Specific Imaging Findings

The finding of an olivary lesion with presence of another abnormality in the contralateral cerebellar dentate nucleus, contralateral superior cerebellar peduncle, ipsilateral red nucleus, or the ipsilateral pontine tegmentum strongly suggests the diagnosis of hypertrophic olivary degeneration (HOD). The earliest finding in HOD is high T2 signal intensity in the inferior olivary nucleus, located anteriorly within the medulla oblongata, which appears within the first month after the symptom onset. This hyperintensity can persist for years and even become permanent. Hyperintensity is followed by hypertrophy, which typically appears 10–18 months after onset, but may be observed by as early as 6 months. Hypertrophy of the inferior olive resolves by 4 years. There is no associated post-contrast enhancement. When the primary lesion is limited to the central tegmental tract, olivary hypertrophy is ipsilateral; when the primary lesion is in the dentate nucleus or in the superior cerebellar peduncle, olivary degeneration is contralateral (the most common and characteristic pattern); when the lesion involves both the central tegmental tract and the superior cerebellar peduncle, HOD is bilateral. Diffusion tensor imaging shows decreased fiber volume and reduced fractional anisotropy in the involved tracts.

Pertinent Clinical Information

Patients with HOD classically present with a palatal tremor or myoclonus, which is characterized by rhythmic involuntary movement of the soft palate, uvula, pharynx, and larynx at one to three cycles per second. Other clinical signs can include dentatorubral tremor, myoclonus of the cervical muscles and diaphragm, and symptoms of cerebellar or brainstem dysfunction. Causes of HOD include infarct, hemorrhage, trauma, demyelinating disease, neoplasm, and it may also be iatrogenic following posterior fossa surgery.