Central Sensitization and Nociplastic Pain
Pain is a frequently under-addressed symptom in ME/CFS. This section covers central sensitization—the amplification of nociceptive processing in the dorsal horn and supraspinal circuits—as the mechanism underlying the widespread, disproportionate pain and allodynia seen in ME/CFS. It distinguishes nociplastic pain (arising from altered nociception without identifiable tissue damage) from nociceptive and neuropathic pain, and connects central sensitization to the neuroinflammatory and glial mechanisms discussed earlier.
1 Wind-up and Dorsal Horn Sensitization
Repetitive C-fiber nociceptor input produces a phenomenon known as wind-up: a slow temporal summation of action potentials in wide dynamic range neurons of the dorsal horn that is experienced as progressively increasing pain. Wind-up depends on activation of \(N\)-methyl-d-aspartate (NMDA) receptors, which become available following sustained depolarization that removes the resting magnesium block; calcium influx then activates kinase cascades that potentiate synaptic efficacy — a process analogous to long-term potentiation in the hippocampus (Woolf 2011). Once established, central sensitization manifests as dynamic tactile allodynia (pain from light touch), secondary hyperalgesia, and aftersensations, all reflecting expanded and lowered pain threshold in central circuits.
2 Evidence for Central Sensitization in ME/CFS
In ME/CFS, multiple lines of evidence demonstrate central sensitization as a disease feature rather than an epiphenomenon. Generalized hyperalgesia has been documented for electrical, mechanical, heat and histamine stimuli across skin, muscle and visceral tissues. Critically, endogenous inhibitory analgesia the conditioned pain modulation (CPM) response that normally suppresses ongoing pain during a second noxious stimulus is absent or blunted in ME/CFS patients (Nijs, Meeus, et al. 2012). Furthermore, exercise that normally activates endogenous analgesia instead exacerbates pain in ME/CFS, accompanied by significant post-exercise upregulation of ASIC3, P2X4 and TLR4 gene expression in leukocytes, persisting for 48h and correlating with fatigue and pain severity (Nijs, Crombez, et al. 2012).
Dolorimetry studies confirm that ME/CFS and Gulf War Illness patients are significantly more tender than sedentary controls across all sex strata, and dolorimetry scores correlate strongly with self-reported pain (Spearman \(R = -0.574\) to \(-0.629\), \(p < 0.001\)) and interoceptive symptoms (Chen et al. 2025). These authors propose injury to midbrain and medullary descending regulatory pathways as the mechanism for simultaneous loss of antinociceptive and antiinteroceptive inhibition.
3 Nociplastic Pain Framework
The International Association for the Study of Pain adopted nociplastic pain as a third mechanistic descriptor in 2017, defined as “pain that arises from altered nociception” not fully explained by nociceptive or neuropathic mechanisms (Kosek 2024). Clinical criteria for musculoskeletal nociplastic pain require: duration exceeding three months; regional, multifocal or widespread distribution; absence of a complete nociceptive or neuropathic explanation; and clinical signs of hypersensitivity in the pain region. Fibromyalgia is the prototype; ME/CFS pain fits the same phenotype.
Mapping IASP nociplastic criteria to ME/CFS evidence:
- Duration exceeding three months: ME/CFS pain is by definition chronic (diagnosis requires ≥ 6 months of symptoms).
- Regional, multifocal, or widespread distribution: Body mapping documents pain across cervical spine (66%), thighs (44–46%), lumbar spine (42%), and calves (38%), with 76.1% having at least one Chronic Overlapping Pain Condition (Fall et al. 2024) (Marshall et al. 2010).
- Absence of complete nociceptive or neuropathic explanation: While 30–80% have SFN (neuropathic component), the widespread pain distribution and central sensitisation exceed what SFN alone explains (Oaklander et al. 2022) (Nijs et al. 2021).
- Clinical signs of hypersensitivity: Dolorimetry confirms generalised hyperalgesia (AUC 0.730–0.816 vs controls); allodynia to light touch documented; conditioned pain modulation absent (Chen et al. 2025) (Nijs, Meeus, et al. 2012).
