Hypertrophic olivary degeneration (HOD) is secondary degeneration of the inferior olivary nucleus (ION) due to a primary lesion in the dento-rubro-olivary pathway. This pathway is known as the Guillain and Mollert triangle, containing the dentate nucleus and the contralateral red and inferior olivary nuclei (figure e-1 on the Neurology® Web site at www.neurology.org). The commonest presenting symptom is palatal myoclonus occurring 8–12 months after the primary insult. MRI of the ION initially has normal results (figure 1). Three phases of HOD exist on MRI: hyperintense signal change without hypertrophy, hyperintense signal change with hypertrophy (figure 2), and regression of hypertrophy with persistent hyperintense signal.1
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