Generalized Chronic Pain: Fibromyalgia and Sensitization

Central Mechanisms: The Neurobiology of Generalized Pain

Generalized chronic pain is not simply "imaginary pain" or the result of psychological fragility. It is a condition with documented neurobiologic alterations that can be visualized on functional neuroimaging and measured by quantitative sensory testing.

The central mechanism is central sensitization: neurons of the dorsal horn of the spinal cord and of the brain become hyperexcitable — they respond with disproportionate intensity to normal stimuli (allodynia) and amplify pain from nociceptive stimuli (hyperalgesia). Two electrophysiologic phenomena are fundamental: wind-up (progressive increase of neuronal response with repetitive stimuli) and long-term potentiation (LTP) of nociceptive synapses.

Fibromyalgia: Diagnosis and Clinical Presentation

The 2016 American College of Rheumatology diagnostic criteria (revised) eliminated the tender points of the older criterion and adopt two índices: the Widespread Pain Index (WPI) — count of painful áreas in the last 7 days (0-19) — and the Symptom Severity Scale (SSS) — severity of fatigue, unrefreshing sleep, and cognitive symptoms (0-12), plus other somatic symptoms.

Diagnosis requires: WPI ≥ 7 AND SSS ≥ 5, OR WPI 4-6 AND SSS ≥ 9; symptoms present for at least 3 months; and absence of another condition that explains all symptoms (although fibromyalgia can coexist with other diseases). Typical presentation includes generalized pain, intense fatigue, unrefreshing sleep, cognitive difficulty (fibro-fog), headache, irritable bowel syndrome, and restless legs syndrome.

Central Sensitization: The Unifying Mechanism

Central sensitization is the neurobiologic mechanism that unifies fibromyalgia and other generalized chronic pain syndromes. Unlike peripheral sensitization (amplification at the peripheral nerve fiber after local inflammation), central sensitization involves functional changes in the CNS that perpetuate pain independently of continued peripheral stimulus.

The role of descending pain inhibition is central in this process. Normally, the brain sends inhibitory signals to the spinal cord (serotonergic, noradrenergic, and endogenous opioid pathways) that reduce nociceptive transmission. In fibromyalgia, this system is compromised — descending inhibition is insufficient, resulting in amplification of the painful signals that reach consciousness.

Comorbidities: The Universe of Fibromyalgia

Fibromyalgia rarely presents alone. Comorbidities are an integral part of the condition and share the same mechanisms of central sensitization and autonomic dysfunction. Understanding and treating comorbidities is essential for therapeutic success.

Irritable bowel syndrome (IBS) coexists in 50-80% of patients with fibromyalgia — both are conditions of visceral hypersensitivity. Chronic headache (migraine and tension-type) affects 50-60% of fibromyalgia patients. Chronic fatigue syndrome and fibromyalgia overlap in 30-70% of cases, with debate about whether they are distinct conditions or spectra of the same process.

Clinical Evaluation and Differential Diagnosis

Evaluation of generalized chronic pain requires comprehensive history-taking: pain map (location, character, intensity), assessment of sleep, fatigue, mood, cognition, and visceral functions. Physical examination includes generalized sensitivity testing, pressure points, and a neurologic exam to exclude neuropathies.

Laboratory tests are necessary to exclude treatable causes of generalized pain: complete blood count, ESR, CRP, TSH, free T4, rheumatoid factor, anti-CCP, ANA, vitamin D, ferritin, and CK. Fibromyalgia has no laboratory marker — diagnosis is clinical, by ACR 2016 criteria.

Fibromyalgia vs. Rheumatologic Disease

Distinguishing fibromyalgia from inflammatory rheumatologic diseases (rheumatoid arthritis, lupus, ankylosing spondylitis) is clinically important but can be difficult, since fibromyalgia can coexist with any of them. Findings that favor a rheumatologic disease: arthritis with objectively demonstrable joint swelling, prolonged morning stiffness (> 1 hour), elevated inflammatory markers (ESR, CRP), positive autoantibodies, and radiologic abnormalities.

Findings that favor fibromyalgia: generalized multisystem pain without arthritis, disproportionate fatigue, unrefreshing sleep, fibro-fog, multiple hypersensitivities, and normal laboratory tests. When the two conditions coexist, treating only the rheumatologic inflammation often fails to resolve the pain — fibromyalgia's central sensitization component requires a specific approach.

The Role of Neuroimaging

Functional neuroimaging (fMRI, PET-CT) has confirmed that fibromyalgia has an objective neurobiologic substrate. Studies show consistent alterations in pain processing: hyperactivity of the anterior insula and anterior cingulate córtex (affective pain processing regions), reduced connectivity in descending inhibitory pathways, and default-mode network connectivity changes that correlate with symptom severity.

In clinical practice, neuroimaging is not routinely requested to diagnose fibromyalgia — diagnosis is clinical, by ACR 2016 criteria. But knowledge of these alterations helps the physician explain to the patient the real basis of their condition, reducing stigma and improving treatment adherence.

Diagnosis by Pain Patterns

Pain patterns in fibromyalgia have features that guide diagnosis. Pain is typically diffuse and migratory — unlike localized regional pain. Unexpected factors such as emotional stress, sleep deprivation, weather changes, and light physical exertion trigger or amplify the pain. Allodynia — pain to normally innocuous stimuli such as light pressure or cold — is frequent.

The medical acupuncturist uses the patient's pain map — which regions hurt, when they worsen, what relieves them — not only for diagnosis, but also to select the most relevant acupuncture points and monitor treatment response over the sessions.

Multimodal Therapeutic Approach

Treatment of generalized chronic pain and fibromyalgia generally requires a multimodal approach — single interventions usually produce inferior results compared with combined strategies. International guidelines (EULAR 2017, ACR) recommend pain education, aerobic exercise (strong recommendation), psychological approach, and pharmacotherapy as pillars of treatment; acupuncture is cited in EULAR 2017 as an option with weak recommendation, and may be considered in the individualized therapeutic plan.

Medical Acupuncture in Generalized Chronic Pain

Medical acupuncture has mechanisms of action particularly relevant to generalized chronic pain and fibromyalgia. Unlike interventions that treat only local pain, acupuncture acts on the central circuits altered in central sensitization.

The central mechanisms of acupuncture in generalized pain include: descending serotonergic inhibition (acupuncture increases the release of serotonin in the inhibitory pathways of the brainstem, restoring the descending inhibition deficient in fibromyalgia), modulation of the prefrontal córtex (fMRI studies show that acupuncture activates the prefrontal córtex — a region that mediates cognitive control of pain and that is hypofunctional in fibromyalgia), and resetting of the HPA axis (acupuncture normalizes the cortisol response to stress, reducing the HPA axis hyperactivation that perpetuates central sensitization).

When to Seek Medical Help

Generalized pain persisting for more than 3 months, especially when associated with significant fatigue, unrefreshing sleep, and cognitive difficulty, deserves specialized medical evaluation. A precise diagnosis guides correct treatment and avoids years of unproductive investigations.