Specific Imaging Findings

Abnormal MRI signal is found with Wallerian degeneration (WD) along the course of a major white matter tract (such as corticospinal tract, corticopontine tract, optic radiation, and corpus callosum), ipsilateral to a proximal cortical or subcortical lesion, most commonly ischemic, hemorrhagic, or traumatic. The abnormality is visible as a round to ovoid area of altered signal on multiple images that are perpendicular to the white matter tract, or as an oblongated or stripe-like lesion on images parallel to the tract. Around 4 weeks after the injury WD is seen as hypointense on PD/T2-weighted images, T1 hyperintense and possibly hypodense on CT, without contrast enhancement. This is followed over the following days to weeks with low T1 signal and the characteristic T2 hyperintensity and volume loss. Reduced diffusion is an early but transient finding, with in 1–2 weeks from the injury, in adults as well as neonates and it should not be confused with a new infarction. Most commonly WD is observed along the corticospinal tract (CST) in the posterior limb of the internal capsule, cerebral peduncle, ventro-lateral pons, and pyramidal decussation. Decreasing ADC values peak at 7 days. The cerebral peduncle and the hemipons are of decreased volume in the chronic stage.

Pertinent Clinical Information

WD indicates irreversible neuronal function loss and death, and its presence suggests poor recovery while the extent of CST WD correlates with the severity of the permanent motor deficit. Awareness of this pathological process should prevent misdiagnosis of new or distant lesions in the settings of infarcts, hemorrhages, demyelinating disease, or gliomas.