Diffusion-Weighted Imaging of the Early Phase of Wallerian Degeneration

Abstract

Wallerian degeneration (WD) affects axons and their myelin sheaths distally from the site of injury of the axon and/or body of the neuron. There is limited knowledge about brain injuries in newborns, mostly because non-invasive imaging sensitive enough to detect subtle changes was not available. Diffusion-weighted magnetic resonance (MR) imaging (DWI) is sensitive in the detection of early injury to the corticospinal tracts (CST) before the occurrence of WD in distal segments. We describe the cases of 14- and seven-day-old newborns in whom MR imaging revealed vascular injury in the territory supplied by the middle cerebral artery, and early development of WD. We performed DWI and faced an unexpected finding because DWI hyperintensities were at the same time hyperintense on T2-weighted images. Hyperintense structures on DWI are supposed to reflect pre-Wallerian injured tissue, which should not be visible on standard MR sequences, while their T2- signal indicated the third, late subacute phase of WD. It seems that time-related stages of WD observed in adults do not necessarily apply to the immature brain of newborns/infants as reflected by differences in signal intensity in various MR sequences. DWI hyperintensity in CST below infarct should not be mistaken for new ischemic foci.