Types of Pain

Overview – Types of Pain

Pain is a complex sensory and emotional experience linked to actual or potential tissue damage. Understanding the numerous types of pain, neurobiology, and treatment is crucial for clinical management. This page outlines pain classifications, mechanisms of sensitisation, neuropathic pain syndromes, clinical reporting challenges, and evidence-based therapeutic targets.

Types and Features of Pain

Altered Pain Sensitivity

  • Hyperalgesia: Mild noxious stimulus causes heightened pain
  • Allodynia: Pain from a non-noxious stimulus

Both may arise from:

  • Healing injury (temporary potentiation): Substance P, Neurokinin A, CGRP, Prostaglandins, Bradykinin
  • Neuropathic origin (permanent potentiation): central wind-up (Substance P + Glutamate)

Altered Pain Sensation

  • Dysesthesia: Unpleasant abnormal sensation (burning, wetness, shock-like) – spontaneous or evoked
  • Paresthesia: Tingling or pricking sensation (e.g. pins and needles)

Neuropathic Pain

  • Chronic, severe pain without current peripheral tissue damage
  • Caused by dysfunction/lesion in the nervous system
  • Mechanisms:
    • Synaptic Remodelling (↓Threshold):
      • Neuromas
      • α-Adrenergic receptor upregulation
      • Altered Na+ channel expression
    • Causes:
      • Mechanical injury
      • Stroke
      • MS
      • Diabetic neuropathy
      • Shingles
  • Treatment:
    • Low-dose TCAs → inhibit reuptake of NE, 5-HT, Enkephalins in dorsal horn

Chronic Pain

  • Ongoing pain from progressive disease or non-healing injury
  • Often opioid-resistant
  • Discordance between self-report and behavioural signs common

Visceral vs. Parietal Pain

Visceral Pain

  • From internal organs
  • C-Fibres → dull, poorly localised
  • Triggers:
    • Ischaemia
    • Chemicals (ulcers)
    • Muscle spasms
    • Distension
  • Highly sensitive to diffuse damage

Parietal Pain

  • From serosal membranes (peritoneum, pleura, meninges)
  • Aδ-fibres → sharp, well-localised
  • Triggered by disease extension to parietal linings

Referred Pain

  • Pain perceived in an area unrelated to the stimulus
  • Occurs via convergent projection of visceral C-fibres and somatic Aδ-fibres in the dorsal horn
Types of pain - referred pain
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Reporting Pain

Paediatric Considerations

  • Common barriers to accurate reporting:
    • Communication difficulty
    • Fear of needles
    • Desire to please caregivers
  • Discordance: conflict between observed and reported pain
  • Assessment Tools:
    • Oucher Scale (visual analogue)
    • Caregiver reports
Paediatric pain scoring
Source: www.oucher.org

Adults

  • McGill Pain Questionnaire: 78 adjectives graded 0–5
McGill Pain Questionnaire

Pain Management

Key Principle

Always start with thorough pain evaluation. Tailor treatment to pain type and mechanism.

1. Peripheral Targets

TRPV1R Receptors

  • Capsaicin: Stimulates TRPV1 → Substance P release → temporary analgesia
  • Uses: Topical arthritis, neuropathic pain

Prostaglandin Pathway

  • NSAIDs (Ibuprofen, Aspirin): ↓PG via COX inhibition → ↓nociceptor sensitisation
  • COX-2 Inhibitors (e.g. Celecoxib): targeted anti-inflammatory action
  • Indications: Hyperalgesia, Allodynia, Mild-moderate pain

Opioid Receptors

  • Drugs: Morphine, Fentanyl, Codeine
  • Peripheral Action:
    • Opens K+ channels → hyperpolarisation
  • Indications: Acute/chronic pain

Use step-down approach to avoid dependence

Paracetamol

  • Mechanism: Unknown
  • Mild analgesic, weak anti-inflammatory
  • Best for non-inflammatory pain

2. Spinal Cord Targets

Key Sites

Therapies

  • Opioids: Close Ca2+ channels → ↓NT release
  • TCAs: Block reuptake of NE, 5-HT, Enkephalins → maintain inhibition
  • Massage therapy: Activates Aβ-fibres → inhibit nociception

3. Brain Targets

Key Sites

Therapies

  • Opioids: Remove PAG inhibition → activate descending pain inhibition
  • General Anaesthesia: CNS shutdown

Summary – Types of Pain

Pain encompasses a range of physiological and pathological mechanisms, from nociceptive stimuli to chronic neuropathic remodelling. Effective evaluation and treatment must consider the types of pain, pathway, and patient-specific context. For a broader context, see our Nervous System Overview page.

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