Evidence for spinal cord hypersensitivity in chronic pain

“This publication from PAIN is an example of increasing findings showing that some people have increased awareness of their musculoskeletal system sensations. This is also described, as here, as “hypersensitivity”, a term I don’t use because it may be prejudicial in tone. There is more data in the fibromyalgia literature that you can access by Internet search. I added emphasis to the last two sentences about conclusions of this research.” Bill Chesnut, MD

 Pain    Volume 107, Issues 1–2, January 2004, Pages 7–15

Evidence for spinal cord hypersensitivity in chronic pain after whiplash injury and in fibromyalgia

Abstract

Patients with chronic pain after whiplash injury and fibromyalgia patients display exaggerated pain after sensory stimulation. Because evident tissue damage is usually lacking, this exaggerated pain perception could be explained by hyperexcitability of the central nervous system. The nociceptive withdrawal reflex (a spinal reflex) may be used to study the excitability state of spinal cord neurons. We tested the hypothesis that patients with chronic whiplash pain and fibromyalgia display facilitated withdrawal reflex and therefore spinal cord hypersensitivity. Three groups were studied: whiplash (n=27), fibromyalgia (n=22) and healthy controls (n=29). Two types of transcutaneous electrical stimulation of the sural nerve were applied: single stimulus and five repeated stimuli at 2 Hz. Electromyography was recorded from the biceps femoris muscle. The main outcome measurement was the minimum current intensity eliciting a spinal reflex (reflex threshold). Reflex thresholds were significantly lower in the whiplash compared with the control group, after both single (P=0.024) and repeated (P=0.035) stimulation. The same was observed for the fibromyalgia group, after both stimulation modalities (P=0.001 and 0.046, respectively). We provide evidence for spinal cord hyperexcitability in patients with chronic pain after whiplash injury and in fibromyalgia patients. This can cause exaggerated pain following low intensity nociceptive or innocuous peripheral stimulation. Spinal hypersensitivity may explain, at least in part, pain in the absence of detectable tissue damage.