Skeletal muscle pain is a common, usually temporary, problem in humans and is often associated with exercise or trauma. Severe or chronic muscle pain is poorly understood and generally not recognised by standard physical examination and investigative procedures in both human and veterinary patients. The basics of myofascial pain and clinical applications of myofascial trigger point (MTrP) therapy will be discussed.

Myofascial Pain

The pain typically associated with skeletal muscle and its fascia (myofascial pain) is deep, dull and aching, although some variation exists. The intensity of the pain may vary from low grade to severe and incapacitating. Skeletal muscle pain is difficult to localise as it is typically a referred pain, and may be difficult to differentiate from pain emanating from tendon, joint capsule, or bone. Human patients with muscle pain also present with complaints of weakness or fatigue of the affected muscles. Subtle alterations of gait due to a primary muscular weakness or fatigue can be observed in veterinary patients.

No standard laboratory test or imaging technique exists at this point for the diagnosis of myofascial pain syndromes (MPS) due to MTrPs. MTrPs are only diagnosed from specific historical data and by skilled palpation. Frank (1998) has published the MTrP diagnostic criteria for the dog, but these are equally applicable to any species. Inter-rater reliability studies have recently led to the establishment of some guidelines for diagnostic criteria for MTrPs. Studies have also found that reliable diagnostic examination for MTrPs requires both training in the diagnostic criteria and clinical experience. Training in palpation and observation skills are particularly important. Absent/poor teaching of MTrP diagnostic criteria and lack of palpation skills are suggested to be important in the failure to correctly diagnose these syndromes. Failure to identify a pain focus, on standard physical exam and conventional testing methods, is suggestive that MTrPs need to be ruled in/out as the source of pain.

Lameness in animals is accepted by veterinarians as being caused by pain. Lameness and chronic pain in animals can occur with no identifiable abnormality, or diagnosis, and may persist after all signs of tissue injury have disappeared. Muscle pain of unknown origin is frequently diagnosed in patients, in which no distinctive abnormality of muscle can be found. MPSs, due to MTrPs, are a very common but poorly recognised and treated, cause of musculoskeletal pain. Travell and Simons have suggested that myofascial pain and dysfunction syndromes should be grouped as a family of related syndromes, with similar terminology, rather than the confused approach which currently exists in pain syndromes of 'unknown' aetiology. Although poorly acknowledged in the mainstream veterinary literature, trained veterinary acupuncturists recognise and treat MTrPs as a common clinical problem in animals with musculoskeletal pain. However, additional training in MTrP therapy is beneficial to improving the results gained by acupuncture alone.

Effectively, MTrPs are a non-diagnosis for the majority of medical practitioners by virtue of education (they have not been taught about them) or paradigm (they choose to ignore them). However, chronic pain is a question of patients not of paradigms. The failure to consider MTrPs as a differential diagnosis results in the failure to confirm MTrPs as the diagnosis, and so failure to identify a potent source of pain. The question becomes how many mystery lameness, arthritis, postoperative, older patients etc. really have undiagnosed MPSs due to untrained or poorly trained clinicians? How often do patients start off with undiagnosed MPSs and get lost in the existing medical paradigm i.e., has multiple unnecessary medical and surgical interventions? Unfortunately, the current author has found the answers to these questions to be "many".

Active and Latent Trigger Points

Myofascial trigger points are classified clinically as active or latent. Active MTrPs cause pain either at rest, or with motion that stretches or loads that muscle. Palpation of an active MTrP is always painful, results in referred pain on compression (reproduces the patient's pain) and a mechanical stimulation elicits a local twitch response. Active MTrPs also cause shortening, weakness and stiffness of the affected muscle, and may initiate referred tenderness and autonomic phenomena in the pain reference zone. The irritability of active MTrPs and the muscular load required to induce the pain are very variable.

Satellite MTrPs may develop in the muscles of the pain reference zone of an active MTrP. Secondary MTrPs may develop in antagonists and synergists of the muscle with an active MTrP. Antagonists directly oppose the action of the agonist muscle, while synergists assist the agonist with the desired movement.

