Inflammatory Pain

Inflammatory pain is the heightened pain that accompanies tissue inflammation, in which mediators released by injured and immune cells sensitize nociceptors and amplify pain signaling. It typically produces tenderness and an exaggerated response to stimuli at and around the inflamed site, and it normally subsides as inflammation resolves.

Definition

Inflammatory pain is pain arising from the sensitization of the nociceptive system by inflammatory mediators released during tissue injury and immune activation, characterized by lowered pain thresholds (peripheral sensitization) at and around the inflamed tissue.

Scope

This topic covers how inflammation sensitizes the pain system: the chemical mediators of the inflammatory soup, peripheral sensitization of nociceptors, the contribution of immune and other non-neuronal cells, and the resulting hyperalgesia. It is a reference treatment of mechanisms and does not address drug therapy or dosing.

Core questions

• Which inflammatory mediators sensitize nociceptors?
• How does peripheral sensitization lower pain thresholds in inflamed tissue?
• What roles do immune and other non-neuronal cells play in inflammatory pain?
• How does inflammatory pain relate to central sensitization?

Key concepts

• Inflammatory mediators (the inflammatory soup)
• Peripheral sensitization
• Primary hyperalgesia
• Immune cell and glial contributions
• Nociceptor transducers (e.g., TRPV1)
• Resolution of inflammation

Key theories

Inflammatory mediators and peripheral sensitization

Tissue injury and immune activation release a mixture of mediators that act on nociceptor terminals to lower their activation threshold and increase their excitability, producing peripheral sensitization and the tenderness characteristic of inflamed tissue.

Mechanisms

When tissue is injured or infected, damaged cells and recruited immune cells release a mixture of mediators, including protons, bradykinin, prostaglandins, cytokines, and growth factors, often referred to collectively as the inflammatory soup. These mediators act on receptors and channels at nociceptor terminals, sensitizing transducers such as TRPV1 and lowering activation thresholds so that the inflamed area becomes painful to stimuli that would normally be innocuous (primary hyperalgesia). Immune and other non-neuronal cells both initiate and sustain this sensitization, and persistent inflammatory input can also promote central sensitization. As inflammation resolves, the mediators subside and nociceptor sensitivity typically returns toward baseline.

Clinical relevance

Inflammatory pain underlies the tenderness of injured, infected, and inflamed tissues and is part of the body's protective response. This entry explains the underlying mechanisms for educational reference and is not a guide to selecting or dosing treatments.

Evidence & guidelines

The mechanistic account draws on reviews of pain neurobiology, nociceptor sensitization, and non-neuronal contributions to inflammatory pain (Basbaum et al., 2009; Dubin & Patapoutian, 2010; Ji et al., 2016), with TRPV1 as an example of a sensitized transducer (Caterina et al., 1997).

History

The idea that inflammation makes tissue painful through chemical sensitization of sensory nerves developed alongside the identification of specific inflammatory mediators and their receptors. The molecular characterization of nociceptor transducers and the recognition of immune and glial contributions clarified how inflammation amplifies pain.

Key figures

• Allan Basbaum
• Ru-Rong Ji
• David Julius
• Ardem Patapoutian

Seminal works

• basbaum-2009
• ji-2016
• dubin-2010
• caterina-1997

Frequently asked questions

Why does inflamed tissue become tender?

Inflammatory mediators released during injury and immune activation sensitize nearby nociceptors, lowering their thresholds so that the inflamed area responds painfully to stimuli that would normally not hurt.

How does inflammatory pain differ from neuropathic pain?

Inflammatory pain results from sensitization of an intact nociceptive system by inflammatory mediators, whereas neuropathic pain arises from damage or disease affecting the somatosensory nervous system itself.