1430 - Characterizing Motor Cortex Interhemispheric Inhibition in Chronic Stroke Survivors with Upper Extremity Impairment

Objectives: Interhemispheric inhibition (IHI) refers to the mechanism by which one motor cortex inhibits the other to enable dexterous unimanual sensorimotor control. Stroke disrupts IHI balance but there is an incomplete understanding of IHI and how it may relate to post-stroke upper limb impairment. Here, we characterized properties of post-stroke IHI and evaluated associations with chronic upper limb motor impairment.

Design: We enrolled 12 stroke survivors in the chronic stage of recovery with mild-to-moderate arm impairment and 13 neurotypical age-similar older adults (NOA). We employed dual-coil transcranial magnetic stimulation (TMS) to measure IHI. Individuals received single TMS pulses and also paired TMS pulses with various interstimulus intervals (ISIs) ranging from 1 to 50ms while the target hand muscle was at rest. We calculated an IHI ratio of the average responses to paired pulse trials versus single pulse trials. We compared IHI ratios for the different ISIs across groups using a two-way ANOVA. For each individual, the ISI that elicited the maximal IHI was entered into correlation analysis with motor impairment assessed using the upper extremity portion of the Fugl-Meyer Assessment (FMA-UE).

Results: IHI was weaker in stroke compared to NOA (F (1,20) = 8.07, p = 0.01). Weaker IHI was consistently observed across multiple ISIs (F (3.66, 73.21) = 3.27, p = 0.02). All participants exhibited the highest level of IHI at a non-standard ISI. Importantly, maximal IHI was not associated with FMA-UE score (r = 0.27, p = 0.46).

Conclusions: Our findings suggest IHI characteristics differ across individuals, but IHI levels are consistently lower in stroke compared to NOA. However, IHI levels were not associated with chronic motor impairment suggesting other factors likely underpin persistent upper limb impairment post-stroke. Future studies should assess the reliability of IHI characteristics over time and explore additional potential biomarkers of post-stroke impairment.