Finger extensor variability in TMS parameters among chronic stroke patients

Abstract

Background: This study determined the reliability of topographic motor cortical maps and MEP characteristics in the extensor digitorum communis (EDC) evoked by single-pulse TMS among patients with chronic stroke.

Methods: Each of ten patients was studied on three occasions. Measures included location of the EDC hotspot and center of gravity (COG), threshold of activation and average amplitude of the hotspot, number of active sites, map volume, and recruitment curve (RC) slope.

Results: Consistent intrahemispheric measurements were obtained for the three TMS mapping sessions for all measured variables. No statistically significant difference was observed between hemispheres for the number of active sites, COG distance or the RC slope. The magnitude and range of COG movement between sessions were similar to those reported previously with this muscle in able-bodied individuals. The average COG movement over three sessions in both hemispheres was 0.90 cm. The average COG movement in the affected hemisphere was 1.13 (+/- 0.08) cm, and 0.68 (+/- 0.04) cm) for the less affected hemisphere. However, significant interhemispheric variability was seen for the average MEP amplitude, normalized map volume, and resting motor threshold.

Conclusion: The physiologic variability in some TMS measurements of EDC suggest that interpretation of TMS mapping data derived from hemiparetic patients in the chronic stage following stroke should be undertaken cautiously. Irrespective of the muscle, potential causes of variability should be resolved to accurately assess the impact of pharmacological or physical interventions on cortical organization as measured by TMS among patients with stroke.

Figure 1

A representative MEP amplitude of 70 μV beginning approximately 20 ms after the stimulus artifact.

Figure 2

Inter-hemispheric variability collapsed across the three mapping sessions for the parameters: average MEP amplitude, normalized map volume, and resting motor threshold. P-values are depicted in the lower right corner of each plot.

Figure 3

2-D representation of the overall COG movement (cm) across three sessions for each participant and both hemispheres. First session is demarcated by a larger symbol. The COG was calculated using mean MEP amplitudes shown for active sites only. Larger numbers on the x-coordinate and y-coordinates represent lateral and anterior scalp stimulus locations, respectively. Note that locations are unadjusted for the repeated measures on hemisphere and session. Each grid location represents one centimeter. The hatched circle represents the COG centroid location for a single subject in one hemisphere. All centroids are displayed in Figure 4.

Figure 4

Centroid location of COG for the affected and less affected hemisphere for individual patients along with 9 able-bodied adults. The left hemisphere corresponds to the dominant arm in able-bodied participants and the affected and less affected hemispheres are of mixed hand dominance for the patients No significant variability exists when comparing left or right hemisphere of right handed able-bodied individuals with affected (p = 0.996) and less affected (p = 0.68) hemispheres of patients. Symbols with asterisks (*) represent centroids for left hemisphere (triangle*) and right hemisphere (square*) of able-bodied individuals. Each grid location represents one centimeter.