White Matter: Beyond Focal Disconnection
Section snippets
Anatomy
White matter comprises roughly half the volume of the adult brain,15 and some 135,000 km of myelinated fibers course within the cerebrum.16 At the macroscopic level, white matter consists of millions of myelinated axons joined in tracts, fascicles, bundles, and peduncles; in the brain, neuroanatomists have usefully distinguished between projection, association, and commissural fiber systems.17 Association and commissural tracts are most important for neurobehavioral function, as they link
White matter dementia
Dementia is an increasing threat to medicine and society, and Alzheimer disease (AD) by itself poses a major challenge, but dementia can also occur in well over 100 other disorders in which cerebral white matter is prominently or exclusively affected.2 These disorders fall into the categories of genetic, demyelinative, infectious, inflammatory, toxic, metabolic, vascular, traumatic, neoplastic, and hydrocephalic.2, 19, 31 Table 1 lists these categories and relatively well-studied examples of
Psychiatric syndromes in white matter disorders
Neurologists have long described psychiatric syndromes in patients with neurologic disorders. These descriptions often derive from patients who have a prominent degree of cerebral white matter neuropathology.2, 19, 31 MRI has proven invaluable in detecting white matter abnormalities, and the advent of diffusion tensor imaging (DTI) in the last decade has begun to add more information by virtue of its capacity to provide detailed depiction of myelinated tracts.62 DTI is based on the diffusion of
Psychiatric diseases with white matter abnormalities
The etiology and pathogenesis of several major psychiatric diseases remain poorly understood despite advances in treatment over the last half century. Standard neuropathological investigation has been applied with little success to the investigation of schizophrenia, depression, and other debilitating psychiatric illnesses. In recent years, largely stimulated by advances in structural and functional neuroimaging, the study of psychiatric diseases has focused more on putative network dysfunction
Discussion
The evidence from both the neurologic and psychiatric disorders reviewed here strongly suggests that white matter plays a major role in cognitive and emotional function. Disruption of myelinated systems in the brain can produce white matter dementia, and may help explain a range of puzzling neuropsychiatric disorders. Implicit in these observations is the notion of multiple neural network disturbance, meaning that the clinical phenomenology of these complex disorders has its basis in widespread
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