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Muscle relaxants for chronic pain

Michele Curatolo
 

Muscle relaxants are largely used to manage chronic pain. Here's how muscle relaxants can help.


Reduction of muscle spasms

Muscle spasms can result from a loop "Painful injury --> Increased activity of pain pathways in the spinal cord and brain --> Activation of motor nerves that supply muscles --> Muscle spasms --> Pain". Muscle relaxants can reduce this loop. For instance, tizanidine produces muscle relaxation by acting on the alpha-2 adrenoceptors in the spinal cord and brain. By binding on these receptors, the medication reduces the flow of signals sent to the muscles, thereby producing muscle relaxation.


Decreased muscle tone

Pain conditions can involve increased muscle tone and stiffness. Muscle relaxants like baclofen, which acts on GABA-B receptors, help decrease muscle tone, providing relief from stiffness and associated pain. This reduction in muscle tone can make daily activities more manageable and less painful.


Improved mobility and function

By alleviating muscle spasms and reducing muscle tone, muscle relaxants may improve overall mobility and function. This can lead to better physical activity levels, which is crucial for managing chronic pain. Improved mobility not only helps in pain management but also enhances overall well-being and quality of life.


However, it is not so simple

  • The proper use of muscle relaxants relies on the detection of muscle tension. However, there are no validated criteria to rule in or out muscle tension. When different examiners evaluate if a patient has muscle tension or not, they can come to different conclusions (poor "inter-rater reliability").

  • Physical examination poorly predicts the efficacy of muscle relaxants, meaning that detecting muscle tension or spasms is poorly associated with efficacy of muscle relaxants.

  • Identifying a tender or tense spot, per se, does not tell us whether that body part is causing the pain. If you are confused and want to know more, read here.

  • Muscle spasms may be secondary to other conditions, such as joint pathology, which are not addressed by muscle relaxants. It is difficult or even impossible to determine whether muscles are the true source of pain, or joints (or other body parts) are primarily responsible for the pain perceived in the muscle. The latter is called "referred pain" meaning that the primary pain signal does not stem from the body part that hurts (in this case, the muscle), but from somewhere else (e.g., a joint). Learn more about referred pain here.

  • A recent systematic review found that while muscle relaxants are effective for neck pain and painful cramps, they are not more beneficial than a placebo for fibromyalgia, low back pain, and headaches. However, caution is needed when interpreting these results. The studies included in the review varied significantly in terms of patient populations, specific muscle relaxants used, duration of treatment, and other factors, which limits the strength of the conclusions.

  • Muscle relaxants can cause side effects, mostly sedation and dry mouth.


Conclusions

Muscle relaxants help manage chronic pain in some patients by reducing muscle spasms and decreasing muscle tone. However, their effectiveness is hard to predict due to the lack of clear criteria for detecting muscle tension and the possibility of referred pain from other body sites. A recent review found they are effective for neck pain and cramps but not for fibromyalgia, low back pain, and headaches. Common side effects include sedation and dry mouth.

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