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Central sensitization and plasticity

TL;DR — Peripheral injury can produce amplified excitation, weakened inhibition, glial activation and altered descending control. Secondary hyperalgesia, temporal summation and allodynia are compatible with central sensitization but do not diagnose it (Latremoliere 2009, PMID 19712899). Central changes can remain input-dependent or become partly self-maintaining. A targeted PubMed search on 2026-08-30 found preclinical perturbation studies and narrative syntheses but no validated clinical test that separates those states (Woolf 2011, PMID 20961685).

Operational definition

Central sensitization is increased responsiveness of CNS nociceptive neurons to normal or subthreshold afferent input; it is a mechanism, not a diagnosis or synonym for chronic pain (Latremoliere 2009, PMID 19712899). Neuropathic pain can occur without demonstrable sensitization, and sensitization can occur without a nerve lesion (Finnerup 2021, PMID 32584191).

Spinal excitation and disinhibition

Repeated C-fiber input recruits glutamatergic AMPA/NMDA signaling, kinases and transcription; reduced GABA/glycine control can permit low-threshold input to engage pain networks (Berger 2011, PMID 21440003). Loss and gain therefore coexist: deafferentation reduces sensation while surviving circuits amplify input (Meacham 2017, PMID 28432601).

“Disinhibition” covers several non-equivalent processes: reduced inhibitory-neuron firing, loss of inhibitory synapses, altered chloride gradients, reduced GABA/glycine release, or impaired postsynaptic receptor function. A 2025 mouse study adds a structural route: after peripheral nerve injury, microglia preferentially engulfed dorsal-horn inhibitory synapses; C3- and C4-deficient mice did not show this synapse elimination, and pharmacologic C1q inhibition prevented inhibitory-synapse loss and attenuated hypersensitivity (Yousefpour 2025, PMID 40382320). This establishes complement-dependent pruning in the model, not in human neuropathic pain.

Plasticity level Candidate process Observable consequence Human-evidence boundary
Synapse NMDA-dependent potentiation Amplified response to repeated input Pharmacologic probes are non-specific
Inhibitory circuit Reduced GABA/glycine control or synapse loss Dynamic allodynia, receptive-field spread No bedside test identifies the molecular route
Glial–neuronal unit Cytokine, trophic, complement and purinergic signaling Persistent excitability after injury Most causal evidence is from rodents
Projection pathway Spinothalamic/thalamocortical reorganization Distorted gain and body representation Imaging shows association, not cellular cause
Descending system Shift in inhibition/facilitation Altered conditioned pain modulation Net psychophysical output, not pathway-specific

Neuroimmune contribution

Microglia and astrocytes signal through purines, cytokines and chemokines after injury, but effects depend on model, time and sex (Inoue 2018, PMID 29416128). Immune pathways also resolve pain, so phase-specific state matters (Fiore 2023, PMID 36859719).

The strongest mechanistic literature is bidirectional rather than uniformly pro-inflammatory. In mice, a CD11c-positive spinal microglial population appeared after hypersensitivity developed; depleting those cells prevented spontaneous recovery, while depletion after recovery or interruption of IGF1 signaling triggered relapse (Kohno 2022, PMID 35357926). By contrast, microglial TRPV4 deletion or blockade reduced nerve-injury hypersensitivity and downstream lipocalin-2-linked excitatory plasticity in another mouse model (Hu 2023, PMID 36701202). Together, the studies argue that microglial identity and disease phase determine direction of effect.

Recent reviews consequently frame microglia as modulators of induction and maintenance, not as a single stable activation state (Malcangio 2025, PMID 40128335). The translational controversy has two defensible sides:

  • For glial targeting: multiple genetic, pharmacologic and cell-state manipulations alter pain behavior and excitatory/inhibitory balance in injury models.
  • Against broad glial suppression: protective resolution states exist, animal sex/model effects are substantial, and no human biomarker currently shows that a given patient's pain is maintained by the targeted glial pathway.

Descending control

Brainstem systems inhibit or facilitate dorsal-horn transmission. Conditioned pain modulation estimates net inhibition but varies with protocol, attention and medication (Arendt-Nielsen 2018, PMID 29105941). Mood and sleep modulate these systems without making lesion-driven pain psychogenic (Cohen 2021, PMID 34062143).

Conditioned pain modulation should be reported as a protocol-specific change score: conditioning stimulus, test stimulus, body site, order, expectation and analgesic exposure all influence the result. A reduced inhibitory effect may reflect weaker inhibition, stronger facilitation, habituation failure or measurement noise. It cannot by itself distinguish neuropathic from nociplastic pain.

Brain and thalamocortical change

Pain engages thalamic, somatosensory, insular, cingulate and prefrontal networks; imaging group differences are not individual diagnostic markers (Peyron 2019, PMID 30318262). A 14-study EEG review found heterogeneous theta/beta changes and no consistent pain-intensity correlation (Mussigmann 2022, PMID 35659993).

