Expansion
Transduction, transmission, modulation and perception
Expansion
- Transduction: noxious stimuli activate nociceptors. Targets: NSAIDs and local anaesthetics
- Transmission: A delta fibres carry fast sharp pain, C fibres slow dull pain, to the dorsal horn and then the spinothalamic tract. Targets: local anaesthetics, regional blocks
- Modulation: in the dorsal horn, by the gate mechanism and by descending inhibition from the periaqueductal grey using endogenous opioids, serotonin and noradrenaline. Targets: opioids, tricyclics, SNRIs, clonidine
- Perception: cortical and limbic processing, influenced by attention, mood and expectation. Targets: psychological therapies, and the placebo response
Types of pain
- Nociceptive: tissue injury, responds well to conventional analgesia
- Neuropathic: nerve damage, with burning, shooting pain, allodynia and hyperalgesia. Responds poorly to opioids and better to gabapentinoids, tricyclics and SNRIs
- Nociplastic: altered central processing, as in fibromyalgia
Wind-up and central sensitisation explain why untreated acute pain becomes harder to control, and why pre-emptive and multimodal analgesia is preferred.