Understanding the pathophysiology of different kinds of pain a cancer patient can develop is the essential first step to logical pain treatment. Cancers can cause both nociceptive and neuropathic pain. Sometimes, both mechanisms are at work to produce the symptoms. Other pains include colic from blockage of a hollow viscus, which may be treated with an anti-spasmodic, and raised intracranial pressure, which is relieved by pressure reduction, such as with corticosteroids.
Nociceptive pain is pain associated with tissue damage. This may be caused by inflammation, trauma, thermal or chemical insults. Cancer cells may infiltrate bone, induce cytokines that break down bone leading to inflammation, weakening or fracture. Metastatic cancer to the liver may cause stretching of the pain-sensitive and non-elastic liver capsule.
Nociceptive pain responds well to the common analgesics, such as paracetamol for mild pain, weak opioids like tramadol or codeine for pain of moderate severity, and strong opioids like morphine or fentanyl for severe pain. Pains that have an inflammatory element would also respond to an anti-inflammatory drug like an NSAID or Coxib, often used in conjunction with the other analgesics as an adjuvant.
Neuropathic pains are pains which are perceived not as a result of stimulation of nerve endings by noxious stimuli, but rather because of disturbance of the function of a peripheral nerve or its connections in the central nervous system. Cancers can cause this by pressure on or infiltration of peripheral nerves, nerve plexuses or nerve roots. The nature of this pain is often different from nociceptive pains.
Some patients may refuse to call this sensation pain, but prefer the description of a severe ache. It may have electric shock-like qualities, like the jarring of the “funny bone” or ulnar nerve. Neuropathic pain may be accompanied by abnormal sensations, such as hypoaesthesia or numbness (like that following a dental nerve block), hyperaesthesia (like the hypersensitivity in normal skin surrounding an area that has been scalded), and allodynia (pain caused by a usually non-painful stimulus like stroking or movement of clothes or air across the skin).
These phenomena help in the diagnosis of neuropathic pain, as does the distribution of the pain, which may follow the distribution of a nerve or dermatome. Further details to pursue in the history include whether the pain is episodic, pain intensity and duration of each episode, frequency and precipitating factors.
Recognition of neuropathic pain enables the practitioner to use adjuvant drugs which modify neurotransmission, such as anti-depressants and anti-convulsants. Frequency, intensity and duration of pain episodes would be used to monitor effect of treatment, whether there is no response, a partial or complete response.
Dr
Cynthia Goh
Head, Palliative Medicine
National Cancer Centre, Singapore
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