Introduction

Demyelinating disorders of the CNS, while bound together by a common process involving damage to or improper laying-down of the myelin sheath, do not share a common aetiology. This is demonstrated by recent advances, which have enabled the classification of these conditions into three groups based on disease aetiology: genetic, viral and autoimmune.

Pelizaeus–Merzbacher disease exemplifies the genetic component of certain demyelination conditions. This X-linked recessive disorder affecting CNS myelination in children is attributed to deficient biosynthesis of the proteolipid protein (PLP) (Koeppen et al., 1987). Lack of PLP, one of the most abundant components of the myelin sheath, contributes to physical symptoms such as spastic extremities and lack of purposeful movement and head control.

The viral aetiology of demyelinating diseases is seen in progressive multifocal leukoencephalopathy (PML). Originally described as a complication of leukaemia and Hodgkin's disease (Aström, Mancall & Richardson, 1958), PML develops mostly in immunocompromised patients with chronic diseases and, more recently, in individuals with AIDS (Levy, Bredsen & Rosenblum, 1985; Niedt & Schinelle, 1985). Symptoms in PML reflect the widespread destruction of the CNS white matter. Neurological deficits include dementia, confusion, aphasia, hemiparesis and ataxia. The viral aetiology of PML has been demonstrated by isolation of the papovavirus JC from the brains of diseased patients (Padgett et al., 1971) and by the demonstration that transgenic mice with part of the JC viral genome develop a neurological disorder that involves dysmyelination of the CNS (Small et al., 1986; Feigenbaum, Hinrichs & Jay, 1992).