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Demyelinating disorders of the central nervous system (CNS) may be divided into primary (cause unknown, eg, multiple sclerosis (MS)) and secondary (eg, infective, hypoxic–ischaemic, metabolic, toxic) processes. The underlying cause damages the myelin sheath and/or oligodendrocyte. MRI is the imaging modality of choice because of its high spatial and contrast resolution, but imaging features are often non-specific.
The most common causes of multifocal white matter lesions are perivascular spaces (not discussed here), ischaemia (small vessel disease) and MS. Small vessel disease is much more common than MS, hence, their distinguishing features are discussed first.
Small vessel disease
Vascular white matter lesions are age-related, asymptomatic foci of ischaemic demyelination (unidentified bright objects or ‘UBOs’), myelin pallor or gliosis. They are uncommon in those below age 40 years (figure 1), thereafter, increasing in frequency with age. These white matter changes occur in 5–10% of those aged 20–40 years. Most patients are neurologically normal. At all ages, the incidence and lesion load increase with cardiovascular risk factors (figure 2). The white matter lesions are small and have high T2 signal but differ from MS plaques because of their irregular shape, poor definition and peripheral location and are most common in the frontal and parietal lobes. Posterior fossa lesions are uncommon, and callosal lesions are rare because of a dual blood supply.
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