Targeting the dorsal root ganglia with local anesthetic may suppress phantom limb pain among amputees, according to results of a recently published study.
“Our working hypothesis was that in amputees the impulses felt by a conscious brain as pain in the missing limb are coming from the peripheral nervous system,” Marshall Devor, professor at the Institute of Life Sciences and the Center for Research on Pain of the Hebrew University of Jerusalem, Israel, told O&P Business News. “That contrasts with the working hypothesis of most people in the field today that the impulses are actually coming from the cerebral cortex. We have no direct way of stopping the impulses in the brain, but it can be done focally for the peripheral nerve.”
Blocking phantom limb pain
With the intention to block access of the nerve impulse discharges originating in the peripheral nervous system from reaching the brain, researchers used lidocaine to treat 16 lower limb amputees with ongoing phantom limb pain by blocking abnormal afferent input by epidural intraforaminal injection, spinal block or by performing local infiltration of stump neuromas or peripheral nerve block. In a fourth group, 15 amputees were treated with a modified protocol of intraforaminal injection.
Researchers found that spinal and intraforaminal block weakened and eliminated phantom limb pain as well as non-painful phantom limb sensations. Blocking solutions also blocked the sensations in lower limb amputees. However, when patients were told to anticipate a pain-relieving injection, researchers found sham injections and intraforaminal injections of non-blocking solutions did not block phantom limb pain. While the effects after spinal block faded within hours, the effects lingered after intraforaminal block. The researchers believe this is a result of anti-inflammatory and membrane-stabilizing effects of the co-injected corticosteroids. Overall, the researchers conclude that neither the dorsal horn of the spinal cord nor the brain are primary generators of phantom limb pain.
Extended pain relief
“Our study, together with earlier work, highlights the dorsal root ganglia as a critical source of ectopic impulse discharge in amputees with phantom limb pain,” the researchers concluded. “Because a low concentration of lidocaine (and other membrane stabilizers) is sufficient, and undoubtedly less toxic than 1% to 2% lidocaine, current implantable pump systems might provide extended pain relief using a single reservoir charge and a slow pumping rate. Novel anesthetic modalities that are selective to small-diameter afferents might be a way to attenuate phantom limb pain while preserving the benefits of non-painful phantom limb sensation in the maintenance of body image.”
According to Devor, the scope of the problem of chronic pain for the general public is enormous. While morphine is effective at alleviating some kinds of chronic pain, it is largely ineffective for phantom limb pain and other types of neuropathic pain. It also carries a serious risk of abuse. An alternative, which Devor and colleagues mention in the study, would be to drip low concentrations of local anesthetics, such as lidocaine, onto the surface of the ganglion with an implantable pump. However, it is a complicated and expensive procedure.
He said more research is needed not only on phantom limb pain, but also on other types of chronic pain, such as low back pain and sciatica, to find which of them stem from the peripheral nervous system and what treatment methods work best.
“Not many people die of pain, but a large number of people die in pain and a much larger number of people live in pain,” Devor said. “If we had a better idea of where these pains come from we would have a better idea of how to handle them.” — by Casey Tingle
Disclosure: Marshall Devor has no relevant financial disclosures.