20.03.1 Introduction to Pain and Analgesia

There are two main types of pain; nociceptive and neuropathic. Nociceptive pain is due to the activation of pain fibres. The pain fibres may be somatic with the pain coming from the skin and muscle or visceral with the pain coming from organs such as the liver.

Neuropathic pain is due to nerve injury and the nerve injury can be due to disease or surgical section occurring in the pain pathway. Peripheral neuropathic pain is associated with disease such as post-herpetic neuralgia (after shingles, herpes zoster can damage nerves), or diabetic neuropathy, where the high levels of sugars lead to damage to the nerves. Phantom limb pain, where a limb has been removed but the subject is still aware of pain from the limb is another example of peripheral neuropathic pain. Central neuropathic pain is associated with spinal cord injury, where the nerve is injured by trauma, and stroke, where the nerve is damaged as a result of the ischaemia (lack of oxygen/nutrients).

Pain can be either acute or chronic. Acute pain is usually rapid onset, short duration, and the cause is often known (e.g. disease, trauma). Pain can also be chronic, which is defined as lasting for more than 3 months. Chronic pain can be cancer or non-cancer pain.

Opioids are not the only drugs used to relied pain (analgesia). The analgesic used depends on the intensity of the pain (Figure 20.10).

Figure 20.10 Treatment of pain (Copyright QUT, Sheila Doggrell)

When the intensity of pain is low, the treatment is the non-opioid analgesics, predominantly the COX inhibitors such as aspirin, paracetamol, and naproxen. When the pain is greater and cannot be managed with the non-opioids, the weak opioids are used e.g. codeine. Codeine is partly metabolised to morphine, and acts as a weak opioid. The weak opioids are often used with the COX inhibitors. If the pain intensity is such that it cannot be managed with the weak opioids, strong opioids such as morphine are used in the treatment. Most pain can be managed be morphine, but if very intense pain cannot be managed with oral, or subcutaneous/ intramuscular/intravenous morphine, the morphine or other agents may be applied intraspinally. In addition to morphine, the local anaesthetics are used intraspinally in intense pain.