Guillain-Mollaret triangle (dentato-rubro-olivary pathway). This functional circuit is responsible for modulating spinal cord motor activity. It is composed of the ipsilateral red nucleus (RN) (red circle) of the midbrain, the ipsilateral inferior olivary nucleus (ION) (green oval) of the medulla, and the contralateral dentate nucleus (DN) (orange circle) of the cerebellum. The RN communicates with the ipsilateral ION via the central tegmental tract (CTT). The ION communicates with the contralateral DN via the inferior cerebellar peduncle. The DN communicates with the contralateral red nucleus via the superior cerebellar peduncle (SCP). The lesions that affect the afferent pathways to the olive result in hypertrophic olivary degeneration (HOD). Therefore lesions resulting in HOD of the right ION would involve the right RN, the right CTT, the left DN, or the left SCP. Lesions affecting efferent pathways to the olive (inferior cerebellar peduncle lesions) are less likely to cause HOD.

Temporal evolution of hypertrophic olivary degeneration (HOD). The evolution of HOD results in three distinct phases on magnetic resonance imaging depending on changing patterns of T2 hyperintensity (blue line) as well as olivary hypertrophy (gray line) . Histologic and imaging changes (T2 hyperintensity) begin approximately 3 weeks after initial injury in most patients but can first appear on imaging up to 6 months after the initial insult. Olivary hypertrophy follows T2 hyperintensity and initially develops approximately 6 months after the initial insult in most patients. It can persist for years but disappears in most patients by 3 to 4 years. Finally, atrophic changes have been noted after several years (not shown), although T2 hyperintensity persists indefinitely.