MS occurs because of damage to the myelin sheath the thin protective layer of fatty membrane that surrounds the nerve tracts in the brain and, more importantly, the spinal cord.

Myelin sheath acts like the insulation around electrical wires, allowing the current or in the case of the human body, the electrical signals to travel through without loss of power or strength. Any damage to this protective layer can cause the electrical signals to "leak" and so lose power and become distorted.

Development and maintenance of a healthy myelin sheath depends on several things, including the presence of dietary essential fatty acids (Neuroepi demiology, 1992; 11: 214-25; Nature, 1967; 215: 821-3). This may be one reason why babies who are breastfed are less likely to develop MS in later life (BMJ, 1994; 308: 1411-2) breastmilk is a rich source of EFAs.

In MS this damage is caused by inflammation, which makes the sheath lose some of its covering a process known as demyelination. Hard scar tissue then forms over these damaged patches or lesions and MS (literally "many scars") is the result.

These patches of demyelination (known as plaques) can occur anywhere. Scarring means that wrong signals go to the muscles via the brain, and so the muscles don't work properly or at all. It can affect any part of the body, but the most usual areas are the eyes, brain, brain stem and spinal cord.

The most commonly reported symptoms of MS are double or blurred vision in one or both eyes, pins and needles in the extremities, slurred speech, difficulty in walking, dragging either foot, loss of coordination and balance, and loss of sensation anywhere in the body.

Thanks to magnetic resonance imaging (MRI) scanning, doctors can now confirm a diagnosis of MS with 95-99 per cent certainty (Radiology, 1991; 178: 447-51). But this doesn't take into account a patient's reactions to MRI, which involves putting the patient into a small enclosed cylindrical magnet and subjecting him or her to a magnetic field 50,000 times stronger than that of the earth. Reactions can range from mild stress to acute claustrophobia, panic attacks, memory loss and nausea (JAMA, Aug 1, 1993; Lancet, Sept 21, 1991). There is also concern about the possible cancer causing effect of MRI (People's Doctor, 1984; 10: 11) and the potential dangers of the magnetically active contrast agents injected into the patient to enhance the picture on the scanner.

In addition, while MRI is used to visualize the progress of the disease, assessing the progress of the disease can be fraught with uncertainties. For instance, there is often no correlation between the extent of the damage seen through MRI and the extent of the disability in MS patients (Ann Neurol, 1989; 25: 43-9; BMJ, 1990; 301: 44). MRI can tell a person how many lesions they have, but not the extent to which this may or may not affect their prognosis, supporting the view that good clinical judgement is the best way to diagnose MS.

Many theories lifestyle, dietary, physical trauma particularly to the head, viral, bacterial, geographical origins and genetic predisposition have been put forward as possible causes of MS. It is widely accepted now that multiple sclerosis has something to do with a response by the immune system (Clin Microb Rev, 1993; 6: 382-427). Because of this, much of medicine's time and energy has been spent looking for a "bug" which causes it. But while it's true that viruses such as shingles and herpes are often present in certain types of MS sufferers (Ann Int Med, 1984; 101: 514-26), it could be that they are fulfilling the role of opportunists in a weakened immune system.