Muscle as pain generator: Myofascial trigger points, recognition, and treatment (Proceedings)
It is not possible to achieve designed goals of physical rehabilitation in the canine patient without adequate pain management. Often in these patients, pain is not only generated from the original injury or trauma of surgery but also additionally from the functional impairment(s) brought about by the primary issues. Changes in weight distribution or posture can overload compensating portions of the body. It is this overload, specifically of muscles, that may result in the development of myofascial pain in areas of the body other than the primary injury. Recognition and management of secondary problems become critical in the achievement of desired outcomes.
Examination for pain is separate but related to the physical, orthopedic and neurologic examinations. While the orthopedic examination mainly focuses on joints the pain examination incorporates this as well as what is between the joints, the myofascial tissues.
With articular dysfunction comes myofascial dysfunction and vice versa. Localization of pain in the myofascial tissues is followed by examination of the joint(s) that those painful muscles exert function. Myofascial pain can also be found in muscles that can become overloaded due to an injury and/or functional impairment. Examples of this the development of myofascial pain in the limb opposite the initial injured limb and myofascial pain in the muscles of the forelimbs seen with hind limb problems. Changes in body posture as the result of injury may result in the development of myofascial pain. Myofascial pain can also develop within muscles innervated by injured peripheral, spinal nerves and spinal cord segments.In many patients the primary source of pain and dysfunction is known, therefore examination can focus on groups of muscles that would be suspect to be affected. Therapy plan can then include treatment directed toward both primary and secondary issues. In so doing restoration of normal function and the full benefit maybe appreciated.
Theories still exist as to the development of myofascial pain due to myofascial trigger points. Gerwin, Dommerholt and Shah expanded one of the original theories of motor endplate dysfunction described by Simons. Their conclusions were that in injured muscle there is release of substances that activate muscle nociceptors and cause pain and facilitate the release of acetylcholine, inhibits it breakdown and up-regulate acetylcholine receptors at the motor endplate. This loss of equilibrium results in a persistent muscle fiber contraction, as is characteristic of the myofascial trigger point (MTrP).
Muscle injury in the rehabilitation patient can possibly occur in one of several ways; 1) Low-level muscle exertions of muscle groups leading to muscle overuse. This can be brought about by adaptations in posture or protection of a painful joint, 2) overload of a muscle groups usually due to increase weight bearing possibly related to eccentric contractions.
In overuse due to low level muscle contractions, the Cinderella Hypothesis could explain the development of myofascial pain and MTrPs in dogs. The Cinderella hypothesis developed by Hagg in 1988 postulates that this type myofascial pain is caused by selective overloading of the earliest recruited and last de-recruited motor units. This results in metabolically overloaded motor units with subsequent activation of autogenic destructive processes and muscle pain.
Myofascial pain in muscle overload maybe the result of maximal eccentric exercise or eccentric exercise in unconditioned muscle. Eccentric contractions often occur during ambulation in a limb(s) that is compensating for decrease or non-weight bearing of another limb.