Interhemispheric Inhibition Measurement Reliability in Stroke: A Pilot Study

Objective: Reliable transcranial magnetic stimulation (TMS) measures for probing corticomotor excitability are important when assessing the physiological effects of noninvasive brain stimulation. The primary objective of this study was to examine test-retest reliability of an interhemispheric inhibition (IHI) index measurement in stroke. Materials and Methods: Ten subjects with chronic stroke (>= 6 months) completed two IHI testing sessions per week for three weeks (six testing sessions total). A single investigator measured IHI in the contra-to-ipsilesional primary motor cortex direction and in the opposite direction using bilateral paired-pulse TMS. Weekly sessions were separated by 24 hours with a 1-week washout period separating testing weeks. To determine if motor-evoked potential (MEP) quantification method affected measurement reliability, IHI indices computed from both MEP amplitude and area responses were found. Reliability was assessed with two-way, mixed intraclass correlation coefficients (ICC(3,k)). Standard error of measurement and minimal detectable difference statistics were also determined. Results: With the exception of the initial testing week, IHI indices measured in the contra-to-ipsilesional hemisphere direction demonstrated moderate to excellent reliability (ICC = 0.725-0.913). Ipsi-to-contralesional IHI indices depicted poor or invalid reliability estimates throughout the three-week testing duration (ICC= -1.153-0.105). The overlap of ICC 95% confidence intervals suggested that IHI indices using MEP amplitude vs. area measures did not differ with respect to reliability. Conclusions: IHI indices demonstrated varying magnitudes of reliability irrespective of MEP quantification method. Several strategies for improving IHI index measurement reliability are discussed.