Muscles can be a source of pain
Everyone, at a certain point of their life, has felt pain in their muscles, and some have muscle pain every day. Muscles are indeed pain-sensitive structures. Nerve endings that can encode noxious stimuli, called "nociceptors", are present in muscles. Substances that are released in the muscles following an injury or inflammation activate the nociceptors and induce pain.
However, pain at a muscle does not necessarily mean pain from the muscle
For instance, experiments with healthy pain-free subjects have shown that stimulation of areas of the spine, such as joints, can induce pain at muscles. This so-called "referred muscle pain" is likely caused by processes within the spinal cord and the brain that transform a pain signal arising from joints into pain perceived at the muscles.
So, where does muscle pain come from?
Let's consider a patient with pain at muscles of the neck. In this individual, the most likely origin of the pain is either: 1) the muscles; or 2) the spine (such joints, discs, or ligaments); or 3) both the muscle and the spine. More rarely, injury to nerves or other structures of the neck or shoulder can cause muscle neck pain.
Diagnostic challenges
If muscle pain can orginate from so many different parts of the body, and not necessarily from the muscle itself, how can we provide a diagnosis and determine the right treatment? When we perform physical examination, we typically palpate muscles. If muscle tenderness is elicited, we may argue that the muscle is a source of pain. Therefore, treatments at those muscles should provide pain relief. However, several studies have demonstrated that injuries of areas other than muscles, such as joints of the spine, can cause muscle tenderness to palpation.
Because injuries of different body structures can cause similar patterns of muscle pain and tenderness, a valid diagnosis cannot rely on the location of pain and tenderness. What about MRI and other imaging? Unfortunately, there is no imaging technique that can confirm or rule out the muscle as primary origin of the pain. Promising research has used advanced imaging such as MRI, ultrasound and PET, but the relationship between imaging findings and pain remains to be elucidated.
Because of our limited understanding of muscle pain, the diagnosis is frequently no more than a working hypothesis that is based on the combined evaluation of the patient's history, physical examination, and imaging results. Since different clinicians may interpret the multiple findings differently, patients frequently receive contradictory diagnoses.
The treatment of muscle pain is frequently based on trial-and-error
The lack of criteria for a valid diagnosis is the result of insufficient research in muscle pain, and limits our ability to identify treatments that address the primary cause of the pain. Nevertheless, there are several therapeutic options for muscle pain, and a list is outside the scope of this article. Because our diagnosis is frequently hypothesis-based, we have a limited ability to predict which treatment will work in which patients.
How to explain muscle pain to patients?
One of the major reasons for patients to seek medical care is to have an explanation for their pain. This is particularly true for patients with chronic pain. Our inability to provide firm explanations is frequently disappointing for patients. Many of them cannot believe that in the 21st century we are unable to explain a "simple" muscle pain.
Patients are mostly comfortable with health care professionals who provide firm explanations. Regrettably, doctor's self-confidence is not a guarantee for a correct diagnosis. In addition, claiming to "know" where the pain comes from can create wrong expectations to patients and cause mistrust if the therapeutic plan is not successful.
I prefer to proceed this way:
Openly discuss with patients the diagnostic challenges and explain that current scientific knowledge is limited;
Present potential causes of their muscle pain;
Reassure patients that there are many treatment options that have been successful in many patients with muscle pain, although we cannot predict which ones will eventually work;
Present a step-wise plan, starting with treatments that match the most likely diagnosis and are least invasive;
Whatever the treatment, stress the importance of physical activity, mental health, and life style for the outcome the proposed treatments.
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