Hypertrophic olivary degeneration (HOD) is a rarely encountered lesion that occurs secondary to focal lesions of the cerebellum or brainstem and results in trans-synaptic degeneration. HOD represents an end result of interruption of the dentatorubral–olivary pathway, the neuronal connections between the dentate nucleus of the cerebellum, the red nucleus, and the inferior olivary nucleus. HOD is considered a unique type of degeneration because it is associated with hypertrophy, rather than atrophy, of the inferior olivary neurons (Figs. 42.1, 42.2).
The dentatorubral–olivary pathway connects the dentate nucleus of the cerebellum, the contralateral red nucleus, and the ipsilateral inferior olivary nucleus. The dentatorubral tract connects the dentate nucleus to contralateral red nucleus via superior cerebellar peduncle, with fibers crossing in the decussation of the peduncle at the lower midbrain. The central tegmental tract connects the red nucleus to the ipsilateral inferior olivary nucleus. The dentatorubral–olivary pathway, also referred to as the “Guillain–Mollaret triangle,” was described by Guillain and Mollaret in 1931 .