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  Nerve Pain       Peripheral Neuropathy      
 
 
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Introduction

Peripheral Neuropathy (Neuralgia, Neuritis) is a type of nerve pain, and means that for some reason the peripheral nerves are not working properly. Peripheral neuropathy is not a disease, but usually a symptom of some other disease process. It is usually associated with loss of function in the nerve, commonly affecting sensations like touch, pain, vibration and position sense (joints etc), and may be associated with dysaesthesias (painful tingling sensations)and numbness.

The longest nerves of the body are most vulnerable to injury, and therefore the signs of peripheral neuropathy start in the hands and feet, the so-called "glove and stocking" distribution. It is usual for the neuropathy to affect each side of the body symmetrically, unlike many other disease processes e.g. sciatica.

Peripheral Nerves consist of three parts:-

  • The Cell Body is the part of the nerve that stores DNA and determines the health of the nerve cell. The Dorsal Root Ganglion is  a collection of cell bodies for all the sensory nerves in one spinal nerve root.
  • The Axon acts as an electrical wire, conducting messages from the peripheral tissues to the Dorsal Horn of the spinal cord.
  • The Myelin Sheath acts as electrical insulation for the axon, improving it's conduction speed, and ensuring that there is no cross-talk between adjacent axons. The thicker the layer of myelin, the faster the conduction speed of the nerve. Myelin is produced by Schwann cells, which wrap themselves around groups of axons in nerve bundles.

Causes of Neuropathy - There are more than 100 known factors that can lead to peripheral nerve damage. Each of the three parts of the nerve can be damaged by separate disease processes:-

  • Disease of the cell body
    • Poisons and toxins may affect the metabolic processes of the cell body, the commonest being alcohol abuse (25-30% of all cases), pernicious anaemia (a deficiency of vitamin B 12), exposure to heavy metals, other vitamin deficiency, and industrial hydrocarbons. 
    • Liver and kidney failure can cause metabolic derangement in the cell bodies, leading to axonal failure.
  • Disease of the axons
    • Interference with nerve blood supply (vasa nervorum) causes progressive axonal damage. Examples are:-
      • Diabetes Mellitus where poor sugar control accelerates arteriosclerosis (narrowing of the arteries) leading to occlusion of the vasa nervorum and progressive loss of peripheral nerve function.
      • Rheumatological conditions like Rheumatoid Arthritis, Systemic Lupus Erythematosis (SLE), Polyarteritis Nodosa (PAN), Sjogren's Syndrome (dry eyes), and Wegener's Arteritis can all produce inflammation of the small arteries (arteritis) supplying the peripheral nerves, leading to arterial occlusion and nerve failure.
      • Chronic peripheral vascular disease can also cause narrowing of the vasa nervorum, producing symptoms similar to diabetic neuropathy.
  • Diseases of myelin
    • Damage to the insulating myelin sheath around the nerve affects nerve function e.g. speed of nerve conduction, and  cross-talk between adjacent nerve fibres. Examples are:-
      • Hereditary conditions like Charcot Marie Tooth Disease where there is faulty myelin production due to a genetic defect.
      • Viral infections can trigger immune reactions to myelin producing syndromes like Guillain Barre Syndrome and Chronic Inflammatory Demyelinating Polyneuropathies (CIDP). Treatment with plasma exchange, immuno-globulins, and immuno-suppression drugs like steroids, azathioprine, and cyclosporin can help.
Treatment

 The mainstay of management in the pain clinic is making a diagnosis and recognition of reversible conditions

  • Withdrawal of alcohol.
  • Industrial toxin exposure recognition and withdrawal
  • Tighter control of blood sugar over a number of years has been shown to reduce the chances of developing diabetic neuropathy in later life.
  • Correcting vitamin deficiencies (vitamin B12 and others).
  • Improving blood flow to the peripheral limbs with a surgical vascular graft or a chemical sympathectomy.
  • Immuno-suppression for inflammatory arteritis and viral induced reactions.

Nerve Pain Suppression

  • Pain relief medications - the commonest combination is amitriptyline (10 - 50 mg) with either gabapentin (2400 mg per day) or pregabalin (600 mg per day).
  • Multiple action drugs e.g. tramadol (Zamadol), tapentadol (Palexia) block morphine, noradrenaline and serotonin receptors
  • Topical capsaicin 0.025% and 0.075% is used frequently for diabetic neuropathy. Qutenza 179 mg (8%) applied topically once every 3 months has also beeen found to be helpful
  • Intravenous lignocaine can be useful in some people.
  • Lidocaine 5% topical patches.
  • Acupuncture may be helpful.
  • TENS seems to aggravate rather than help.
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