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Those suffering from chronic neck pain may be interested to know that researchers have, for the first time, detailed the involvement of the spinal cord in pain hypersensitivity in humans. Spinal cord involvement has long been suspected in conditions such as fibromyalgia and neck and back pain where no other cause can be pinpointed for the pain but imaging the human spinal cord is somewhat problematic.
Animal research has previously demonstrated the link between spinal cord sensitization and chronic pain but this is the first human trial to show such a link using functional magnetic imaging. The study took place at the Pain Management Division, Stanford University, California, and may offer insights into new ways to treat chronic neck pain and other afflictions. Further tests, this time on patients with fibromyalgia, are planned by the same research team.
The Experience of Pain
The spinal cord receives and transmits nerve impulses throughout the body by way of the nerves that branch out through the neural foramina. Some of these impulses travel all the way from the skin on the fingertips up to the brain and others only to to the spinal cord where some nerve impulse feedback loops occur without signals ever reaching the brain for processing. Patients with fibromyalgia or conditions that affect the spinal cord itself may have overly sensitive reactions to ordinary stimuli, meaning that some find it painful even to wear clothing with zippers and/or have hypersensitive reactions to pain (hyperalgesia).
Fibromyalgia and Pain
A variety of mechanisms have been proposed to explain this oversensitivity in fibromyalgia patients, including small fiber polyneuropathy, abnormally high levels of substance P in the spine, as well as abnormally low levels of serotonin, norepinephrine and dopamine. These neurotransmitters are all involved in pain sensitivity. Fibromyalgia sufferers have also been found to have increased levels of excitatory amino acids in the cerebrospinal fluid (CSF) with an association noted between glutamate and nitric oxide metabolites and clinical assessments of pain.
Research into the Spinal Cord and Chronic Pain
Recognizing that something is likely going on in the spinal cord of patients with chronic pain has led researchers to carry out innumerable animal experiments but these, as with all animal research, have little bearing on human pain and disease and may actually prove to be deceptive and misleading in many cases. This latest research carried out tests on human subjects in order to improve our understanding of chronic pain and spinal cord involvement, using resting-state functional magnetic resonance imaging (MRI) on patients with and without induced central sensitization to pain.
The Spinal Cord and Pain Sensitivity
Presenting the findings of the new study, research assistant Brittney R. Reyes, noted that this work highlights the role of the spinal cord in chronic pain syndromes as being as important as that of the brain. The research team used a capsaicin (hot pepper) cream on the forearm of volunteers in order to block nerve signals transmitting pain to the brain. This was done after an initial application of heat to the area for five minutes, followed by measurements of mechanical pain. The cream remained in place for half an hour and then patients had the heat applied again for five minutes before mechanical hyperalgesia was measured once more.
Mapping the Brain in Pain
A second group had heat applied for 30 seconds to their left forearm, then had 40 seconds without heat and then had the process repeated seven times. No capsaicin cream was used for this group. Both groups had two scans performed, the first to map out the brain’s dorsal horn as the volunteers carried out a task and the second as a baseline for a resting state where the participant simply lay inactive in the MRI scanner.
How the CNS Talks to Itself
The purpose of these scans was to spot any signal fluctuations in the spinal cord and potentially isolate any connections between regions that may indicate a functional relationship. Signals and communication in the spinal cord continues at a low frequency when the participant is not performing a task and the researchers hoped to find which areas of the central nervous system were talking to each other and to what extent after administration of pain and when nerve signals were blocked.
Hypersensitivity to Pain and Spinal Cord Abnormalities
What the researchers found was that the subjects who had not been sensitized to pain had signs of functional connectivity in the C6 area of the spinal cord, while those in the group sensitized to pain had a wider spread of activity in the C6 to C5 regions of the dorsal horn. The results were indicative of activity occurring even when subjects reported having no pain, with ramifications for those with hyperalgesia and allodynia.
Fibromyalgia and Chronic Pain
The research team that carried out this study now intends to test those with fibromyalgia to determine if this increased functional connectivity is present in the spinal cord. Whether this will eventually lead to new treatments for fibromyalgia remains to be seen but these researchers are certainly a step closer to understanding the role of the spinal cord in chronic pain.
American Academy of Pain Medicine (AAPM) 29th Annual Meeting. Abstract 107. Presented April 12, 2013.