Myofascial Trigger Points, Sensitization, and Chronic Musculoskeletal Pain: Evaluation and Management
Chronic musculoskeletal pain is highly prevalent and presents a major burden to individuals and society. Myofascial pain syndrome (MPS) is one of the most common musculoskeletal disorders and encompasses the surrounding fascia and connective tissue. MPS can be either acute or chronic and is highly associated with myofascial trigger points (MTrPs) in the affected muscles. Active (spontaneously painful) MTrPs, which comprise the gold standard for the diagnosis of myofascial pain, are one of the major peripheral pain generators for musculoskeletal pain conditions. The continual nociceptive bombardment from primary afferent activity over time leads to abnormal functional and structural changes in the dorsal root ganglia and dorsal horn, a phenomenon known as central sensitization. Sensitization is consistently associated with musculoskeletal pain states, underscoring its significance. Moreover, the sensitized spinal segments and somatic dysfunction may be explained, in part, by viscerosomatic convergence and viscerosomatic reflex; both highlighting the contributions of visceral pathologies to somatic structure pain. In addition, the limbic forebrain and hypothalamus likely play a role in myofascial pain. For example, the limbic system has a bidirectional relationship with negative affect, which may contribute to the amplification or maintenance of the pain. These central factors illustrate the importance of a systematic evaluation in addition to palpation of the skeletal muscle for the objective physical findings of active MTrPs. Comprehensive management of MPS focuses on deactivating painful MTrPs and addressing sensitization at both the peripheral and central level through the use of manual therapies, dry needling, paraspinous injection blocks, electrical modalities, local botulinum toxin injection, centrally acting pharmacologic agents, and biofeedback, among others.
Myofascial pain is the most common component of musculoskeletal pain conditions. As a form of muscle pain, myofascial pain can often be described as aching, cramping, deep, and difficult to localize. Several biological processes differ between muscle and cutaneous pain. For example, muscle pain involves nociceptive-specific neurons in the brainstem and spinal cord to a greater extent than observed in cutaneous pain.1-2 In addition, muscle pain activates midbrain areas that are associated with affective or emotional components of pain.3 Although muscle nociception is inhibited more intensely by descending pain-modulating pathways,4-5 persistent muscle nociception is more effective than cutaneous nociception at inducing maladaptive neuroplastic changes within the dorsal horn.6 Such neuroplastic changes underlie the clinical observation that chronic muscle pain is often persistent and difficult to resolve.
Myofascial pain syndrome (MPS) is a term used to describe a pain condition which can be acute (less than 3 months in duration) or, more commonly, chronic, and stems from the muscle and its surrounding connective tissue and fascia. For many clinicians and investigators, a necessary component to the diagnosis of MPS is the finding of one or more MTrPs. An MTrP is palpable on physical examination as a discrete hyperirritable nodule in a taut band of skeletal muscle (Figure 11.1). The pain experienced with MPS is associated with but not necessarily caused by an active MTrP. An active MTrP is associated with spontaneous pain in the surrounding tissue and with referred pain. Strong digital pressure on the active MTrP intensifies the patient’s pain and mimics the patient’s typical pain experience. MTrPs can also be latent, such that the MTrP is present but not associated with spontaneous pain. However, pressure on a latent MTrP often elicits pain at the nodule even in individuals who are not experiencing pain in that region. Both latent and active MTrPs can be associated with muscle dysfunction, muscle weakness, and limited range of motion.
FIGURE 11.1 Schematic of a trigger point complex. A trigger point complex in a taut band of muscle is composed of multiple contraction knots. (Adapted from Simons, D.G.. Travell, J.G. Myofascial Pain and Dysfunction: The Trigger Point Manual, vol. 1; second ed., and Anvandare: Chrizz.)
Origin and History of MPS
The clinical study of MPS and its associated MTrPs has spanned the past two centuries in which scientists and physicians have striven to characterize MPS, proposing distinguishing characteristics and associations for the disorder. However, the scientific literature appears disjointed and confusing, as the terminology, theories, concepts, and diagnostic criteria are inconsistent, incomplete, and, at times, controversial. As a result, MPS and its relationship to MTrPs are not fully understood.
The term “myofascial” conveys that both muscle and fascia likely contribute to the symptoms. In the past, “chronic muscle pain” and nomenclature such as “fibrositis” reflected inflammation of connective tissue lining muscle. These terms have been replaced by the term “myofascial pain.”
The study of myofascial pain and dysfunction was pioneered by the U.S. physician Janet Travell, whose work is arguably the most comprehensive to date. Travell and Rinzler coined the term “myofascial trigger point” in the 1950s, reflecting their finding that the nodules can be present and refer pain to both muscle and overlying fascia.7 The two-volume book, Myofascial Pain and Dysfunction: The Trigger Point Manual, which she co-authored with her colleague, David Simons, represents decades of keen observations and study of myofascial pain and MTrPs.8
This manual, along with over 40 papers Travell published, remains crucial in defining and popularizing the diagnosis and treatment of MPS and MTrPs within the healthcare community, which includes physical therapists, allopathic and osteopathic physicians, chiropractors, dentists, pain specialists, massage therapists, and myofascial trigger point therapists. Among the various allopathic medical specialties, physiatrists currently have the most comprehensive understanding of MTrPs, in part because physiatrists see MPS and the MTrP as related to muscle and musculoskeletal dysfunction. Physiatrists are perhaps unique among the medical specialties as they do not regard muscle as an “orphan organ.” Simons used this term to describe the medical community’s lack of understanding and consideration of muscle as a contributor to non-articular musculoskeletal pain syndromes. Now, Simons’ comments along with Travell’s myofascial pain concepts are gaining ground in mainstream medicine.
The contemporary use of the term “MPS” implies a specific condition which is distinguished from other soft tissue pain disorders such as fibromyalgia, tendonitis, or bursitis.9 For instance, unlike fibromyalgia, which is widespread, symmetrically distributed, and frequently affects sleep and mood, MPS presents as local or regional pain, sometimes with referred pain, and is often accompanied by increased tension and decreased flexibility. The pain of MPS may present independently of mood or sleep abnormalities and has been reported with other diseases and syndromes associated with pain, such as rheumatologic diseases and fibromyalgia.10 Other pain conditions associated with MPS include radiculopathies, joint dysfunction, disk pathology, tendonitis, cranio- mandibular dysfunction, migraines, tension type headaches, carpal tunnel syndrome, computer- related disorders, whiplash-associated disorders, spinal dysfunction, pelvic pain and other urologic syndromes, post-herpetic neuralgia, and complex regional pain syndrome.10 In addition, MTrPs have been associated clinically with a variety of medical conditions including those of metabolic, visceral, endocrine, infectious, and psychological origin."
MPS has generally been characterized by a physical finding and symptom cluster that lacked demonstrable pathology and attracted little research attention until recently. Additionally, characteristics such as pain quality, distribution, and whether the pain radiates have never been required for the diagnosis of MPS. The inter-subject variability and subjective nature of each individual’s symptoms present a challenge for standardization and validation if these are to be used as diagnostic criteria, outcome measures of improvement, or in clinical trials. However, the nature of the symptoms of muscle pain in general is highly dependent upon the individual’s perception of its intensity, distribution, and duration. Centralized mechanisms have been suggested to be involved in MPS, potentially altering the pain threshold, pain intensity, and emotional affect.12 Thus, it is recommended that examiners take a systematic approach to evaluating the individual’s pain perception along with related central factors when diagnosing and treating MPS.