Phantom Pain is a form of nerve pain (neuropathy, neuralgia, neuritis) appearing to arise from an area of the body that has been removed or amputated, either surgically or traumatically. It is most commonly seen following amputation of the arm (upper limb) and leg (lower limb), but may arise following surgery to remove breasts, testicles, and even internal organs. Common amputee pain complaints including cramping, burning, shooting, stabbing as outlined in the diagram above. Some patients also feel as if their amputated digits are held in a fixed clawed position.
Phantom pain should be clearly distinguished from phantom experiences and stump pain:-
Phantom experiences are sensations that occur following amputation which are not painful, but which may cause distress through lack of understanding e.g. a patient may go to scratch the ankle in the leg which no longer exists. Phantom experiences occur due to the persistence of sensory maps for the amputated limb in the sensory cortex of the brain.
Stump pain is pain perceived locally in the limb stump, which is usually made worse by the pressure of wearing an artificial limb (prosthesis). This type of pain is often due to the presence of a neuroma (benign cauliflower shaped growth) which has sprouted at the cut end of a major nerve in the limb (femoral and sciatic nerves in the lower limb, median, ulnar, and radial nerves in the upper limb). When nerves are cut surgically, the end nearest the spinal cord sprouts out trying to re-establish connection with the other cut end. In the case of an amputation this can never happen, and the result is a neuroma. Neuroma's can give rise to spontaneous pain due to random inappropriate electrical discharge, and also pressure pain when the neuroma is stimulated mechanically. In some cases the stump neuroma is causing the phantom pain in the leg also - a type of referred pain.
Risk Factors - The following have been associated with an increased risk of developing phantom pain:-
Poorly controlled pre-amputation pain
Persisting stump pain afterwards
Bilateral amputations (both legs)
Lower limb more than upper limb amputations
Chronic Sciatica - There is a weak suggestion that phantom pain may occur more commonly in those who have had chronic sciatica in the leg prior to amputation. In some patients MRI scanning of the lumbar spine reveals a disc prolapse large enough to be causing lower lumbar nerve root irritation and referred pain to the leg. Treatment with epidural steroid injections can help in some of these patients. Lumbar disc surgery carries significantly higher risks in patients already suffering from widespread peripheral vascular disease, and is often not an option.
Pain Mechanisms - why does phantom pain occur ? The exact reason is not known, but theories about the pain can be divided into three areas:-
The periphery
Random ectopic nerve firing at the end of a cut / damaged peripheral nerve may lead to dorsal horn sensitisation
Increased neuroma sensitivity to mechanical stimuli (pressure, rubbing)
Dorsal horn nerve reorganisation secondary to injury to peripheral nerves may occur. Small C-fibre sensory nerves die away after they are cut in the periphery. Large A-beta sensory nerve fibres then unplug from their usual sockets in the dorsal horn telephone exchange, and link up with where the C-fibres originally were. This may explain why relatively harmless sensations like light touch can be perceived as painful afterwards.
Persisting dorsal horn hyper-excitability (sensitisation) may occur secondary to it receiving a constant barrage of nerve impulses from the periphery. Having the dorsal horn in this state causes an exaggerated response to all painful impulses.
Excitatory chemicals like glutamic acid and aspartic acid may cause dorsal horn sensitisation through activation of NMDA receptors. Other substances like substance P and calcitonin gene related peptide may also have a role to play.
Activation of dormant silent ascending nerves in the spinal cord may occur when the dorsal horn has become sensitised. Once these silent ascending nerves have been activated they are very difficult if not impossible to switch off again. Intravenous lignocaine infusions have been said to help by inhibiting sodium channels present in the silent nerve membrane.
The brain
Reorganisation of the nerve fibres in the cerebral cortex may occur following amputation. Cortical Sensory Mapping shows where sensations from different parts of the body are normally processed in the brain. The size of the area in the sensory cortex of the brain is proportional to the number of sensory nerves per square centimetre in the sensory area of the skin supplied by those nerves. The resultant cortical sensory map is often referred to as The Homunculus .
The area in the sensory cortex of the brain responsible for receiving messages from a normal limb may begin to receive message from other areas after the limb has been amputated (Cortical remapping). This may explain why phantom pain sufferers feel an increase in their phantom arm pain by touching part of their face on the same side of the body, as on the Homunculus the face and hand are very close together.
Managing the Pain - There is evidence to suggest that the severity of phantom pain is worsened by poor pain management before, during and after amputation. Better pain management can prevent dorsal horn sensitisation, and also prevent the opening up of silent pain pathways (peripheral and central), which once activated are very difficult to close down. There are three areas where pain relief could be improved:-
Before amputation - Peripheral vascular disease leading to gangrene is a common cause of amputation, and it is clear that many patients have prolonged pain in the limb for many months before the eventual decision to amputate. Ischaemic pain (lack of oxygen to the limb) may have somatic and nerve components, and therefore patients may need combinations of paracetamol, morphine, anti-depressants, and anti-convulsants. Ketamine (NMDA Antagonist) is being used increasingly by intravenous infusion in hospital, to help difficult to control ischaemic pain. Consult your acute pain team for further advice.
During amputation - Peri-operative use of epidural infusions, local anaesthetic nerve blocks and patient controlled analgesia (PCA) morphine, can help to reduce the severity of pain and the degree of dorsal horn sensitisation, but have not been shown to decrease the risk of developing phantom pain. Consult your anaesthetist for further advice. Good surgical technique is also important during amputation. Adequate trimming back of the major nerves to the limb is important to prevent a neuroma being exposed to excessive load bearing when wearing a prosthesis.
After amputation - Involvement of the acute pain team in hospital is important in the early stages after amputation. Pain control techniques (IV ketamine, epidural infusion, IV PCA morphine) need to be continued until pain scores are down to mild levels. Some of the drugs used before amputation may need to be continued afterwards to control somatic and nerve pain symptoms. Early referral to the local chronic pain clinic is advised if pain continues to be difficult to manage afterwards.
Treatment
Drugs - all the usual anti-convulsants and anti-depressants have been used in phantom pain. Currently gabapentin seems to be the most successful with the least number of side effects.
Injecting around a stump neuroma with local anaesthetic and depot steroids in the pain clinic may be useful when it is thought that the neuroma is responsible for the phantom pain.
Sympathetic nerve blocks have been used. The current advice is against permanent phenol blocks in this area.
Mirror Box Therapy - in patients where the digits of the amputated limb seem to be held in a clawed position, using a mirror box can help the digits release. This involves observing and exercising the normal limb side by side a reverse mirror image of it. This fools the brain into thinking that the amputated limb is still there. Patients are encouraged to think that they are moving the digits of the amputated limb at the same time as the normal side. It appears to work through the Cortical Re-mapping Theory (see www.MirrorBoxTherapy.com and www.ReflexPainManagement.com for more information).
