Hypertrophic Olivary Degeneration

Hypertrophic olivary degeneration occurs as a result of lesions in the dentatorubral-olivary pathway (also known as the Guillain-Mollaret triangle): neural connections between the dentate nucleus of the cerebellum, the red nucleus, and the inferior olivary nucleus. Axons from the dentate nucleus travel up via the superior cerebellar peduncle cross the midline at the level of the midbrain at the decussation of the superior cerebellar peduncle to reach the contralateral red nucleus. This is the dentatorubral tract.

Axons from the red nucleus travel down via the central tegmental tract to reach the ipsilateral inferior olivary nucleus. There are no direct connections between the inferior olivary nucleus and the contralateral dentate nucleus, making the Guillain-Mollaret triangle an incomplete one.

Damage to the dentate or red nuclei can lead to hypertrophic olivary degeneration, which commonly manifests clinically as palatal myoclonus, dentatorubral tremor and ocular myoclonus. The actual olivary hypertrophy typically appears 4–6 months after the original injury (infarction, demyelination, tumor, etc.) and resolves by 10-16 months. The T2 hyperintensity that accompanies olivary hypertrophy, however, persists for years. Unfortunately, the clinical symptoms rarely improve.

Reviewing the diagram can help predict the pattern of olivary degeneration based on the location of the lesion.

• Lesions limited to the central tegmental tract lead to ipsilateral olivary degeneration.
• Lesions limited to the dentate nucleus or superior cerebellar peduncle lead to contralateral olivary degeneration
• Lesions involving the central tegmental tract and the superior cerebellar peduncle on the same side, lead to bilateral olivary degeneration.

Differential considerations for T2 hyperintensity in the anterolateral medulla include:

• Hypertrophic olivary degeneration
• Infarction
• Demyelination
• Tumor: Astrocytoma, metastases, and lymphoma
• Infection: Tuberculosis, AIDS, rhombencephalitis
• Inflammatory process: Sarcoidosis

References

Salamon-Murayama N, Russell EJ, Rabin BM. Diagnosis please. Case 17: hypertrophic olivary degeneration secondary to pontine hemorrhage. Radiology. 1999 Dec;213(3):814-7.