Most ME/CFS patients exhibit nociplastic features (central sensitisation, widespread pain, hypersensitivity to multiple stimulus modalities) while 30–80% also have a confirmed neuropathic component (SFN, biopsy-proven reduced IENFD). Fibromyalgia also has a substantial neuropathic subgroup—systematic review estimates ~49% SFN prevalence in FM (Grayston et al. 2019) (Oaklander et al. 2013) — so the hybrid is not unique to ME/CFS. What may distinguish ME/CFS is the interaction of the nociplastic-neuropathic hybrid with PEM-driven pain amplification (see PEM-Driven Pain Amplification as Prominent ME/CFS Feature): post-exertional immune gene upregulation (ASIC3, P2X4, TLR4) re-sensitises both central and peripheral pain pathways in a manner not observed in FM alone. This interaction remains a hypothesis pending direct head-to-head comparison. The hybrid model predicts that: (1) SFN-positive ME/CFS patients should respond differently to peripheral nerve treatments (IVIG, immunotherapy) than SFN-negative patients; (2) SFN-negative patients should respond preferentially to centrally acting agents (LDN, NMDA antagonists, pregabalin); (3) patients with both components should have worse pain outcomes than those with either alone; and (4) pain phenotype should predict treatment response more reliably than disease severity alone (Oaklander et al. 2022) (Kosek 2024) (Nijs et al. 2021). (Certainty: 0.40; individual components well-supported; the hybrid framework as a clinical stratification tool has not been tested; Barhorst 2022 meta-analysis pools ME/CFS and FM cohorts, preventing ME/CFS-specific effect extraction.)
Elevated quinolinic acid (an NMDA agonist produced by the kynurenine pathway under neuroinflammatory conditions see Section Kynurenine Pathway and Quinolinic Acid Excitotoxicity: The “Fog Machine”) may provide sustained NMDA receptor activation that maintains wind-up and dorsal horn sensitization in ME/CFS. If confirmed, this would mechanistically link neuroinflammation, kynurenine pathway activation, and central sensitization in a single causal chain. (Certainty: Low; no direct human measurement of spinal QUIN in ME/CFS.)
4 Peripheral Sensitization: Substance P and CGRP
Substance P and calcitonin gene-related peptide (CGRP) released from primary afferent C fibers sensitize peripheral nociceptors and maintain neurogenic inflammation in the periphery. In the context of mast cell activation (see Section Mast Cell Mediators and Histaminergic Symptom Generation), mast cell-released histamine and tryptase further lower nociceptor thresholds, creating a peripheral–central sensitization loop. CGRP-driven neurogenic inflammation can be sustained via ROS-dependent TRPA1 activation in Schwann cells, independently of direct neuronal injury.
5 Why Nerve Sheaths Become Painful
A subset of ME/CFS patients report deep, burning, or lancinating pain along nerve trunks—distinct from the diffuse aching of central sensitization and from the distal burning of small fiber neuropathy. This pattern suggests pathology at the level of the nerve sheath itself, where Schwann cells, endoneurial vasculature, and local immune cells create a microenvironment vulnerable to several converging ME/CFS mechanisms.
Schwann Cell Activation as Nociceptive Amplifier. Schwann cells are not passive insulators. They express TRPA1 channels that, when activated by reactive oxygen species (ROS) or CGRP, trigger the release of pro-algesic mediators including prostaglandins, cytokines, and additional CGRP, sustaining neurogenic inflammation independently of direct axonal injury (Macpherson et al. 2007). In ME/CFS, the combination of elevated systemic ROS (Section Oxidative and Nitrosative Stress as Symptom Amplifier) and circulating pro-inflammatory cytokines creates conditions for chronic Schwann cell activation along nerve trunks. The resulting neurogenic inflammation produces pain that is localized to nerve distribution but does not require demyelination or axonal degeneration.
Endoneurial Hypoxia and Metabolic Stress. The endoneurium—the connective tissue layer immediately surrounding individual nerve fibers—is supplied by vasa nervorum, small blood vessels vulnerable to the endothelial dysfunction documented in ME/CFS (Heng et al. 2025). Microvascular impairment reduces oxygen delivery to the metabolically active Schwann cells and axons within the nerve sheath. Nerve fibers have exceptionally high energy demands: maintaining resting membrane potential, sodium-potassium pump activity, and axonal transport all require substantial ATP. When endoneurial oxygen supply falls below demand, local ischemia activates acid-sensing ion channels (ASICs) on nerve fibers, producing aching pain along nerve trunks, and hypoxia-inducible factor (HIF-1\(\alpha\)) upregulation in Schwann cells further promotes pro-inflammatory signaling.
Autoimmune Targeting of Nerve Sheath Components. Autoantibodies against neuronal and glial antigens have been identified in ME/CFS subsets (Oaklander and Nolano 2019). In autoimmune small fiber neuropathy, antibodies target components of the nerve sheath including gangliosides, sodium channels, and contactin-associated proteins. Complement fixation at the node of Ranvier disrupts saltatory conduction and recruits inflammatory cells to the endoneurium, creating focal painful neuropathy. The 30–38% prevalence of biopsy-confirmed SFN in ME/CFS (Oaklander et al. 2022) likely underestimates nerve sheath involvement, because standard IENFD measurement captures only the most distal epidermal fibers and misses proximal nerve trunk pathology.
