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MRI and Tractography in Hypertrophic Olivary Degeneration

A 50-year-old man suffered from acute lacunar infarction of the right thalamus, midbrain and pons 3 months earlier and was left with blurred speech and unstable gait. Neurological examination revealed mild dysarthria, left-sided central facial paralysis, left-sided hypoglossal paralysis and gait ataxia. There was no palatal myoclonus or nystagmus. The remaining neurological examinations were unremarkable. Serologic and metabolic workup were unrevealing. Brain MRI showed chronic infarction of right thalamus, midbrain and pons, and bilateral olivary hyperintensity on T2 and fluid-attenuated inversion recovery, indicating bilateral hypertrophic olivary degeneration (HOD) (Figures 1A-1B). Diffusion tensor imaging showed decreased volume of right central tegmental tract (Figure 1C). Hypertrophic olivary degeneration is secondary trans-synaptic degeneration of the inferior olivary nucleus (ION) owing to primary lesions in the dento-rubro-olivary pathway, also named as Guillain-Mollaret Triangle (GMT). Guillain-Mollaret Triangle comprises connections among the inferior olive nucleus, the red nucleus and the contralateral dentate nucleus, which was first described by Guillain and Mollaret in 1931. The common causes of HOD include infarction, hemorrhage, arteriovenous malformation, tumor, trauma, prior surgery, inflammation and demyelination. In addition, other disorders including abscesses, Wilson’s disease, metronidazole intoxication and neuro-Behcet disease may also cause HOD. Pathological changes of HOD are characterized by enlargement and vacuolation of neurons, astrocytic hyperplasia and demyelination, which are considered secondary to loss of afferent input to the ION. Bilateral HOD is more common than unilateral HOD. Palatal tremor, Holmes tremor, nystagmus and cerebellar ataxia are classically associated features of HOD, but they are often not present. About 61%-77% of the patients are asymptomatic and could be found incidentally on MRI scans. Differential diagnoses of HOD include brain stem infarction, inflammatory diseases and neoplastic diseases.