Latent MTrPs are not responsible for clinical pain syndromes, although they are locally tender on palpation. Latent MTrPs can have all of the remaining clinical characteristics of active MTrP, e.g., weakness, stiffness and restriction of movement.

MTrPs are activated either directly or indirectly. Direct activation occurs via microtrauma such as in acute overload, overwork fatigue and chilling or gross trauma of the affected muscle. Indirect activation of MTrP may occur as a result of other MTrPs, visceral disease, arthritic joints and emotional distress.

History

Human patients presenting with MPSs typically fall into two categories. Patients with an abrupt onset give precise detail of the initiating event and movement which caused the pain. Individuals describing a gradual onset, when questioned, often have a history of chronically overloaded muscles.

Dogs with a primary diagnosis of a MPS also fit into these categories. Owners often describe an incident in detail that coincides with the start of pain/lameness. A subset of dogs, in this category, has chronically overused muscles which present with an acutely painful condition. In these dogs, MTrP activation is typically associated with a movement that overloads (sometimes with very little effort) the affected muscle. Owners of these animals often remark, once the animal is improving, that in hindsight a gradual decline in function had occurred which they had not noticed or had put down to advancing age.

Dogs with a gradually worsening condition often have a particular type of activity in which they repeatedly engage, e.g., ball chasing, running in circles, leash pulling, running along or jumping up at a fence. Careful questioning of most owners will reveal activities which specifically relate to the affected muscle/s that must be eliminated or modified. Failure to do so results in treatment failure or rapid relapse of the clinical condition. Acupuncture failure, or the requirement for frequent "top ups", is due in part to failure to identify these and other perpetuating factors (see below).

Acute and Chronic Syndromes

Acute activation of a single muscle myofascial pain syndrome is generally easily treated when recognised quickly. Incorrect diagnosis in the presence of perpetuating factors (which compromise the affected muscle) may lead to a progressive, complex, multi-muscle chronic pain syndrome.

Animals presenting with acute injuries, or post-operative healing, which do not resolve in the expected time frame should be routinely examined for MTrPs. Owners of these animals should be advised to present the animal for re-examination if he/she does not return to full function within the estimated time for healing to occur. Myofascial pain syndromes which are correctly treated in the early stages after initiation, or which are only causing stiffness and mild dysfunction, are generally easily treated.

Myofascial pain syndromes are activated by one set of factors and perpetuated by completely different factors. Perpetuating factors (see below) often result in further activation of other MTrPs and the development of complex, multi-muscle chronic pain syndromes. Thus, a localised pain syndrome may develop into a regional, or sometimes generalised, pain syndrome depending on the number and severity of the perpetuating factors present. Patients with long term chronic myofascial pain can present with an extremely complex combination of MTrPs and perpetuating factors to unravel. Such cases, particularly where myofascial pain is secondary to other conditions, require considerable clinical experience.

Travell has identified three phases of patient presentation for chronic myofascial pain syndromes in humans. Phase 1 patients are in constant pain due to very irritable active MTrPs. Patients are unable to identify movements or activities which exacerbate the pain due to the existing intensity of the pain. Phase 2 produces pain with motion or effort, these MTrPs are less irritable, are worse with specific movements and pain free days may occur. A lag time of up to 24 hours between movement and the initiation of pain can make identification of the inciting action difficult. Phase 3 patients have latent MTrPs which are responsible for referred tenderness, stiffness and restricted range of motion, but no pain. Animals similarly present with these 3 phases of chronic myofascial pain.

Myofascial trigger point pain can be augmented or decreased by a number of factors. Augmentation occurs with pressure, cold, stretching, keeping the muscle in a shortened position for a period of time and strenuous or repeated use of the affected muscle. Pain may be decreased by a short rest, slow passive stretching, moist heat over the MTrP, short periods of movement and specific myofascial therapy. Long term resolution requires specific MTrP therapy.

