Hypertrophic olivary degeneration (HOD) is s a rare condition characterized by unique pattern of trans-synaptic degeneration. It is usually caused by primary lesions in dento-rubro-olivary pathway (triangle of Guillain and Mollaret).

The triangle of Guillain and Mollaret has three corners (Fig. 1):

• Red nucleus
• Inferior olivary nucleus 
• Contralateral dentate nucleus

These corners communicate by tracts; the central tegmental tract, the inferior cerebellar peduncle and the superior cerebellar peduncle. Depending on the location of interruption, cerebellar atrophy or hypertrophic olivary degeneration can ensue.

Clinical presentation

The most common symptoms are palatal tremor or/and myoclonus, dentatorubral tremor (Holmes tremor), ocular myoclonus, severe myoclonus of cervical muscles and diaphragm, symptoms of cerebellar or brain stem dysfunction (Fig. 2).

Etiology

A variety of lesions involving the dentato-rubro-olivary pathway have been implicated in development of HOD. They include primary hemorrhage, ischemic infarction, demyelination, vascular malformation, diffuse axonal injury following severe head trauma, tumor, posterior fossa surgery (Fig. 3), and idiopathic.