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Neuroradiology

Wallerian degeneration

(Augustus Volney Waller, 1816 - 1870, English physiologist), degenerative changes which occur in the distal axonal segments and their myelin sheath secondary to proximal axonal injury or death of the cell body.

This was first described at the histological level by Waller, almost 150 years ago; he observed different stages of the process, from interruption of axonal transport, to collapse of the axon, physical breakup and degradation of the surrounding myelin sheath, cell proliferation and oedema, breakdown and removal of myelin. Finally, there is resultant volume loss with gliosis.

This process is observed in the pyramidal tract following cerebral infarct, trauma to the motor cortex or in neurodegenerative diseases like amyotrophic lateral sclerosis.

Within 5 to 12 weeks after acute insult, the Wallerian degeneration of the pyramidal tract can be detected as a high signal intensity area on T2-weighted MR scans (Fig.1). Ipsilateral brain stem atrophy, well seen at the level of the cerebral peduncle, appears within 8 - 12 months after the ictus.

 

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Fig.1

a, b, c, d, e. MR, T2-weighted images. Hyperintensity of the left corticospinal tract extending from the internal capsule (c) to the pons (e) indicating Wallerian degeneration secondary to a focus of previous autoimmune encephalitis in the left corona radiata (b)
Wallerian degeneration, Fig.1 (a)
Wallerian degeneration, Fig.1 (b)
Wallerian degeneration, Fig.1 (c)
Wallerian degeneration, Fig.1 (d)
Wallerian degeneration, Fig.1 (e)