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Pain is an unpleasant emotional and sensory sensation which is actually or potentially damages the tissue or described in terms of such damage.
Etiology and Neurophysiology
Neuroanatomy of pain transmission
The majority of tissues and organs are innervated by special sensory receptors (nociceptors) connected to primary afferent nerve fibers of differing diameters. Small myelinated Aδ fibers and unmyelinated C fibers are responsible for the transmission of painful stimuli to the spinal cord where these afferent primary fibers terminate in the dorsal horn.
Neurotransmitters and pain
Various neurotransmitters in the dorsal horn of the spinal cord are involved in pain modulation. These include amino acids such as glutamate and γ-aminobutyric acid (GABA), monoamines such as noradrenaline and 5-hydroxytryptamine (5-HT, serotonin) and peptide molecules, of which the opioid peptides are the most important. Opioid receptors are found in both the CNS and the periphery; in the CNS they are found in high concentrations in the limbic system, the brainstem, and the spinal cord.
Assessment of pain
Evaluation of pain should include a detailed description of the pain and an assessment of its consequences.
There should be a full history, psychosocial assessment, medication history and assessment of previous pain problems, paying attention to factors that influence the pain. Diagnostic laboratory tests, imaging, including plain radiography, computer tomography (CT) and magnetic resonance imaging (MRI), and diagnostic nerve blocks may aid confirmation of the diagnosis.
Pain is a subjective phenomenon and quantitative assessment is difficult. The most commonly used instruments are visual analog and verbal rating scales.
Management Of Pain
Acute pain usually results from noxious stimulation as a result of tissue damage or injury. It can be managed effectively using analgesic drugs and is often self-limiting.
Persistent pain may be considered as pain which continues beyond the usual time required for tissue healing.
Treatment may involve specialist pain management services, hospices and a multidisciplinary approach that assesses and manages patients using a biopsychosocial approach.
Initial treatment is usually directed at the underlying disease process where possible, for example, medication, surgery or antitumour therapy. However, non-medical treatments such as physical therapy and various psychological techniques including cognitive behavioral therapy may also form part of a multimodal treatment program.
Pain can be modulated using non-pharmacological techniques: for example, stimulation produced analgesia such as transcutaneous electrical nerve stimulation (TENS), acupuncture and massage, or invasive procedures such as neurosurgery or neurolytic nerve blocks.
Treatment Of Pain
Analgesic drugs
Paracetamol
Despite being used in clinical practice for over 50 years and much investigation, the mechanism by which paracetamol exerts its analgesic effect remains uncertain. Inhibition of prostaglandin synthesis within the CNS has been proposed, although this is probably not the only mechanism. Interaction with the serotonin and endocannabinoid neurotransmitter systems have been demonstrated in animal models.
Non-steroidal anti-inflammatory drugs
NSAIDs exert their analgesic and anti-inflammatory effects through inhibition of the enzyme cyclooxygenase. NSAIDs are used widely to relieve pain, with or without inflammation, in people with acute and persistent musculoskeletal disorders. In single doses, NSAIDs have superior analgesic activity compared to paracetamol. Higher doses of NSAIDs have both analgesic and anti-inflammatory effects, which makes them particularly useful for the treatment of continuous or regular pain associated with inflammation.
Codeine is the prototypical drug in this group. It is structurally similar to morphine and about 10% of the codeine is demethylated to form morphine, and the analgesic effect may be due to this, at least in part.
Dextropropoxyphene Historically, dextropropoxyphene was prescribed in combination with other analgesics such as paracetamol (co-proxamol). There are few data on its therapeutic value, and at least one review concluded that analgesic efficacy is less than aspirin and barely more than placebo. At best, dextropropoxyphene failed to show any superiority over paracetamol. At worst, it is a dangerous drug which has the potential for steadily developing toxicity.
Strong opioids
Morphine: Morphine is the ‘gold standard’ strong opioid analgesic. It is available for administration by a range of administration routes, including oral, rectal and injectable formulations and has a duration of action of about 4h after oral administration. There is no ceiling effect when the dose is increased.
Other strong opioids
Opioids such as pethidine and dextromoramide offer little advantage over morphine in that they are generally weaker in action with a relatively short duration of action (2h).