Three inferential errors recur in central-mechanism studies: regional activation is treated as pain-specific, cross-sectional connectivity is treated as injury-induced plasticity, and a group classifier is treated as an individual biomarker. Longitudinal within-person designs with lesion timing, medication control and external validation are needed before a network signature can separate cause, consequence and coping.

Central lesion syndromes

AAPT criteria require a plausible CNS lesion, matching pain distribution, neurologic signs and confirmation (Widerström-Noga 2017, PMID 28666966). After SCI, at-level pain may mix root and cord injury, whereas below-level pain indicates central pathway damage (Widerström-Noga 2017, PMID 28451808).

Clinical probes

Secondary pinprick hyperalgesia probes receptive-field spread; temporal summation probes facilitatory gain; conditioned pain modulation probes net inhibition. None is lesion-specific (Bannister 2020, PMID 31914896). Entrapment surgery changed QST phenotype at six months, showing state dependence (Kennedy 2021, PMID 33769367).

Probe Minimum reproducible report Compatible inference Invalid shortcut
Secondary hyperalgesia Stimulus, mapped area, reference site, timing Spread beyond primary input zone “Central sensitization diagnosed”
Temporal summation Stimulus count/rate, first and last ratings Facilitatory gain under that protocol NMDA dependence proven
Conditioned pain modulation Conditioning/test stimuli, order, change score Net descending modulation Specific brainstem pathway localized
Dynamic allodynia Brush force/speed and area Low-threshold input evokes pain Spinal disinhibition uniquely identified
EEG/fMRI Prespecified feature, correction and external test Group-level neural correlate Clinical diagnostic biomarker

Treatment implications

Gabapentinoids reduce transmitter release, antidepressants support descending monoaminergic control, and stimulation changes networks; response does not prove a mechanism (Finnerup 2015, PMID 25575710). High-frequency motor-cortex rTMS has guideline support, but durability and maintenance remain uncertain (Lefaucheur 2020, PMID 31901449).

Evidence matrix

Observation Compatible process Key limitation
Dynamic allodynia Disinhibition Also peripheral sensitization
Temporal summation Spinal gain Context-dependent
Secondary hyperalgesia Receptive-field expansion Experimental injury also causes it
Abnormal CPM Descending control Protocol-sensitive
EEG slowing Thalamocortical rhythm Heterogeneous, non-specific
fMRI connectivity Network reorganization Reverse inference

Evidence interpretation map

The table makes the evidence role and inferential boundary explicit; it is not a replacement for the full reports.

PMID Year Evidence role What it cannot establish alone
40382320 2025 Complement-dependent inhibitory-synapse pruning in mice Human disinhibition mechanism or treatment efficacy
35357926 2022 Resolution-associated CD11c-positive microglia in mice That broad microglial activation is protective
36701202 2023 TRPV4–lipocalin-2 neuroimmune axis in mice Target engagement or benefit in people
40128335 2025 Mechanistic synthesis of microglia induction/maintenance A patient-selection biomarker

Minimum reporting controls

Domain Required report
Case definition Possible, probable or definite neuropathic pain
Etiology Lesion/disease and diagnostic evidence
Distribution Focal, length-dependent, dermatomal, at-level or below-level
Baseline phenotype Negative and positive sensory signs
Comparator Placebo/sham, active care or natural history
Exposure Dose, duration, adherence and co-interventions
Benefit Mean change plus ≥30% and ≥50% responders where applicable
Function Sleep, mobility, participation and patient global change
Harm Adverse events, withdrawals and serious events
Durability Follow-up after treatment and attrition
Subgroups Prespecified interaction test, not within-group significance
Missingness Denominator and imputation method

Reporting cautions

  • Do not infer lesion presence from a symptom descriptor.
  • Do not convert a group-average association into an individual diagnostic rule.
  • Do not treat statistical significance as clinically important benefit.
  • Do not compare NNTs without checking outcome threshold, duration and population.
  • Do not interpret an inactive or completed registry record as proof of efficacy.
  • Do not merge painful and painless neuropathy outcomes.
  • Do not omit adverse-event withdrawals from responder interpretation.
  • Do not call a post hoc subgroup predictive without an interaction test.
  • Do not generalize a focal peripheral result to central neuropathic pain.
  • State when evidence is short-term, indirect or restricted to a selected cohort.

Open questions

  • OQ-1: Can peripheral-block response quantify ongoing afferent dependence?
  • OQ-2: Which multimodal marker identifies disinhibition specifically?
  • OQ-3: Can early treatment prevent consolidation after nerve injury?
  • OQ-4: Do central phenotypes predict rTMS or SCS response?

References

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