Stimulation of the spinal cord is effective in treating secondary hyperalgesia and is worthy of further study, researchers reported here at the American Academy of Pain Medicine 26th Annual Meeting.

Secondary hyperalgesia is brought about by damage to nocireceptors and peripheral nerves and is a component of many chronic pain disorders. It is thought to be related to central nervous system sensitization and hyperexcitability, although its etiology is unknown.

The study involved 15 patients with functioning implanted spinal cord stimulators and stable pain conditions of various causes. They were randomized to two visits in a crossover design. The device was turned off for four hours prior to one visit, and turned on for four hours prior to the other. At each visit, a 2 X 4 cm probe was placed on the thigh and heated to 45 C for five minutes. Investigators then measured the area of secondary hyperalgesia around the thermal application.

Investigators observed a significant reduction in the area of pain from secondary hyperalgesia in patients undergoing stimulation. The size was reduced from 58.9 to 37.6 cm² on average (P < .05), but this varied widely from patient to patient, Tyson Riesenberg, MD, a fellow in pain management in the Department of Anesthesiology at the University of California at San Diego, told meeting attendees.

"We found that spinal cord stimulation significantly reduced experimentally induced hyperalgesia, which we feel supports the mechanism of descending inhibitory control on the dorsal horn cells," Dr. Riesenberg said in an interview with Medscape Neurology.

The study was small and unblinded but was praised by meeting organizers as one of the few to explore hyperalgesia in humans.

"The stimulation study is one of the first studies that tries to show the hyperalgesia issue in humans," Jeffrey Tiede, MD, a pain management physician from Jacksonville, Florida, and a member of the meeting's poster review committee, told Medscape Neurology after Dr. Riesenberg's presentation. "We see a lot of it in rat models."

Dr. Tiede said blinded studies and studies that stimulated one leg while leaving the other unstimulated would be a good next step.

Hyperalgesia is a condition of a lowered threshold for pain and an increased response to stimuli that cross that threshold. Primary hyperalgesia occurs in the area of an injury, and secondary hyperalgesia occurs in uninjured tissue around it.

It is believed that primary hyperalgesia is brought about by a combination of peripheral and central mechanisms; secondary hyperalgesia results from central sensitization and hyperexcitability.

Dr. Riesenberg's study was conducted with no outside funding, drawing the praise of meeting coordinators.

He said he's working on a grant now to study whether there is a way to predict whether potential study subjects will respond to spinal cord stimulation during a trial.

"Implanting permanent devices is in excess of $30,000, and being able to reduce this cost in someone who probably won't be a responder would be a great advantage," he said.

Future work could include the effect of spinal cord stimulation on primary hyperalgesia intensity, examining the association between parasthesia coverage and secondary hyperalgesia and exploring the cutaneous changes that help with secondary hyperalgesia.

American Academy of Pain Medicine (AAPM) 26th Annual Meeting: Abstract 143. Presented February 4, 2010.

Thomas R. Collins • Medscape Medical News © 2010 Medscape, LLC