Hypertrophic olivary degeneration (HOD) is a rare condition characterised 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.
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.
Pathologically, this is characterised 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 HOD, as anatomically there are no direct connections between the inferior olivary nucleus and the contralateral dentate nucleus (fibres 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.
General imaging differential considerations include:
- 1. Goyal M, Versnick E, Tuite P et-al. Hypertrophic olivary degeneration: metaanalysis of the temporal evolution of MR findings. AJNR Am J Neuroradiol. 21 (6): 1073-7. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 2. Salamon-Murayama, N. et al "HOD secondary pontine haemorrhage" Radiology 1999, 213:814-817
- 3. Salamon-Murayama N, Russell EJ, Rabin BM. Diagnosis please. Case 17: hypertrophic olivary degeneration secondary to pontine hemorrhage. Radiology. 1999;213 (3): 814-7. doi:10.1148/radiology.213.3.r99dc43814 - Pubmed citation