Neuropathic pain

When a nerve outside of the central nervous system (i.e. outside of the brain and spinal cord) functions abnormally, and that abnormal function causes pain, then the pain is said to be neuropathic - literally pain due to pathology of a nerve. The central nervous system consists of nerves, and disease of central nervous system neurons which leads to pain could therefore be called neuropathic pain, but the causes for central nervous system nerve dysfunction that leads to pain, the experience of the pain itself, the possible course of the illness, and the treatments that may be useful, differ so much from that of peripheral nerve pain, that the former is referred to as central pain, and will be described in a separate article. Neuropathic pain is thus here discussed, as it refers to dysfunction of the peripheral nerves.

Symptoms

No language has ever developed an accurate terminology for the varied way in which humans experience pain. The most obvious reason for this is that it is not possible for a person to observe what another is experiencing, but that we can only observe actions, or express feelings by analogy with common external experiences (the Problem of the Other Mind). This is both a medical and philosophical problem. Nevertheless, there are certain words and descriptions which tend to be used for the experience of neuropathic pain, some of which express pain as such, and others which express associated sensations. These together give clues as that the pain may be neuropathic. Common descriptors include words such as hot, burning, stabbing, lightning pains, shooting pains, electric shocks, tingling, itching, excruciating sensitivity, pins and needles, crawling under the skin, hot and cold, and bursting. To an outsider, these descriptions may seem quite bizarre, especially since they are complained of without one being able to see any physical event that correlates with the description.

Neuropathic pain can vary rapidly from being virtually absent, to being intolerable, without there being any external cause for why the sufferer has suddenly developed such an acute attack of pain. Associated symptoms of nerve damage may include loss of normal sensation, and muscular weakness. A notable experience may be that pain is felt in an area where no normal sensation is present. This is in sharp contrast to the usual experience that a body part that has no sensation, also has no pain. The person suffering from neuropathic pain frequently experiences pain in response to a stimulus which would normally be felt as completely innocuous. He can be intolerant of a sensation as mild as a cool breeze, the touch of clothing, or a gentle stroking of the skin. This altered feeling, where non-harmful stimulation is experienced as pain, is called allodynia, meaning a "different feeling". This allodynia can normally be experienced in the area surrounding and acute injury, or severe tissues infection, but in neuropathic pain the body structures in the affected area may appear quite normal. On the other hand, the person may notice that there are definite changes in temperature, colour, sweating and swelling in the painful area, which alters as the pain changes, and which have no clear local cause. This is related to the normal function of the nervous system to control blood flow and blood vessel permeability, as a function of normal circulatory and metabolic homeostasis.

In the longer term, the abnormality of nerve function which causes the pain also leads to changes in the way that the body tissue in the area grows. There may be loss of hair, or abnormal growth, dryness and thinning of the skin, brittleness of the nails and atrophy of the muscles in the affected area.

The area where the pain is felt can vary according to where the damage to the nerve is located. Where a nerve is injured where it exits from the spinal cord (e.g. following a herniation of an intervertebral disk - or "slipped disk") the pain is experienced in specific spinal dermatomal areas. Where a mixed peripheral nerve, or a plexus of such nerves is affected, the distribution is typical for that nerve, while a diffuse generalised peripheral pain, not localised to any known spinal nerve root or peripheral nerve distribution, generally follows disease of the smaller nerve fibres in the body. Neuropathic pain can be devastatingly severe, and intractable to all but drastic medications or procedures. This realisation is important both for the sufferer, and for those close to the person, since the typical case of neuropathic pain has very little external evidence of injury that would "explain" the degree of suffering, and the person may be quite unfairly accused of psychological illness, or of exaggerating or faking symptoms (malingering) in order to obtain some psychological, social or financial benefit.

Chronic neuropathic pain tends to be associated with significant psychological disturbances, especially complaints of depression. Expand a bit on the loss of quality of life, out of proportion to the apparent injury.

Signs

The signs are those of the cause, sensory and motor defects, altered sensitivity to normal and painful stimuli, sympathetic and trophic changes.

Causes and general pathophysiology.

See neuropathy. The causes are those of neuropathy: metabolic (including diabetic neuropathy), traumatic, infective, neoplastic, toxic, etc. Why pain? Theories of peripheral and central changes.

Treatment

Underlying cause (medical, surgical), medications (pain, adjuvants, associated pathology), invasive treatments. Psychological support. Education.

Combined nortriptyline and gabapentin, alone and in combination may be better than either medication alone.

Topical capsaicin may benefit about 1 of every 12 patients treated (number needed to treat = 12).

Recent advances

Newer drugs, surgery, electrical treatment.

See also

• Pain