Activation-Induced Rigidity in Neurologically and Cognitively Healthy Individuals Aged 18-90 Years: A Cross-Sectional Study

Background: Rigidity is a key clinical feature of Parkinson's disease (PD), but in a very early phase of the disease it may be absent and can be enhanced through active movements of the arm contralateral to the one being tested. Objective: To evaluate in a large cohort of neurologically and cognitively healthy (NCH) subjects aged 18-90 years if activation-induced rigidity (AR) is present in all age classes, and if there are biological differences between subjects showing AR (AR+) and not showing AR (AR-). Methods: 2,228 NCH subjects categorized as young adult (18-44 years), adult (45-64 years), elderly (65-74 years), and old/oldest-old (75-90 years) were included in the analysis, and underwent brain MRI. White matter hyperintensities were assessed through two visual rating scales. Lacunes were also rated. Atrophy of the caudate nuclei and ventricular enlargement were assessed through the bicaudate ratio and the lateral ventricles to brain ratio. To elicit AR, the Froment's maneuver (FM) and the instructions of the UPDRS-ME were used. Results: Among the sample, 1,689 (75.81%) subjects showed AR, of which 1,270 (57.00%) subjects showed AR by using FM, and 419 (18.81%) showed AR by using UPDRS-ME instructions. The latter subjects also showed AR by using FM. The number of AR+subjects significantly increased with increasing age, regardless of the activation maneuver used. In each age class, the number of AR+subjects was significantly higher by using the FM than the UPDRS-ME instructions. Conclusion: Our findings suggest that AR is likely to be one of the signs of the prodromal phase of PD.