CANCER PAIN: DEFINITIONS AND CLASSIFICATIONS-NOCICEPTIVE, NEUROPATHIC AND PSYCHOGENIC PAIN
Nociceptive pain. Nociceptive or physiological pain is produced by stimulation of specific sensory receptors ornociceptors located in the tissues. The neural pathways involved are normal and intact. Somatic pain from the skin and superficial structures is usually well localised and described as aching, sharp, throbbing or pressure-like. Visceral pain from deep seated structures is less well localised and felt over a larger area, and there is often referred pain to cutaneous sites. It may be described as a deep aching or throbbing pain which may be sharp if organ capsules are involved. Obstruction of hollow viscera causes gnawing or colicky pain.
Neuropathic pain. Neuropathic pain is caused by peripheral or central nervous system injury. Neuropathic pain from a lesion involving the central nervous system is referred to as central pain and is non-dermatomal in distribution. Pain from peripheral nerve lesions, sometimes referred to as deafferentation pain, is dermatomal in distribution. Pain occurs because the injured nerves react abnormally to stimuli or discharge spontaneously. Neuropathic pain is most often described as a burning, stinging feeling or as a shooting, lancinating pain. Terms used to describe neuropathic pain, such as hyperalgesia, dysaesthesia and allodynia.
Damage to sympathetic nerves may lead to sympathetic-type pain. It is characterised by burning pain and allodynia, similar to deafferentation pain. There are signs of sympathetic dysfunction in the affected area including vasomotor instability (erythema, pallor, oedema), sudomotor (sweating) abnormalities and trophic changes (thinning of the skin and atrophy of the subcutaneous tissue). Sympathetic pain is less sensitive to non-opioid and opioid analgesics but can often be relieved by a regional sympathetic nerve block.
Psychogenic pain. Psychogenic pain is pain for which there is no physical basis, in patients who have other evidence of psychopathology. Whilst psychological factors greatly influence the perception of pain, pure psychogenic pain does not occur in patients with advanced cancer.
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