Wallerian Degeneration of the Superior Cerebellar Peduncle

Wallerian degeneration (WD) occurs after nerve damage in both the peripheral nervous system and central nervous system (CNS). Wallerian degeneration is named after Augustus Volney Waller (1816-1870), a British neurophysiologist who observed distal nerve changes after experimental lesions of the hypoglossal nerve in frogs.¹ The distal part of the axon of the damaged nerve degenerates, a process called orthograde degradation. Histologically, WD is characterized by structural loss of the cytoskeleton, a process that takes roughly 24 hours in the peripheral nervous system and days to weeks in the central nervous system.

Wallerian degeneration of the corticospinal tract is common after ischemia in the primary motor cortex or internal capsule. On imaging, hypointensity on T2 sequences is present in the corticospinal tract during 4 to 12 weeks, after which a permanent T2 hyperintensity is seen. After several months to years, atrophy of the involved tract can be observed.² On diffusion-weighted imaging (DWI) sequences, diffusion restriction is found. If present, poor motor outcomes are likely.³ Wallerian degeneration can occur in every nerve tract.

We describe a man in his early 80s who had a deep cerebellar hemorrhage with damage to the dentate and interposite nuclei (Figure 1A). On brain magnetic resonance imaging obtained 5 days after the event (Figure 1B and C), there was a marked hyperintense signal on DWI of the ipsilateral superior cerebellar peduncle (SCP). The apparent diffusion coefficient showed a subtle hypointense signal of the ipsilateral SCP. There were no other hyperintensities on DWI. These changes were due to WD of the dentato-rubral-thalamic-(cortical) tract, which is the main output of the dentate nucleus and which travels through the SCP, crosses the midline in the SCP, and runs to the contralateral red nucleus. From the red nucleus, there are projections to the thalamus and to the inferior olivary nucleus (Figure 2).

A new brain magnetic resonance image (3-dimensional T2-weighted imaging; 0.6-mm slice thickness) was performed 4 months after the stroke (Figure 1D-F). On T2-weighted magnetic resonance sequence images, there was a hyperintense signal and atrophy of the ipsilateral SCP, compatible with WD. The contralateral red nucleus was smaller, with some hyperintensity, and the contralateral inferior olivary nucleus was hypertrophic, with marked hyperintensity. These findings confirmed the anatomical brainstem circuit, which is also known as the triangle of Guillain-Mollaret or myoclonic triangle.⁴ Lesions involving this circuit, such as ischemia involving the central tegmental tract, may cause palatal myoclonus. Palatal myoclonus is typically associated with temporary hypertrophic degeneration of the inferior olivary nucleus. Clinically, our patient experienced nausea and truncal ataxia but palatal myoclonus was not observed.

To our knowledge, this is the first report of early WD of the SCP. However, late WD has been described after cerebellar surgery involving the deep nuclei or after hemorrhage in the dentate nucleus.^5,6 Several neurodegenerative diseases, such as progressive supranuclear palsy and Friedreich ataxia, are also associated with atrophy of the SCP.

Hyperintense lesions on DWI occurring at a distance from the initial cerebral infarction or hemorrhage are not necessarily due to accompanying ischemia but may reflect early WD and may illustrate complex anatomical relationships.

Corresponding Author: Thomas Decramer, MD, Department of Neurosurgery, University Hospitals Leuven, Herestraat 49, 3000 Leuven, Belgium (thomas.decramer@uzleuven.be).

Conflict of Interest Disclosures: None reported.