Over the last 10 years GPs have become increasingly confident in using morphine for pain control in patients in the palliatve care setting. The use of morphine suspension every four hours or the longer acting controlled release morphine 12 hourly seems straightforward to use on the surface, but there are a number of subtleties that can cause problems. This paper will outline some of the more common problems associated with morphine use.
To begin with, it is worth emphasising again the limitations of the analgesic ladder approach to pain control, which determines choice of analgesics solely on pain level rather than the cause of the pain. The other problem is that sometimes the pain escalates despite the use of high doses of morphine. To sort through the complexities of a patient in severe pain it is useful to have a model to work by.
The four step approach described by Professor Norelle Lickiss and her team from the Department of Palliative Care, Royal Prince Alfred Hospital, is very helpful. It takes into account the cause of the pain as well as attacking the pain pathway at different levels.
STEP 1
Dampen the noxious stimulus
Always attempt to decrease the noxious stimulus at the periphery and thus decrease nociception. This is done by dealing with pain at its source where free nerve endings may be stimulated in the skin, connective tissue and viscera by pressure, heat, inflammation, tissue injury, etc. The tumour may be dealt with by DXT or chemotherapy; if infection is present antibiotics are used; or sometimes paracetamol (just a gram of the humble panadol taken 4-6 hourly can work wonders with severe cancer pain); or NSAIDs. Other measures are dexamethasone, local blocks, immobilisation, massage, etc. The penalty for not addressing Step 1 is to move onto the opioid drugs in unnecessarily higher doses than would be needed if the above measures were used. The side effect rate and potential for morphine toxicity could increase.
STEP 2
Always raise the pain theshold by reducing anguish.
Anguish and suffering undoubtedly affect the way pain is perceived. We all know that our patients can feel the pain more when they are depressed, anxious, angry, lonely, frightened, etc. Care, comfort and concern are very much in the hands of GPs, nursing staff, family and other supporters, and are a higher priority than pharmaceutical intervention. However, sometimes anxiolytics are
useful and benzodiazepines are usually the best to use.
Pain caused by anguish is relatively morphine resistant and to use morphine to help a patient settle is not a good idea. Many a patient has become morphine toxic by morphine being used as a sedative.
MORPHINE IS A BAD SEDATIVE. Given in high enough doses it will eventually cause drowsiness, agitation, confusion, irritability, hallucinations and possibly myoclonic jerks.
STEP 3
Sometimes add in (not replace by) an opioid.
Start with a weaker opioid like codeine and titrate up to the appropriate dose. Move on to morphine if needed, again starting with smaller doses. Some people, especially the elderly and frail only require very small doses, say 2.5 mgm. Others sensitive to opioids are those with renal failure which is often seen in the terminal stages of life and morphine doses and frequency may need reducing to avoid toxicity.
The other problems seen with morphine use are when the controlled release tablets are used without regard to their long duration of action. Just as it is wrong to use long acting insulin to stabilise a newly diagnosed diabetic so it is wrong to start a patient on say MS Contin without first finding the correct 24 hour requirement using four hourly suspension of morphine. This may take a few days.
Similarly it is wrong to use MS Contin for breakthrough doses. It may result in a large build up of opioid blood levels and a high risk of toxicity.
Another problem seen with morphine is the escalation of dose when clearly there is little analgesic benefit. Not only is it a bad sedative, it is also useless trying to use it to shorten life. Quite aside from legal and ethical considerations such usage rarely achieves the aim and morphine toxicity is a major risk. Morphine toxicity is not only distressing for all concerned but often requires a number of days and possibly hospital admission to sort it out.
STEP 4
Recognise neuropathic pain and treat accordingly.
This problem arises from irritation or destruction of a peripheral nerve or plexus. It results in severe burning or shooting pain and may only be partially helped by morphine. It is important to recognise this type of pain and introduce a neuropathic agent rather than escalating the dose of morphine. Commonly used neuropathic agents are anticonvulsants, tricyclic antidepressants,
corticosteriods and local anaesthetic congeners.
Don't forget the bowels
It sounds elementary to always use laxatives when introducing opioids but it is surprising how often preventable constipation is caused by this omission. It is very easy to be so focused on the pain that such measures are forgotton.
Nausea
This can usually be managed with anti-emetics but if not it is worth trying a switch to subcutaneous morphine, remembering to at least halve the dose.
High bowel obstruction
In these cases morphine is unpredictable in its absorption and it may be prudent to use the subcutaneous route in these instances.
Overall morphine is a highly effective analgesic with few problems if used logically. However there are a few traps for the unwary.
Dr Andrew Binns is the director of the PCU at St Vincent's Hospital, Lismore, NSW, Australia.
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