Fibromyalgia Research Library

Fibromyalgia (FM) is a chronic pain syndrome characterised by widespread musculoskeletal pain, fatigue, sleep disturbances, cognitive dysfunction ('fibro fog'), and heightened sensitivity to touch, light, and sound. It is classified in ICD-11 (code MG30.01) as 'chronic primary widespread pain' and as 'nociplastic pain' by the International Association for the Study of Pain (IASP, 2017). Unlike inflammatory arthritis or tissue-damage-driven pain, fibromyalgia arises not from injury to muscles or joints, but from a fundamental dysregulation of how the central nervous system processes pain signals.

The term 'nociplastic pain' — introduced by the IASP in 2017 — captures this distinction precisely. It refers to pain arising from altered nociception without clear evidence of tissue damage or nerve injury. In fibromyalgia, blood tests return normal, X-rays show no pathology, and nerve conduction studies are unremarkable. Yet the pain is entirely real: functional MRI studies consistently show abnormal activity in pain-processing brain regions, particularly the insula, anterior cingulate cortex, and prefrontal cortex. FM patients experience pain because their nervous system is generating it — not because something is broken in the periphery.

Fibromyalgia was formally recognised in modern medicine when the American College of Rheumatology (ACR) published its first diagnostic criteria in 1990, based on tender point examination. In 2010 and again in 2016, the ACR updated these criteria to remove the tender point requirement, replacing it with the Widespread Pain Index (WPI) and Symptom Severity Scale (SSS). This shift reflected a deeper scientific understanding: FM is not primarily a muscle disorder but a disorder of pain amplification in the central nervous system. Today, diagnosis is entirely clinical — based on symptom pattern and duration — and does not require laboratory tests or imaging.

One of the most significant challenges in fibromyalgia is the diagnostic delay. On average, patients wait 8 years from the onset of symptoms to receiving a confirmed diagnosis. This delay is not simply a failure of clinical recognition; it reflects the fact that fibromyalgia symptoms — fatigue, widespread pain, poor sleep, cognitive fog — are non-specific and overlap with dozens of other conditions. Patients are frequently dismissed, misdiagnosed with depression or hypochondria, or subjected to extensive workups that return normal results. This invalidation itself contributes to the psychological burden of the condition and can worsen central sensitisation by increasing stress, disrupting sleep, and activating the HPA axis.

Core Symptoms Explained

Who Gets Fibromyalgia and Why?

Fibromyalgia affects an estimated 2–4% of the global population — roughly 200 million people worldwide. The condition shows a striking sex disparity: women are diagnosed approximately 6 times more often than men, though post-COVID research suggests this ratio may partly reflect different triggering mechanisms. Peak onset occurs between ages 30–55, overlapping precisely with perimenopause — not coincidentally, since estrogen and progesterone are powerful modulators of serotonin and GABA pain pathways.

Risk factors include female sex, middle age, a personal or family history of depression or anxiety, prior trauma, and prior infections. FM is frequently triggered by an identifiable event — viral illness, physical injury, extreme stress, or a hormonal transition. This 'trigger hypothesis' aligns with the central sensitisation model: a stressor activates the HPA axis, depletes monoamines, disrupts sleep, and primes the nervous system for amplified pain signalling. Once established, this state becomes self-sustaining even after the original trigger resolves.

FibroCOVID: A New Clinical Entity

In 2021, Italian rheumatologist Giacomo Ursini and colleagues published a landmark study examining 616 COVID-19 survivors at 6 months post-infection. They found that 30.7% — nearly 1 in 3 — met ACR 2016 fibromyalgia criteria. They coined the term 'FibroCOVID' for this post-COVID fibromyalgia syndrome, characterising it as a distinct manifestation within the post-COVID-19 spectrum. This was not pre-existing FM being revealed: the criteria were specifically not met before infection.

One of the most scientifically important findings was the unexpected sex distribution. Classical FM affects women at 6:1 compared to men. But in the FibroCOVID cohort, 43% of cases were male — dramatically higher than classical FM. This confirms that COVID-19 disease severity — not female hormonal vulnerability — is the driving trigger for post-COVID FM. Male vulnerability here is explained by higher rates of severe COVID-19 in men (ACE2 expression differences, testosterone-driven immunosuppression, metabolic comorbidities), not inherent male FM susceptibility.

