Fibromyalgia and myofascial pain (MP) are among the most common musculoskeletal disorders from which older adults suffer. These disorders represent opposite ends of the pain spectrum with the discrete character of MP at one extreme and the widespread symptoms of fibromyalgia at the other. MP may be acute or chronic, and is associated with taut muscle bands and hypersensitive areas called trigger points. Fibromyalgia syndrome includes symptoms of sleep disruption, fatigue, and psychological distress in addition to widespread pain. Both fibromyalgia and MP syndromes may result in significant functional impairment and cause suffering and disability comparable to that of rheumatoid arthritis and osteoarthritis. Diagnosis of these disorders is grounded in appropriately targeted history and physical examination; these are the tools required to avoid unnecessary ordering of “diagnostic” tests and foster implementation of appropriate management strategies.
Definition and Epidemiology
While a number of fibromyalgia classification criteria have been proposed, the criteria developed by the American College of Rheumatology are used most commonly. These criteria, which are 81% sensitive and 88% specific, allow fibromyalgia patients to be distinguished from patients with widespread pain caused by other rheumatological disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis). They include a history of generalized body pain (i.e., pain in at least three of four body quadrants) for at least 3 months duration and at least 11 out of 18 specific tender points on physical examination. Although initially developed for classification of fibromyalgia, practitioners tend to regard them as required for diagnosis, although this is not accurate. Older adults who present with widespread pain and other supportive clinical features (see below) should be considered to have fibromyalgia even if they do not precisely fulfill the ACR criteria. These criteria are best used as a general guide and to allow for study enrollment, not for strict use in the office setting.
The incidence of fibromyalgia syndrome (the proportion of new cases or first ever episodes) is difficult to measure in part because symptoms seem to ebb and flow over time. According to five large population studies, approximately 10% of the population has widespread pain. Of those with widespread body pain, approximately 2% meet ACR diagnostic criteria for fibromyalgia. Women are four to seven times as likely to have fibromyalgia compared to men, with the greatest prevalence in those 60 to 79 years of age. Patients with fibromyalgia also have been estimated to have a two- to sevenfold greater risk of suffering from depression, anxiety, headache, irritable bowel syndrome, chronic fatigue syndrome, systemic lupus erythematosus, and rheumatoid arthritis compared to healthy individuals.
While recent studies have added to our understanding of the pathogenesis of fibromyalgia, the exact cause is still unknown. Most studies suggest that abnormal central nervous system pain processing, known as central sensitization, plays a key role in fibromyalgia pathogenesis. Abnormal peripheral pain processing, peripheral sensitization, also contributes to fibromyalgia pathogenesis. The cause of sensitization is not known, but a variety of neuroendocrine and biochemical abnormalities are believed to be involved.
Peripheral Tissue Abnormalities
Early studies of fibromyalgia patients failed to consistently show abnormalities in the peripheral tissues. However, reexamination of this issue has uncovered differences between muscle samples from fibromyalgia subjects and healthy controls. One difference is higher levels of nitric oxide in muscles of fibromyalgia patients that may result in increased cell death. Other abnormalities that have been identified in muscles of fibromyalgia patients as compared with healthy individuals include lower phosphorylation potential and oxidative capacity as evidenced by lower levels of muscle phosphocreatine and ATP, as well as increased substance P, DNA fragmentation, interleukin-1, and perfusion deficits (see Staud reference). While the exact meaning of these abnormalities is unclear, the findings suggest an underlying difference in muscle metabolism of fibromyalgia patients as compared to people without fibromyalgia. Further study is needed to establish a relationship between these findings and the pain and fatigue fibromyalgia patients report.
Central and Peripheral Sensitization and Pain Amplification
Abnormalities of peripheral and central pain processing are well established in fibromyalgia. In the periphery, tissue sensitization results from changes in primary nociceptive afferents, increased neuronal excitability, and enlarged neuronal receptive fields. Central sensitization involves neuroplasticity in the brain and spinal cord. “Windup,” a normal finding of increased pain sensations after repeated exposures to a painful stimulus, is an example of central sensitization. In studies of fibromyalgia patients, the “windup” response is exaggerated compared to controls. Staud and colleagues studied fibromyalgia patients and healthy controls after repeated exposure to heat stimuli. While both groups had higher pain ratings after repeated exposures to heat, the degree of windup and temporal summation was significantly greater in fibromyalgia subjects. In addition, the fibromyalgia subjects had more prolonged after sensations compared with control subjects. Peripheral and central sensitizations contribute to the exaggerated pain response of fibromyalgia patients.
Altered activity of the hypothalamic–pituitary–adrenal axis (HPA), and abnormal levels of adrenocortical trophic hormone (ACTH) and urinary cortisol have been demonstrated in patients with fibromyalgia. Evidence suggests that the HPA axis may be less resilient than normal in fibromyalgia patients and that this and other HPA axis abnormalities may underlie the impaired response to stress that many of these patients exhibit. The review by Crofford provides an expanded discussion of the neuroendocrine abnormalities in fibromyalgia syndrome.
While there are no serologic tests to assist practitioners with making a diagnosis of fibromyalgia, a number of biochemical abnormalities have been identified in the context of research studies. Russell and colleagues have identified lower levels of serum serotonin and norepinephrine in patients with fibromyalgia compared to controls. Low platelet serotonin levels have also been identified. While cerebrospinal fluid (CSF) serotonin has not been measured in patients with fibromyalgia, its precursor and metabolic products have been demonstrated ...