Hypertrophic olivary degeneration

Last revised by Dr Daniel J Bell on 10 Jan 2022

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:

Oculopalatal tremor is a distinctive part of the presentation of hypertrophic olivary degeneration, although is also seen in many other brainstem and cerebellar conditions 4. Oculopalatal tremor comprises ocular, palatal and other muscular involuntary movements.

  • 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 is pathognomonic
  • oscillopsia may be present due to the pendular nystagmus caused by the ocular tremor 4

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 transneuronal 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). 

Oculopalatal tremor results from disinhibition of the inferior olivary nucleus, following the pathology in dentato-rubro-olivary pathway which is further modulated by the cerebellum 4

It is often seen several months after the original insult.

The inferior olivary nucleus gets larger and increases in signal intensity on T2 weighted and T2-FLAIR sequences.

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:

  1. T2 hyperintense without olivary swelling
  2. T2 hyperintense and olivary swelling occur concomitantly, developing after 6 months and persisting for nearly 3-4 years
  3. olivary swelling subsides, but T2 hyperintensity persists

General imaging differential considerations include:

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Cases and figures

  • Figure 1: tiangle of Guillain and Mollaret
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  • Case 1
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  • Figure 2: illustration
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  • Case 2: bilateral
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  • Case 3
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  • Case 4
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  • Case 5
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  • Case 6: bilateral
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  • Case 7
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  • Case 8
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