Degeneration of the myelin sheath and axon distal to the most proximal site of axonal interruption secondary to axonal disease has been called Wallerian degeneration. On MR imaging, Wallerian degeneration of the pyramidal tract can be observed as an abnormal signal intensity, showing prolonged T1 and T2 relaxation times that correspond to the corticospinal tract, with or without shrinkage of the ipsilateral cerebral peduncle and pons.

Thirty-one patients with Wallerian degeneration seen on MR imaging after CVA were studied. The purposes of this study were to identify the somatotopic localization of the descending cortical tract, to demonstrate their relationship with the severity of motor deficit.

With small lesion confined to the precentral gyrus, corona radiata, or the posterior limb of the internal capsule there was an abnormal signal at the centre of the peduncle, suggesting degeneration of the corticospinal tract. Patients with a lesion of the parietal, temporal or occipital lobes, not including the paracental or precentral gyri, corona radiata, or the posterior limb of the internal capsule, had an abnormal area laterally in the peduncle, suggesting degeneration of the parietopontine, temporopontine or occipitopontine tract. A lesion in the involving frontal lobe, not including the precentral gyri, showed an abnormal area medially in the peduncle, suggesting degeneration of the frontopotine tract.

The maximum extent of shrinkage of the ipsilateral cerebral peduncle was measured on T2W imaging. The width of shrinkage of cerebral peduncle was closely related to the severity of motor deficit.