Mast Cell Enrichment of the Perineurium. Mast cells are concentrated in the perineurium (the connective tissue sheath surrounding nerve fascicles) and epineurium of peripheral nerves. In the context of mast cell activation syndrome (MCAS), which overlaps with ME/CFS (Section Mast Cell Mediators and Histaminergic Symptom Generation), perineurial mast cell degranulation releases histamine, tryptase, and nerve growth factor (NGF) directly into the nerve sheath microenvironment. NGF sensitizes nociceptive fibers by upregulating TRPV1 expression (Moriyama et al. 2005), while tryptase activates proteinase-activated receptor 2 (PAR2) on Schwann cells and sensory neurons, lowering pain thresholds. This mechanism explains why nerve trunk tenderness can fluctuate with mast cell activation episodes.
Nerve sheaths represent a site of convergent pathology in ME/CFS, where four disease mechanisms—oxidative Schwann cell activation, endoneurial hypoxia from microvascular dysfunction, autoimmune targeting of nerve sheath antigens, and perineurial mast cell degranulation—simultaneously lower pain thresholds and sustain neurogenic inflammation. This convergence predicts that nerve trunk pain in ME/CFS should correlate with composite biomarkers of oxidative stress, endothelial dysfunction, autoantibody burden, and mast cell activation, rather than with any single marker. (Certainty: Low-Medium; individual mechanisms are supported by evidence from other conditions; convergence in ME/CFS specifically is not yet empirically demonstrated.)
6 Why Pain Localizes Around Joints and in Muscles
ME/CFS patients frequently report diffuse pain around joints (without inflammatory arthritis) and deep muscle aching (without exercise-induced injury). This periarticular and muscular pain distribution—clinically overlapping with fibromyalgia—has specific mechanistic explanations beyond the generalized central sensitization described above.
Periarticular Mast Cell Density. Mast cells are not uniformly distributed in connective tissue: they concentrate heavily in the synovium, joint capsule, and periarticular ligaments, where tissue mast cell densities substantially exceed those in subcutaneous tissue. In MCAS-associated ME/CFS, periarticular mast cell degranulation releases histamine, prostaglandin D2, and substance P directly into the joint microenvironment, producing pain, local edema, and stiffness that mimic inflammatory arthritis but without the neutrophilic infiltrate or cartilage erosion of true arthritis. Patients often describe “joints feeling inflamed” despite normal inflammatory markers and imaging—consistent with mast cell-mediated neurogenic inflammation rather than autoimmune synovitis.
Muscle Metabolic Nociception. In ME/CFS, mitochondrial ATP synthesis impairment (Chapter Energy Metabolism and Mitochondrial Function) produces a characteristic metabolic profile in exercising and even resting muscle: elevated lactate from premature glycolytic shift, intramuscular acidosis from impaired proton buffering, and accumulation of metabolic intermediates (succinate, reactive oxygen species) (Lien et al. 2019) (Jammes et al. 2021). These metabolites directly activate muscle nociceptors through three receptor families:
- Acid-sensing ion channels (ASICs): ASIC3, expressed on muscle afferents, responds to the combination of low pH, elevated lactate, and elevated ATP that characterizes ischemic muscle metabolism. Importantly, ASIC3 responds to the combination of these signals rather than any one alone, functioning as an integrative metabolic danger sensor (Nijs, Crombez, et al. 2012). Post-exercise ASIC3 gene expression is significantly upregulated in ME/CFS patients and persists for 48h, correlating with pain severity.
- Purinergic P2X receptors: Extracellular ATP released from metabolically stressed muscle fibers activates P2X3 receptors on muscle afferents, producing deep aching pain. In ME/CFS, where ATP metabolism is disturbed, resting extracellular ATP may be chronically elevated.
- TRPV1 (vanilloid receptors): Proton accumulation and elevated temperature in metabolically active muscle sensitize TRPV1 on muscle nociceptors, lowering pain thresholds for mechanical and thermal stimulation (Moriyama et al. 2005).
Fascial and Connective Tissue Innervation. The thoracolumbar fascia, intramuscular septa, and periosteum are richly innervated by C-fiber and A-delta nociceptors and contain substantial mast cell populations. In hypermobile Ehlers–Danlos syndrome (hEDS)—overrepresented among ME/CFS patients—connective tissue laxity produces microtrauma at fascial attachment sites during normal movement, generating nociceptive input that would not occur with normal tissue resilience. Even in non-hEDS ME/CFS patients, deconditioning-related changes in fascial stiffness and hydration may alter the mechanical environment of fascial nociceptors.
