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Background
Transneuronal degeneration (TND), including anterograde Wallerian degeneration (WD) and retrograde degeneration, are maladaptive pathophysiologic processes that compound the effects of initial neural injuries. Modern imaging techniques can identify their unique characteristics and patterns, which radiologists should be aware of. This educational exhibit will introduce the pathophysiology of TND and present a multimodal, case-based approach to its identification and understanding.

Educational Goals / Teaching Points
Highlight the unique imaging characteristics and patterns of TND in various disease processes utilizing CT, MRI, and PET. Describe and exemplify the use of advanced imaging techniques like diffusion tensor imaging and tractography (DTI/DTT) for detecting changes resulting from TND.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Many underlying conditions can activate the cascade of transneuronal degenerative changes, including but not limited to stroke, traumatic brain and spinal cord injury, neoplasms, postsurgical lobectomy, autoimmune and neurodegenerative diseases, mesial temporal sclerosis, and chronic seizures. Patterns of TND include the following: Wallerian degeneration of the corticospinal tract; crossed cerebellar diaschisis, which refers to decreased metabolism and function from a supratentorial lesion; and hypertrophic olivary degeneration, characterized by injury to the dentato-rubro-olivary pathway. The exhibit will focus on both anterograde and retrograde degeneration.

Conclusion
Understanding the pathophysiology of TND and its unique imaging characteristics helps determine the possible extent of the initial insult, chronicity, and disease progression. Advanced imaging techniques like DTI/DTT in the setting of spinal cord injury allow for the detection of TND changes, which may not appear on traditional MRI sequences and can aid in prognosis.