Initiating and Perpetuating Factors

Myofascial pain syndromes are typically initiated by trauma, inflammatory, spinal or discogenic diseases. However, these syndromes may also be caused by repetitive minor trauma or chronic muscle tension due to poor posture, occupational overload or emotional stress.

The perpetuating factors which prolong, and complicate, myofascial pain syndromes include: mechanical stresses (structural asymmetries), nutritional inadequacies (vitamin and mineral insufficiencies), metabolic and endocrine inadequacies (thyroid insufficiency, hypoglycaemia and anaemia), chronic infection or infestation (bacterial, viral and parasitic) and emotional stress. Mechanical stresses serve to overload the affected muscles, while the systemic perpetuating factors may not only perpetuate MTrPs in the affected muscle, but also make all muscles more irritable and susceptible to MTrP formation.

Treatment

Modes of therapy used for myofascial pain syndromes involve inactivation of the MTrP followed by stretching of the affected muscle. Stretching of the muscle is important in order to restore full range of movement and normal muscle strength and function. A number of methods are commonly used to inactivate myofascial trigger points including: trigger point pressure release, dry needling with an acupuncture needle, vapocoolant spray, transcutaneous electrical nerve stimulation (TENS), injection with 0.5% procaine and ultrasound.

Identification and elimination of perpetuating factors are important to the resolution of myofascial pain syndromes and prevention of their recurrence. In conjunction with MTrP inactivation, owners are given instructions on the use of heat and massage on the affected area and specific muscle stretches. Client education provides the owner with the means to control secondary MTrPs with chronic underlying conditions.

Exercise is carefully controlled in order to maintain the animal in the pain free range at the start of treatment. The initial level of exercise is dependent on the severity of the pain and often the duration of the problem. Absolute rest is not recommended. Dogs which present with a long history of pain often have poor levels of fitness, and multiple muscle involvement, sometimes necessitating prolonged treatment protocols. Any animal which does not respond completely after 3-4 treatments, or in which the pain is severely exacerbated by the treatment, is reassessed for underlying perpetuating factors. Once a consistent improvement is evident, the animal is started on a graded programme of exercise to increase strength and fitness. The final level of fitness is usually determined by the amount of time that the owner wants to spend exercising the animal. Arthritic animals are stabilised at a constant daily level of exercise which is within their functional limits. This level will vary with the individual animal.

References

1.  Baldry PE. Acupuncture, Trigger Points and Musculoskeletal Pain second ed. Churchill Livingston, London, 1993.

2.  Frank EM. Myofascial Trigger Point Diagnostic Criteria in the Dog. In Muscle Pain, Myofascial Pain, and Fibromyalgia: Recent Advances. L. Vecchiet and M.A. Giamberardino eds. The Haworth Press, N.Y. 1999: 231-237.

3.  Hong Chang-Zern. Considerations and Recommendations Regarding Myofascial Trigger Point Injection. J Musculoskel Pain, 2, 1, 1994, 29-59.

4.  McNulty H, Gervitz Richard N. et al. Needle electromyographic evaluation of trigger point response to a psychological stressor. Psychophysiology 31, 1994, 313-316.

5.  Mills KR, Newman DJ, Edwards RHT. Muscle Pain. In Textbook of Pain second ed. PD Wall and R. Melzack eds. Churchill Livingstone, London. 1989.

6.  Simons DG. Myofascial Pain Syndromes Due To Trigger Points. In Rehabilitation Medicine. J Goodgold ed. C.V. Mosby Co., St Louis. 1988: 386-723.

7.  Simons DG, Travell JG. Myofascial Pain Syndromes. In Textbook of Pain second ed. PD Wall and R. Melzack eds. Churchill Livingstone, London. 1989: 368-385.

8.  Simons DG. Clinical and Etiological Update of Myofascial Pain from Trigger Points. J Musculoskel Pain 4, 1/2, 1996, 97-125.

9.  Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual Vols.1&2. Williams and Wilkins, Baltimore 1983 and 1992.

10. Travell JG. Chronic Myofascial Pain Syndromes: Mysteries of the History. J Man Med, 6, 1991, 46.