Myofascial Pain

Dr. John C. Lowe

We can define the term "myofascial pain" in at least two ways. First, it may be pain that actually arises from skeletal muscle and/or its connective tissues. Second, it may be pain that we only perceive arises from these tissues.

Pain that Actually Arises from Myofascial Tissue

When myofascial pain actually arises from a myofascial tissue, the tissue is either damaged or there is a threat of damage. A myofascial trigger point is a good example of threatened or real damage to myofasicial tissue. At a trigger point, an excess of the transmitter substance acetylcholine induces a contraction of muscle fibers.

When the contraction increases the pressure inside the muscle beyond some threshold, local arteries, veins, and lymph channels are compressed. Compression of veins will reduce the elimination of metabolic wastes from the tissue region.

If the wastes accumulate, their high concentration will lower the firing threshold of local nerves (specifically, type C and A delta fibers). The receptors of the nerves respond to pressure. The increased pressure, caused by the contraction, within the tissue activates the nerves. The resulting increased nerve flow into the spinal cord and brain are termed "nociceptive" because humans perceive it as unpleasant or painful.

The increased nerve flow activates two types of nerves cells within the spinal cord that can intensify the adverse conditions of the local tissue. The signals from the tissue activate sympathetic nerves, and an increased flow of signals through these nerves to the tissues induces contractions of arterioles. When the arterioles contract, arterial blood flow to the local tissues further decreases.

Also, the signals from the tissue activate alpha motor nerves. Signals passing through these nerves back to the involved myofascial tissue increase the contraction of muscle. The increased contraction heightens local muscle pressure. The increased pressure further reduces blood flow through arteries and veins, intensifying the adverse process even further.

If muscle cells are destroyed by this process, muscle proteins are replaced with an accumulation of connective tissue. We then say that the tissue is "dystrophic." The perception of pain in this case arises first from changes in the myofascia that threaten damage, and second, from actual damage of the tissue.

Pain that is Mainly Perceived to Arise from
Myofascial and Other Peripheral Tissues

The best example of an individual who only mainly perceives that her pain arises from peripheral tissues is the patient with severe fibromyalgia. Fibromyalgia patients have an increased perception of pain. They appear to do so because of two biochemical changes in the central nervous system: (1) an increase of substance P, and (2) a decrease of norepinephrine. (Studies have failed to show the serotonin deficiency in the central nervous system that some researchers, such as I. Jon Russell, hypothesized. So, the available evidence doesn’t support the related hypothesis that fibromyalgia patients’ increased pain perception is underlain by a low central nervous system serotonin deficiency. For practical purposes, the serotonin deficiency hypothesis of fibromyalgia is falsified.) The increased substance P and low norepinephrine levels in fibromyalgia can be induced by too little thyroid hormone regulation of central nervous system cells.

The increased substance P augments the flow of nociceptive nerve signals into the dorsal horns of the spinal cord. At the same time, low secretion of norepinephrine into the dorsal horns (by the endings of descending inhibitory nerves) results in a failure to properly inhibit incoming nociceptive nerve signals. As a result, an excess of nerve signals from myofascial and other peripheral tissues enter the spinal cord and ascend to the brain. The excess signals give rise to the perception of pain. It is important to emphasize that the increased pain perception in fibromyaglia doesn’t result from damage or the threat of damage to peripheral tissues. Instead, the patient perceives pain because her central nervous system pain modulating system is impaired.