Mechanical Nerve Compression from Skeletal Asymmetry. Beyond fascial nociception, skeletal asymmetry (pelvic obliquity, rotoscoliosis, leg length discrepancy) can produce direct mechanical compression of peripheral nerve trunks. Asymmetric fascial tension from chronic postural compensation alters the glide planes of nerves against bony structures: the lateral femoral cutaneous nerve at the inguinal ligament, the sciatic nerve at the piriformis, and the pudendal nerve in Alcock’s canal are mechanically vulnerable. Chronic low-grade compression produces a distinct sensory phenotype — fluctuating paresthesia, positional allodynia, and a sensation of “wrongness” rather than lancinating pain — that differs from both the metabolic muscle pain and the hypermobile fascial microtrauma described above. The resulting nociceptive input, amplified by central sensitization (Central Sensitization and Nociplastic Pain), contributes to the diffuse hypersensitivity characteristic of ME/CFS. This mechanism is distinct from small fiber neuropathy (Peripheral Nervous System) — it is a compression neuropathy, not a degenerative one — and would be expected to improve with postural correction. Evidence note: Schulte et al. (Schulte et al. 2010) documented that mechanical disruption of the paravertebral sympathetic chain causes autonomic dysfunction; whether spontaneous skeletal asymmetry produces analogous nerve compression is untested. Falsifiable prediction: In ME/CFS patients with pelvic obliquity >10 mm, nerve conduction studies of the lateral femoral cutaneous nerve will show reduced sensory nerve conduction velocity \((< 45 \text{m/s})\) compared to spinally aligned ME/CFS patients; shoe lift correction for 4 weeks should increase conduction velocity by ≥10%. Falsified if no group difference or if post-correction change ≤2%. This pathway-3 mechanism is further developed in Skeletal Asymmetry as a Primary Mechanical Trigger of the ME/CFS Cascade. Consequence: A treatable mechanical source of pain that does not require centrally acting analgesics — postural correction could reduce nociceptive input at source. (Origin: Gerlier 2026-07-21, personal communication.)
Microvascular Dysfunction in Muscle. Endothelial dysfunction in ME/CFS extends to the intramuscular microvasculature (Heng et al. 2025). Reduced capillary recruitment during even mild activity creates focal ischemia–reperfusion cycles within muscle tissue. Each ischemia–reperfusion episode generates ROS, activates local mast cells, and releases metabolic danger signals (lactate, succinate) that activate the nociceptor populations described above. The intermittent, unpredictable nature of microvascular dysfunction—varying with posture, blood volume, and autonomic state—may explain why muscle pain fluctuates without apparent relation to activity level.
Interaction with Central Sensitization. The periarticular and muscular nociceptive inputs described above converge on dorsal horn circuits that are already centrally sensitized (Section Central Sensitization and Nociplastic Pain). Central sensitization amplifies these tissue-specific inputs and, critically, expands the receptive fields of dorsal horn neurons: a nociceptive signal from one joint capsule activates neurons that represent adjacent joints and surrounding muscle, producing the “spreading” quality of ME/CFS pain. This expansion of receptive fields explains why patients report pain that seems to migrate between joints and muscles—the peripheral generator may be focal, but central amplification makes it appear diffuse.
Diffuse periarticular and muscular pain in ME/CFS arises from genuine tissue-specific nociceptive generators—mast cell-mediated periarticular inflammation, metabolic nociceptor activation in ischemic muscle, and fascial microtrauma—whose signals are then amplified and spatially expanded by central sensitization. This dual-mechanism model predicts that: (1) peripheral nerve blocks should temporarily reduce pain in the targeted region even in centrally sensitized patients (confirming peripheral generators); (2) central sensitization reduction (via LDN, NMDA antagonists) should reduce pain spread but not eliminate pain at the primary generator site; and (3) patients with higher mast cell activation markers should have more pronounced periarticular pain, while patients with worse metabolic profiles should have more pronounced muscle pain. (Certainty: Medium; individual mechanisms are well-supported; the integrated model generates testable predictions but has not been directly tested.)
7 Treatment Implications
Targeting central sensitization requires centrally acting approaches:
- NMDA antagonism: Low-dose ketamine, memantine and dextromethorphan reduce wind-up; see also LDN (low-dose naltrexone) which attenuates microglial activation (Polo et al. 2019).
- Alpha-2-delta ligands: Pregabalin and gabapentin reduce calcium channel-mediated neurotransmitter release in sensitized dorsal horn circuits; widely used in fibromyalgia with modest effect sizes.
- Pain neuroscience education: Reconceptualizing pain as a central amplification phenomenon rather than peripheral tissue damage reduces catastrophizing and improves function in short-term studies in ME/CFS (Nijs, Crombez, et al. 2012).