hypertrophic olivary degeneration
Hypertrophic olivary degeneration (HOD) is a rare condition characterized by a unique pattern of trans-synaptic degeneration. It is caused by a lesion in the triangle of Guillain and Mollaret, resulting in hypertrophy of the inferior olivary nucleus. The three corners of the triangle are:
Palatal myoclonus, rhythmic involuntary movement of the soft palate, uvula, pharynx, larynx and upper extremity are classically described clinical features.
Palatal myoclonus may not always be present, but when it is present, it's pathognomonic.
The connection between the red nucleus, ipsilateral inferior olivary nucleus and the contralateral dentate nucleus forms the triangle of Guillain and Mollaret. Interruption of either connection between the dentate nucleus and contralateral red nucleus (dentatorubral tract, superior cerebellar peduncle) or the connection between the red nucleus and ipsilateral inferior olivary nucleus (central tegmental tract) leads to changes in the olive. The olive receives inhibitory (GABAergic) signals within the dentato-rubro-olivary pathway, with trans-neuronal degeneration causing enlargement rather than atrophy.
Pathologically, this is characterized by "trans-synaptic degeneration resulting in vacuolation of the neurons" and an increase in astrocytes. Isolated lesions of the inferior cerebellar peduncle do not cause hypertrophic olivary degeneration, as anatomically there are no direct connections between the inferior olivary nucleus and the contralateral dentate nucleus (fibers from the inferior olivary nucleus project to the cerebellar cortex via the olivocerebellar tracts and then to the dentate nucleus).
It is often seen several months after the original insult.
In radiology laymen terms, the inferior olivary nucleus gets larger and increases in T2 signal intensity.
Typically, within a few months, T2 signal increases and lasts for 3-4 years, whereas hypertrophy occurs later (at about one year), and resolves by 3-4 years. There are three stages:
- first stage: T2: hyperintense without olivary swelling
- second stage: T2 hyperintense and olivary swelling occuring concomitantly; develops after 6 months and persists for nearly 3-4 years.
- third stage: olivary swelling subsides, however, T2W and FLAIR hyperintensity persist.
General imaging differential considerations include: