-inhibiting mechanisms.
  • Then the test stimulus is applied again, and the participant is asked to rate the percieved pain again. In participants with normally-functioning natural pain-inhibition mechanisms, the test stimulus should be perceived as less painful to them after having experienced the conditioning stimulus (cold water).
  • The conditioned pain modulation of participants was measured over the course of rehabilitation to see if their natural pain-inhibition mechanisms improved or worsened.

What were the results of the study?

The study found that those participants who were admitted with neuropathic pain had a decrease in conditioned pain modulation over time, while those who did not have neuropathic pain stayed the same. This means that following rehabilitation, the participants with prior neuropathic pain reported a decrease in pain sensation to the cold water following the heated patch.

This finding is important for two reasons. Firstly, it overturns the common idea that poor conditioned pain modulation results in the worsening of neuropathic pain, by providing evidence in favour of the reverse. Secondly, the finding will inform future research on therapeutics aimed at reducing neuropathic pain, which greatly affects quality of life for many spinal cord injury patients

Key Terms:

  • Conditioned Pain Modulation (CPM): A procedure for measuring the strength of natural pain-inhibiting mechanisms
  • Neuropathic pain: Pain that results from injury or disease that damages the nervous system
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