What is the difference between allodynia and hyperalgesia?

Distinguishing Allodynia from Hyperalgesia

Allodynia is pain caused by stimuli that normally do not provoke pain (such as light touch or gentle pressure), while hyperalgesia is an exaggerated or increased pain response to stimuli that are normally painful (such as pinprick or heat). 1

Core Definitions

• Allodynia: An abnormal state where innocuous (non-painful) stimuli are perceived as painful 1

  • Examples include pain from light touch, gentle brushing of skin, or in gastrointestinal contexts, pain from normal food components, bile acids, or intestinal distension 1
  • Represents a fundamental shift in pain threshold where normally harmless stimuli cross into the pain domain 2

• Hyperalgesia: An exaggerated or prolonged response to noxious (already painful) stimuli 1

  • Examples include excessive pain from pinprick, heat, or in inflammatory contexts, amplified pain from low-grade inflammation 1
  • Represents heightened sensitivity where painful stimuli produce disproportionately intense pain 1

Mechanistic Differences

Peripheral Sensitization

• Both conditions can arise from peripheral sensitization, where inflammatory mediators (prostaglandins, leukotrienes, serotonin, histamine, cytokines) act on nociceptor terminals 1
• Primary hyperalgesia occurs at the site of injury or inflammation due to this peripheral sensitization 1
• Allodynia typically requires additional central nervous system changes beyond simple peripheral sensitization 2

Central Sensitization

• Secondary hyperalgesia and allodynia develop in surrounding uninjured tissue through central sensitization—increased excitability of spinal neurons and expanded receptive fields 1
• Allodynia specifically involves recruitment of normally non-nociceptive pathways (A-fiber low-threshold mechanoreceptors) that become capable of transmitting pain signals 3
• Hyperalgesia to punctate stimuli involves heterosynaptic facilitation where C-fiber input amplifies A-fiber nociceptor responses 3

Clinical Context Examples

In Inflammatory Bowel Disease

• Allodynia: Patients experience pain from innocuous intestinal stimuli like certain foods, normal intestinal contractions, or mild distension—stimuli that would not cause pain in healthy individuals 1
• Hyperalgesia: Patients have exaggerated pain responses to low-grade inflammation or mild strictures that would cause minimal discomfort in others 1

In Neuropathic Pain

• Allodynia: Light touch or clothing contact causes pain in 15-50% of neuropathic pain patients 2
• Hyperalgesia: Pinprick or thermal stimuli produce disproportionately severe pain 2, 4

In Opioid-Induced Pain Sensitization

• Both allodynia and hyperalgesia can develop as paradoxical responses to opioid therapy through NMDA receptor-mediated neuroplastic changes 5, 6
• Remifentanil particularly causes clinically significant hyperalgesia with higher postoperative pain intensity 5, 6

Sensory Modality Classification

Both phenomena are classified by the sensory modality that elicits them 2:

• Mechanical allodynia/hyperalgesia: Touch, pressure, or punctate stimuli 2, 7
• Thermal allodynia/hyperalgesia: Cold or heat stimuli 2, 4
• Cold allodynia is particularly common in neuropathic pain, where cold detection thresholds are significantly decreased 4

Clinical Pitfalls

• Do not confuse with tolerance: Hyperalgesia represents actual increased pain sensitivity, not simply requiring higher doses for the same effect 6
• Recognize the temporal relationship: Allodynia often indicates chronic pain states where central sensitization has become established, while hyperalgesia can occur in both acute and chronic contexts 1
• Assess the stimulus quality: If the patient reports pain from normally non-painful stimuli (gentle touch, room temperature), this is allodynia; if they report excessive pain from clearly noxious stimuli (firm pressure, hot/cold extremes), this is hyperalgesia 1