The strongest predictors of FibroCOVID were obesity (OR 82.82; 95% CI 32.19–213.08) and male sex (OR 9.95; 95% CI 6.02–16.43). BMI correlated with FM severity at R=0.763 (p<0.0001). The researchers proposed that obesity facilitates the COVID→FM transition via adipose-tissue-driven chronic low-grade inflammation, priming the nervous system for central sensitisation — consistent with obesity being a risk factor for classical FM via adipose-derived cytokines (leptin, adiponectin, TNF-α) that cross the blood-brain barrier and modulate neuroinflammatory tone.

The Biological Mechanisms: How COVID Triggers FM

SARS-CoV-2 triggers FM through at least six converging pathological pathways, each attacking a different component of the pain regulation system. This explains why post-COVID FM persists long after viral clearance.

1. Direct CNS neuroinvasion: SARS-CoV-2 enters the CNS via ACE2 receptors on neurons and glial cells, directly infecting brainstem nuclei and triggering microglial activation. Activated microglia produce pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) that lower pain thresholds and disrupt descending inhibitory pathways. Post-mortem studies confirm SARS-CoV-2 RNA in CNS tissue with persistent neuroinflammation months after acute infection.

2. Fascial myofibroblast activation (Plaut 2023): COVID-19's cytokine storm activates fascial myofibroblasts — contractile cells embedded in connective tissue throughout the body. This elevates intramuscular pressure from 12.23 mmHg to 33.48 mmHg — a threefold increase that directly stimulates nociceptors throughout the musculoskeletal system, producing the diffuse deep burning muscle pain characteristic of FM.

3. Mitochondrial dysfunction & oxidative stress: COVID-19 impairs mitochondrial function directly (viral proteins interfere with the electron transport chain) and indirectly (hypoxia). Mitochondrial dysfunction in sensory neurons impairs serotonin and norepinephrine production and reduces the neuronal capacity to maintain normal membrane potentials, further lowering pain thresholds. Elevated oxidative stress markers are consistently found in both COVID-19 and FM.

4. Gut microbiome dysbiosis: SARS-CoV-2 extensively infects the GI tract, disrupting the gut microbiome — the primary source of tryptophan (serotonin precursor) and short-chain fatty acids that modulate CNS function. Post-COVID dysbiosis depletes serotonin precursors, increases intestinal permeability, and allows bacterial endotoxins into systemic circulation, triggering low-grade inflammation that amplifies pain sensitisation. Gut microbiome abnormalities persist ≥6 months after COVID-19 recovery.

Shared Mechanisms: Long COVID = Fibromyalgia?

In 2023, Clauw & Calabrese published a landmark paper in Annals of the Rheumatic Diseases drawing explicit parallels between long COVID and fibromyalgia. Their central argument: after excluding patients with identifiable organ damage, the remaining long COVID patients present with a symptom profile indistinguishable from FM — and show the same neuroimaging abnormalities (increased insula and anterior cingulate cortex activity), the same autonomic dysfunction (heart rate variability abnormalities, orthostatic intolerance), and the same pharmacological response profile (benefit from SNRIs and gabapentinoids; no benefit from opioids or NSAIDs).

Goldenberg (2025), reviewing the latest evidence in Expert Review of Neurology, went further: proposing that long COVID, fibromyalgia, and ME/CFS are three clinical presentations of one underlying disorder — central sensitisation syndrome. This has direct treatment implications: the entire FM evidence base becomes applicable to long COVID, and decades of FM pathophysiology research provides a ready-made framework for treating the post-COVID epidemic of chronic fatigue and pain.

The Friston View: Collapsed Inference After COVID

From Friston's Free Energy Principle perspective, COVID-19 is a catastrophic assault on the brain's generative model. During acute infection, the virus floods the body with unprecedented conflicting interoceptive signals — fever oscillations, hypoxia, severe fatigue, widespread pain, GI distress, and neuroinflammation — all simultaneously. The brain's prediction machinery is overwhelmed; it cannot generate a coherent prior. The generative model destabilises.

