CHRONIC MUSCULOSKELETAL PAIN
Pain originating in the muscles and fascia is very common. Nearly everyone at some point suffers from this type of pain, known as myalgia fasciitis or myofascitis. Chronic myofascial pain, however, involves pain that is chronic, or long lasting and is associated with specific trigger points.
Fibres extending from the trigger point to the muscle attachments shorten and form a tight band. From a tight band, the persistent contraction of muscle fibres compresses blood vessels and decreases their blood supply leading to oxygen starvation and tissue damages. The tissue damages result in the release of inflammatory mediators from damaged cells including ions (K+, H+), bradykinin, histamine, 5‐hydroxytryptamine (5‐HT), ATP and nitric oxide and this irritates nerves and causes inflammation and pain. The pain signal sends back to the CNS, and it induces vasoconstriction and muscle contraction over again and again.
As the diagram is shown below, musculoskeletal pain may fall into a vicious cycle when the initial pain and damage have not properly modulated also recovered. A chronic musculoskeletal pain eventually induces central sensitization, and it makes patient to suffer from various health disorders.
Pain mechanism and chronic pain disorders of central sensitization
Nociceptive pain results from the activation of receptors (nociceptors) sensitive to noxious stimuli (inflammation or disease).
Prolonged or intense exposure to these stimuli, for example, chemical mediators released during inflammation, enhances the responsiveness of nociceptive nerve fibres. This process, termed peripheral sensitization, involves a shift in the activation threshold of nociceptors and upregulation of voltage-gated sodium channels.
Peripheral sensitization leads to increased action potential firing and transmitter release in the dorsal horn of the spinal cord, where somatosensory information is processed. Dorsal horn neurons react to the rising input with heightened excitability; a process termed central sensitization.
Central sensitization has two main characteristics. Both involve a heightened sensitivity to pain and the sensation of touch. They are called allodynia and hyperalgesia.
Allodynia occurs when a person experiences pain with things that usually are not painful. For example, chronic pain patients often experience pain even with things as simple as touch or massage. In such cases, nerves in the area that was touched send signals through the nervous system to the brain. Because the nervous system is in a persistent state of heightened reactivity, the brain doesn't produce a mild sensation of touch as it should, given that the stimulus that initiated it was a simple touch or massage. Instead, the brain produces a feeling of pain and discomfort.
Hyperalgesia occurs when a stimulus that is typically painful is perceived as more painful than it should. An example might be when a simple bump, which ordinarily might be mildly painful, sends the chronic pain patient through the roof with pain. Again, when the nervous system is in a persistent state of high reactivity, it produces pain that is amplified.
Central sensitization can lead to heightened sensitivities across all senses, not just the sense of touch. Chronic pain patients can sometimes report sensitivities to light, sounds and odours. As such, normal levels of light can seem too bright or the perfume aisle in the department store can produce a headache.
Central sensitization s also associated with cognitive deficits, such as poor concentration and poor short-term memory.
Central sensitization also corresponds with increased levels of emotional distress, particularly anxiety. After all, the nervous system is responsible for not only sensations, like pain, but also emotions. When the nervous system is stuck in a persistent state of reactivity, patients are going to be nervous – in other words, anxious. Lastly, central sensitization is also associated with sick role behaviours, such as resting and malaise, and pain behaviour.
Acupuncture treatment for Central Sensitization
Acupuncture is used extensively by patients worldwide for a variety of illnesses and researches show the effectiveness of acupuncture in reducing central sensitization. Acupuncture enhances the descending inhibitory effect and modulates the feeling of pain, thus modifying central sensitization. The possible mechanisms underlying the analgesic effects of acupuncture include segmental inhibition and the activation of the endogenous opioid, adrenergic, 5-hydroxytryptamine, and N-methyl-D-aspartic acid, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate pathways. Moreover, acupuncture can locally reduce the levels of inflammatory mediators. In clinical settings, acupuncture can be used to treat headache, neuropathic pain, low back pain, osteoarthritis, and irritable bowel syndrome. These mechanisms of acupuncture analgesia may be involved in the alleviation of central sensitization.
Scholz, J. (2014). Mechanisms of chronic pain. Molecular Pain, 10(Suppl 1). doi:10.1186/1744-8069-10-s1-o15
McAllister, M. J. (n.d.). What is Central Sensitization? Retrieved from http://www.instituteforchronicpain.org/understanding-chronic-pain/what-is-chronic-pain/central-sensitization
Lai, H., Lin, Y., & Hsieh, C. (2019). Acupuncture-Analgesia-Mediated Alleviation of Central Sensitization. Evidence-Based Complementary and Alternative Medicine, 2019, 1-13. doi:10.1155/2019/6173412
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