hypertrophic olivary degeneration

A radiologic

review of hoarse voice from anatomic and neurologic perspectives. Hypertrophic olivary degeneration. A 69-year-old man with history of hypertension presenting with new onset palatal myoclonus, hoarse voice, and ataxia. Axial T2 (a) and susceptibility-weighted (b) MR images reveal a lesion at the left parasagittal pontomedullary junction at the level of the facial colliculus demonstrating bubbly high T2 signal internally surrounded by a rim of low T2 signal intensity (arrow) and associated blooming (solid arrowhead), compatible with a cavernous malformation. Thin section axial T2 image (c) reveals hyperintensity and expansion of the left inferior olivary nucleus (dashed arrowhead), consistent with hypertrophic olivary degeneration. Lesions involving the dento-rubro-olivary tract (such as this cavernous malformation) can lead to hypertrophic olivary degeneration

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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:

Clinical presentation

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.

Pathology

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).

Radiographic features

It is often seen several months after the original insult.

MRI

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.