[Pathophysiology of multiple sclerosis]

Patients with multiple sclerosis (MS) can exhibit an exceptionally wide variety of symptoms. This is largely due to the semi-random distribution of the lesions in the central nervous system (CNS). Most lesions occur in apparently "silent" areas in the brain, and so cause no detectable symptoms. The disease is therefore much more active than a mere clinical monitoring would suggest. Most symptoms are related to a loss of function. During the relapses, this is due to a failure of axonal conduction at the site of the lesion(s). The conduction block is caused by the local demyelination which prevents the saltatory conduction but also seemingly, to some extend, by the inflammation per se. Remissions are related to a recovery of function of the affected axons owing to a spreading of sodium channels along the demyelinated axolemma but also to cerebral functional plasticity and remyelination. However, nerve conduction remains slower and less secure than normal, easily altered by physico-chemical changes such as the increase in body temperature (Uhthoff's phenomenon). Remission is incomplete when the lesion has led to axonal transaction and therefore axonal loss. Progression in MS is mainly related to "slow-burning" diffuse and chronic axonal loss in a toxic inflammatory milieu. Lastly, some symptoms in MS are so-called "positive" arising from an acquired hyperexcitability of demyelinated axons and occur either spontaneously (e.g. paresthesias) or mechanically (e.g. Lhermitte's sign).