Acute pain (defined as of < 3 months duration) is
7 Neuralgia is pain felt in the distribution of a peripheral nerve.
transmitted principally by fast conducting A-delta fibres (but to a lesser extent involves slow conducting type C fibres) and has major nociceptive input (physical trauma, pleurisy, myocardial infarct, perforated peptic ulcer). Patients perceive it as a transient, though sometimes severe threat and they react accordingly. It is a symptom that may be dealt with unhesitatingly and effectively with drugs, by injection if necessary, at the same time as the causative disease is addressed. The accompanying anxiety will vary according to the severity of the pain, and particularly according to its meaning for the patient, whether termination with recovery will soon occur, major surgery is threatened, or there is prospect of death or invalidism. The choice of drug will depend on the clinician's assessment of these factors. Morphine by injection has retained a preeminent place for over 100 years because it has highly effective antinociceptive and anti-anxiety effects; modern opioids have not rendered morphine obsolete.
Neuropathic pain follows damage to the nervous system. Acute pain without nociceptive (afferent) input (some neuralgias) is less susceptible to drugs unless consciousness is also depressed, and any frequently recurrent acute pain, e.g. trigeminal neuralgia, poses management problems that are more akin to chronic pain.
Chronic pain is transmitted principally by slow conducting type C fibres (but to a lesser extent by fast conducting A-delta fibres). It is better regarded as a syndrome8 rather than as a symptom (see above) for it is a collection of disparate pains of long duration, often sharing common emotional and behavioural aspects. It presents a depressing future to the victim who sees no prospect of release from suffering, and poses for that reason long-term management problems that differ from acute pain. Suffering and affective disorders can be of overriding importance and the consequences of poor management may be prolonged and serious for the patient. Analgesics alone are often insufficient and
8 A set of symptoms and signs that are characteristic of a condition though they may not always have the same cause (Greek: syn: together, dramein: to run).
adjuvant drugs as well as nondrug therapy gain increasing importance. Although dependence is less of a problem than might be feared, continuous use of high efficacy opioids, e.g. morphine, pethidine, is generally is best avoided in chronic pain (except that of palliative care). But the lower efficacy opioids (codeine, dextropropoxyphene) may often be needed and used.
Sedation should be avoided and therapy should be oral if possible; regimens should be planned to avoid breakthrough pain. Antidepressants can often be useful. Sedative-hypnotic drugs, e.g. benzodiazepines, may be needed for anxiety but may induce depression.
Chronic pain syndrome is a term used for persistence of pain when detectable disease has disappeared, e.g. after an attack of low back pain. It characteristically does not respond to standard treatment with analgesics. Whether the basis is neurogenic, psychogenic or sociocultural it should not be managed by intensifying drug treatment. Opioid analgesics, which may be producing dependence, should be withdrawan and the use of psychotropic drugs, e.g. antidepressants or neuroleptics, and nondrug therapy, including psychotherapy, should be considered.
Transient pain is provoked by activation of nociceptors in skin or other tissues in the absence of tissue damage. It has evolved to protect humans from physical damage from the environment or excessive stressing of tissues. It is a part of normal life and not a reason to seek medical help. Nevertheless, it is partly through the production of transient pain in physiological experiments that present concepts of pain have evolved.
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