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Neuropathic pain - the 'invisible illness'

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Neuropathic pain - the 'invisible illness'

The man opposite you on the train has one leg in plaster and one arm in a sling. Each time he moves, he grimaces with pain. Few of us could fail to have a degree of sympathy for his suffering. The apparently normal man seated next to him may be experiencing excruciating burning and stabbing pain in his legs and feet as a result of his MS. Who is to know, though? There is nothing to see to explain his pain. Neuropathic pain is truly an invisible illness.

What is pain?

All of us have experienced pain. For most that will have been related to some sort of injury or operation. This is called acute or nociceptive pain. As the affected tissues heal, so the pain will go away. Conventional painkillers and antiinflammatory drugs may help and speed up this process.

Chronic pain is not simply acute pain but lasting longer - it is a very different clinical and physiological problem. Not only is there constant input of pain over a long period of time, the pain is often periodic in nature - increasing and decreasing for no apparent reason.

We now know that the continual pain can actually increase the sensitivity of the pain receptors and can even radically change the way in which the brain and spinal cord process that pain input. For some people, even acute pain from an area of the body unrelated to the site of their ongoing chronic pain can be increased in severity. These processes are termed 'peripheral and central sensitisation'.

What sort of pain occurs in MS?

Nociceptive pain - from damage to muscles and joints - can be either acute or chronic. This may not be directly due to MS itself, but be as a consequence of having MS eg from changes in posture or walking, from the over-use of crutches or a wheelchair or where weakness in one group of muscles leads to strain in another group.

Neuropathic pain affects up to 25% of people with MS. It is a consequence of damage to the myelin that surrounds nerves in the central nervous system.

Very commonly the pain is not due to physical damage to the nerve but as a result of a physiological change within it. This means that even when the initial cause is treated or removed, the nerve remains permanently changed and the resulting painful symptoms can become a long-term problem. Once present, neuropathic pain is invariably chronic.

Spasmodic pain is frequently associated with spasticity and muscle spasms. It can be severely debilitating.

Paroxysmal pain is short unpredictable bursts of severe shooting pain, normally neuropathic in nature. This pain can either be 'spontaneous', where the nerves fire off for no apparent reason, or 'evoked', where a physical movement or stimulus causes the nerve to fire off.

Pain is a common symptom for people with MS and many report the effects as being severe1. People with MS usually have a mixture of both nociceptive and neuropathic pains. Many people can learn to cope with the constant background pain, but it is the unpredictable paroxysms that are most distressing. MS pain is commonly under-reported, under-recognised and under-treated.

Why is a part of my body numb but painful as well?

Damaged nerves prevent sensory information from reaching the relevant area of the brain. The brain thus thinks that this area of the body is numb. However, many of these same pathways are also made hypersensitive so that the nerve itself can fire off impulses and start the pain impulse to the brain from the area of damage. The result is that the brain perceives pain from an area that it also perceives to be numb - known as anaesthesia dolorosa. In addition, hypersensitive nerves can mean that normally nonpainful stimuli (such as lightly brushing the skin) can be perceived as severe pain - called allodynia.

Why don't normal painkillers help me?

The answer is simple - because your pain is not just nociceptive pain but is neuropathic as well. Analgesics such as paracetamol and antiinflammatories such as ibuprofen help nociceptive pain but have no benefit in neuropathic pain. Even very strong drugs such as morphine have very limited effects.

We need to choose the right sort of drug for the right sort of pain. Some of these drugs may be associated with other conditions.

Why am I being given anti-depressants when I am not depressed?

Clinical depression is associated with insufficient levels of two chemicals in the brain - serotonin and noradrenaline. Anti-depressants raise the levels of these chemicals, which lifts mood and reduces depression. The same chemicals are involved in parts of the brain and spinal cord that deal with pain - the Descending Inhibitory Pain Pathways. Thus for many people, there is a crossover of effects - not only can these drugs act as anti-depressants but can reduce pain as well. The drugs most commonly used are known as tri-cyclic anti-depressants eg amitriptyline, nortritptyline, imipramine. The dosage needed to help chronic pain is only 10 - 20% of that needed to act as an anti-depressant.

Why am I being given anticonvulsant drugs when I don't have epilepsy?

In simple terms, people with epilepsy have a hypersensitive focus of nerves in their brain which fire off and cause a seizure. Anti-convulsant drugs reduce the sensitivity of these nerves, preventing them from spontaneously firing off. The oversensitive nerves causing neuropathic pain can similarly be treated. Gabapentin and pregabalin are the most common drugs used. Carbamazepine, the drug of choice for trigeminal neuralgia, is another such drug.

Is cannabis the answer?

Sadly, no. Whilst there are some encouraging results from the use of the cannabis based drug Sativex, the evidence as yet is not overwhelming. It is licensed to treat spasticity, and there is also some limited evidence that it reduces central neuropathic pain, but it has no analgesic effect on nociceptive pain.

It's not just about pain...

The perception of any pain is not just the result of a physical stimulus. Many other factors are equally important in the perception of that pain - psychological factors, ongoing stresses, someone's understanding and beliefs about their condition, the environment in which someone lives, etc. We now talk in terms of a 'biopsychosocial' model of chronic pain where each of these factors is given relevant status. For successful management of chronic pain it is essential that individuals, carers and healthcare professionals all fully embrace this concept. Just thinking that a pill, operation or injection will cure it all is neither helpful nor true.

Treatment or management?

We can rarely cure chronic pain - the best we can often do is to reduce it. As for many people the expected result of treatment is a cure, most chronic pain specialists now use the term management. This can encompass the whole range of individual therapies that will be needed if we are to fully use the biopsychosocial approach to chronic pain.

As with many other complex diseases, people with MS experience a whole range of symptoms - of which pain is just one. Also there will be many different elements to just that one symptom of pain. By carefully teasing out each individual component and providing specific therapy, a satisfactory management plan can then be devised for each individual. It may take time to find the right combination of treatment and dosage to best manage the pain. Optimal therapy always demands understanding, time and patience.

Written by Dr Steve Allen, Consultant in Chronic Pain Management, Oxford University Hospitals Trust for Open Door - August 2012

More references

  • O'Connor AB, et al. Pain associated with multiple sclerosis: systematic review and proposed classification. Pain 2008;137(1):96-111. Summary

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