Acute Pain Mechanisms

Acute pain is the protective pain that arises from an immediate noxious event or tissue injury and normally resolves as healing proceeds. Its mechanisms include the activation of nociceptors, transmission of nociceptive signals to the spinal cord and brain, and short-lived sensitization that promotes guarding and recovery of the injured region.

Definition

Acute pain is pain of recent onset and limited duration that is usually associated with identifiable tissue injury or a noxious stimulus and that typically subsides as the underlying cause resolves.

Scope

This topic covers the physiology of normal, protective acute pain: nociceptor activation, signal transmission, the brain's processing of acute nociceptive input, and the early sensitization that accompanies tissue injury. It also notes how acute pain can, in some circumstances, transition toward persistence. It is reference material on mechanisms, not clinical management.

Core questions

• How does tissue injury activate nociceptors and generate acute pain?
• How is acute nociceptive input processed in the spinal cord and brain?
• What short-term sensitization accompanies acute injury, and why is it adaptive?
• Under what conditions does acute pain transition to persistent pain?

Key concepts

• Protective nociceptive pain
• Tissue injury and inflammation
• Peripheral sensitization
• Primary and secondary hyperalgesia
• Ascending pathways and cortical processing
• Transition to chronic pain

Key theories

Protective acute pain with adaptive sensitization

Acute pain serves a protective function: nociceptor activation and a reversible increase in the gain of the pain system promote behaviors that protect injured tissue and support healing, with sensitization normally subsiding as the injury resolves.

Mechanisms

An acute noxious event activates peripheral nociceptors, whose signals are conducted to the dorsal horn and relayed through ascending pathways to brain regions that generate the sensory and affective dimensions of pain. Local tissue injury releases mediators that sensitize nociceptor terminals, lowering thresholds at and around the injury (primary and secondary hyperalgesia) so that the area is protected during healing. This reversible amplification is normally self-limited. However, intense or sustained nociceptive input, as can occur with surgery, may trigger longer-lasting changes that contribute to persistent postsurgical pain in a subset of cases.

Clinical relevance

Acute pain is a near-universal experience after injury and surgery, and its mechanisms explain why injured areas become temporarily more sensitive. This entry provides background on those mechanisms for education and does not offer guidance on assessing or treating pain in individuals.

Epidemiology

Acute pain accompanies most injuries and surgical procedures. A recognized minority of patients develop persistent pain after surgery, which is why the mechanisms linking acute and chronic pain are an active area of study.

Evidence & guidelines

The mechanistic account follows reviews of pain neurobiology and cortical pain processing (Basbaum et al., 2009; Tracey & Mantyh, 2007; Woolf & Salter, 2000), with the acute-to-chronic transition discussed in the context of persistent postsurgical pain (Kehlet et al., 2006).

History

Acute pain was long understood mainly as a symptom of injury, but research on activity-dependent plasticity reframed it as a state in which the pain system can rapidly and reversibly increase its sensitivity. Recognition that poorly controlled acute pain, especially after surgery, can presage chronic pain further sharpened interest in acute pain mechanisms.

Key figures

• Clifford Woolf
• Henrik Kehlet
• Irene Tracey
• Allan Basbaum

Seminal works

• basbaum-2009
• woolf-2000
• kehlet-2006
• tracey-2007

Frequently asked questions

What distinguishes acute pain from chronic pain?

Acute pain is of recent onset, usually tied to identifiable injury or a noxious stimulus, and typically resolves as healing occurs, whereas chronic pain persists beyond expected healing and often involves maladaptive nervous-system changes.

Why does an injured area become extra sensitive?

Tissue injury releases mediators that sensitize nociceptors in and around the site, producing hyperalgesia that protects the area during healing; this heightened sensitivity is usually temporary.