Multiple sclerosis I
Multiple sclerosis (MS, disseminated sclerosis) is a common disorder affecting about 1 in 1000 individuals in the UK. It is a major cause of disability in young adults. The diagnosis of MS requires two separate episodes of central nervous system demyelination separated in space and time.
Pathology
The pathological hallmark of MS is the plaque. This is an area of demyelination, with loss of myelin and relative preservation of axons (Fig. 1). Active lesions may have an associated inflammatory response and oedema. In more chronic lesions, the oedema and inflammation have resolved and there is a demarcated area of gliotic scarring, with atrophy and axonal loss.
Pathophysiology
The area of demyelination disrupts the conduction of a nerve impulse (Fig. 2). This initially blocks conduction, but with recovery conduction is slowed and the refractory period is prolonged. Conduction along such segments is particularly sensitive to temperature changes and may fail if the temperature rises (which leads to Uhtoff’s phenomenon; see below).
Clinical features
The peak age of onset of MS is 25–35 years. It is rare below 15 years and over 60 years. It is more frequent in women than men (about 1.5 : 1). There are essentially two patterns of disease (Fig. 3):
