An exploration of how acupuncture to achieve analgesia, an ancient modality, is used to treat a variety of pain levels in animal patients.
Acupuncture is defined as the stimulation of specific predetermined points near the surface of the human or animal body. This stimulation produces therapeutic effects by evoking homeostatic mechanisms within the nervous, endocrine, cardiovascular and other body systems, and in turn, promoting self-healing.1 The practice of acupuncture stimulates the endings of small nerves and other tissues around the points, leading to both local and systemic effects. This article explores how acupuncture achieves analgesia, which occurs on many levels,2 in veterinary patients.
Mechanisms of action
Current knowledge of the mechanisms of action in acupuncture pain control involves a number of areas, both local and global.
- Pain perception may be altered locally. The response to a painful stimulus may be dampened, secondary muscle spasms and tissue drag can be reduced, and neurochemical alteration of inflammation can occur throughout the body.3 Electroacupuncture or EA (see sidebar) has been shown to be superior to dry needling for achieving analgesia when Functional MRI (fMRI) is used to measure the brain’s response to a painful stimulus. Because of this, EA is more often used for analgesia in research studies than dry needling.3,4
- Pain relief at the level of the spinal cord has been explained using the Gate Control Theory.5 This theory is based on the stimulation of A-beta and A-delta fibers. These fibers compete with the pain transmission of the slower-firing C fibers at the level of the spinal cord. Acupuncture-stimulated A-beta and A-delta fibers fire onto the receptors of neurologic circuits at the local spinal cord level and block the reception from C fibers, thereby closing the “gate” to further pain transmission because these synapses are occupied. This is a proposed mechanism for the local inhibition of pain transmission and is fast-acting and of short-duration.5
- The effects of acupuncture occur at more than just the local spinal cord level. Impulses from both A-delta and C fibers synapse in the dorsal horn gray matter of the spinal cord interneurons, which leads to the release of inhibitory neurotransmitters that reduce pain transmission both up and down the spinal cord. Stimulation of A-delta fibers leads to activation of enkephalinergic interneurons that inhibit the pain response by blocking C fiber transmission. Acupuncture also decreases the release of neurotransmitters that increase the pain response at the level of the spinal cord.
- Central pain sensitization in the brain from peripheral nerve damage or tissue injury can be an important source of chronic pain. Patients that experience central sensitization have been shown to have pain receptors with increased activity in the spinal cord. In mice and rats, acupuncture has been demonstrated to reduce the pain response in these receptors, showing that acupuncture can be effective for pain control even after central sensitization has occurred.7
- Acupuncture can have effects on tendons, ligaments, muscles and other connective tissues. Relief from painful muscle spasms can be achieved by inserting a needle directly into the muscle. Reducing primary muscle pain and spasm with acupuncture also serves to reduce nerve pain secondary to tissue drag from the spastic muscle. It is especially effective when the acupuncture point used is close to where the tendon attaches to a muscle.8
- Increases in sympathetic nervous system tone have been demonstrated in patients with chronic pain.9 Vasoconstriction is a by-product of this increased sympathetic tone, which leads to a variety of problems including hyperesthesia, allodynia, burning and aching.These issues are not related to inflammation and are resistant to treatment with anti-inflammatory medication.9 In horses, it has been demonstrated via thermography that EA increases blood flow to muscles with reduced blood circulation.10 Research in rats has shown that blood flow to the rear limb is increased when acupuncture is performed at the level of the nerve root next to the spine, instead of at a location further down the limb.10
- Patients experiencing pain often have secondary anxiety as well. Studies using fMRI have shown that acupuncture stimulates calming neurochemistry in painful as well as depressed patients.11
- Acupuncture in humans has been shown to be effective for the treatment of back pain when compared to sham treatment (stimulation of points on the body not known to be acupuncture points). Analgesia was greater when patients received EA at the nerve root compared to dry needle acupuncture in the same locations.3
- Acupuncture in humans for osteoarthritis of the stifle and hip was shown to be effective over three months of treatment, but the cost often exceeded routine medical treatment because of the need for multiple sessions. Acupuncture treatment has been shown to be superior to sham treatment for stifle arthritis.12
- Pain relief for cancer patients is an important part of therapy. Traditionally, opioids have been used for the relief of cancer pain. The side effects of opioids in humans include depressed respiratory function, decreased appetite, constipation and addiction. The use of EA for nerve pain secondary to experimentally-induced tumors in mice was shown to be beneficial at reducing this cancer-related pain.13
There have been a few quality studies assessing the effectiveness of acupuncture for pain relief in animals. It has been shown to be effective in the treatment of lameness, back pain, thoracolumbar pain and hoof pain in horses.
The acupuncture point prescription
Acupuncture point selection is important for achieving a positive treatment outcome. Selecting points that stimulate both A-delta and A-beta receptors in the dorsal horn of the spinal segment of the painful area will maximally compete with the C fiber pain, thus inhibiting it.16The needle should be inserted as close as possible to the source of pain — “Put the needle where it hurts.”
From a neurological perspective, formulating an acupuncture point prescription is based on a physical and neurologic examination of the patient. Points are chosen close to the primary area of discomfort and to the spine, in order to more efficiently affect the transmission of pain at the level of the spinal cord, as well as distal to the primary area of concern. These distal points often lie close to nerve bundles that can provide pain relief in a powerful manner.5
Acupuncture has a broad range of effects on the neurologic system which have been widely studied and documented. The specific effects of acupuncture on the neurologic system should be taken into account when deciding if it will benefit a particular patient, along with which points and method would best suit that patient.
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