As the acute phase resolves, the brain rebuilds its generative model — but the template it has available is the one it just learned: 'my body is sick, painful, and exhausted.' This pathological prior becomes the new baseline. Even when the virus is cleared, the brain continues predicting pain and fatigue, because that is what recent experience taught it to expect. From a computational perspective, this is a hyperprior of illness: a deeply entrenched belief about the body's state that requires enormous updating evidence to revise. Without targeted therapeutic interventions — CBT to update catastrophic beliefs, graded exercise to provide corrective sensory evidence of function, sleep improvement for nightly recalibration — the hyperprior of illness persists indefinitely.

The Hormonal Pain Connection

The epidemiology of FM presents a long-standing puzzle: why does it affect women 6× more often than men, and why does onset peak precisely in the 30–55 year range? The answer lies in the profound role that sex hormones — estrogen, progesterone, and testosterone — play in regulating pain thresholds, serotonin synthesis, and CNS excitability. These are not merely reproductive chemicals; they are powerful neuroactive compounds that modulate the very systems that fail in FM.

A landmark study by Carranza-Lira & Villalobos (2014) made this concrete: 29% of premenopausal women with climacteric symptoms (hot flashes, sleep disturbance, mood changes) also met FM criteria, compared to just 4% without climacteric symptoms — a sevenfold difference. The perimenopause represents the single most significant FM onset risk window in the female lifespan, because it is characterised by maximum hormonal instability — not simply declining levels.

The Estrogen-Serotonin-Pain Axis: A Molecular Explanation

The estrogen-serotonin-pain axis is the central mechanistic pathway linking menopause to FM. Estrogen directly stimulates transcription of tryptophan hydroxylase-2 (TPH2) — the rate-limiting enzyme for brain serotonin synthesis. When estrogen declines at menopause: TPH2 activity falls → raphe nucleus serotonin production decreases → the descending serotonergic pain inhibitory pathway weakens → less inhibitory control over ascending pain signals → more pain reaches the brain.

Beyond synthesis, estrogen also regulates serotonin receptor density. Estrogen maintains high 5-HT2A receptor density in the prefrontal cortex and limbic system — critical for mood regulation and pain modulation. After menopause, 5-HT2A density falls, impairing serotonin signalling even at the same synaptic serotonin concentrations. Women undergoing surgical menopause (bilateral oophorectomy) show even more dramatic FM risk increases than natural menopause, because the abrupt estrogen withdrawal leaves no time for neural adaptation.

The Kynurenine Pathway: Tryptophan Hijacked

Tryptophan (serotonin's amino acid precursor) can be metabolised through two competing pathways. The serotonin pathway (preferred when estrogen is present) produces serotonin via TPH2. The kynurenine pathway (activated by inflammation and estrogen withdrawal) instead converts tryptophan into kynurenine → quinolinic acid. Quinolinic acid is an endogenous NMDA receptor agonist — the same receptors involved in central sensitisation and the induction of long-term potentiation in pain pathways.

At menopause, estrogen withdrawal simultaneously suppresses the serotonin pathway AND activates the kynurenine pathway — a double hit: serotonin availability falls (pain inhibition weakens) AND quinolinic acid accumulates (NMDA-driven central sensitisation is induced). Lang et al. (2025) demonstrated that menopausal women have elevated kynurenine-to-tryptophan ratios, correlating with reduced BDNF (brain-derived neurotrophic factor). Lower BDNF means reduced capacity for the nervous system to 'recover' from sensitisation — explaining why post-menopausal FM is often particularly persistent.

GPER: Receptor-Level Dysfunction

A 2019 study by Koca et al. uncovered a finding that challenges the simple 'low estrogen → FM' model. FM patients had elevated GPER (G-protein coupled estrogen receptor) — 0.11 vs 0.059 ng/mL in healthy controls (p=0.037) — despite normal circulating estrogen. GPER is a membrane-bound receptor that mediates pro-nociceptive effects when overactivated: it promotes neuroinflammation, lowers pain thresholds, and amplifies sensory signalling. Elevated GPER in FM suggests the problem is not simply estrogen deficiency but estrogen receptor dysfunction — the estrogen signal is being misprocessed at the receptor level even when hormone levels are normal.

Hormone Therapy Evidence: What Does It Mean for FM?

Given the clear biological mechanisms linking estrogen withdrawal to FM pathophysiology, the question of whether hormone replacement therapy (HRT) benefits fibromyalgia is clinically important. The evidence is encouraging, with significant caveats.