Medial accessory olivary nucleus asymmetry and its selective degeneration in Parkinson’s disease: A human neuropathological study

OBJECTIVE: The medial accessory olivary nucleus (MAO), a distinct but inconsistently present subnucleus of the human inferior olivary complex, is poorly characterized despite its proposed role in cerebellar motor circuitry. We aimed to investigate its structural asymmetry, functional correlates, and pathological vulnerability in humans. MATERIALS AND METHODS: We performed high-resolution histological analysis (Nissl and immunohistochemistry) in 35 neurologically normal brains to quantify MAO volume and neuronal numerical density and relate these to handedness and lifetime manual dexterity. In a separate cohort of 20 Parkinson's disease (PD) brains (Braak stages 4 - 6), we compared MAO neuronal numerical density and α-synuclein pathology to age-matched controls. RESULTS: The MAO was identifiable in 74% (26/35) of neurologically normal cases. A significant volumetric asymmetry was present (p < 0.01), with larger MAO volume contralateral to the dominant hand, and the asymmetry index correlated with lifetime manual dexterity scores (r = 0.54, p = 0.002). PD brains showed a 43% reduction in MAO neuronal numerical density versus controls (p < 0.001) and more severe α-synuclein pathology (median grade 3 vs. 0, p < 0.001). CONCLUSION: The MAO may be a neuroanatomically variable but functionally lateralized structure that appears linked to fine motor control and may exhibit selective vulnerability in PD.