MEP as predictor of motor recovery in chronic stroke patients after a 4-week daily physical therapy

Introduction: Motor evoked potentials (MEP) that can be elicited by single pulse transcranial magnetic stimulation (TMS) applied to the primary motor cortex (M1) reflect cortico-spinal tract (CST) integrity [1]. It was shown that presence of MEP in contra-lesional upper limb muscles within the first 30 days after stroke predicts motor recovery [2]. In severely affected chronic stroke patients (time since stroke >6months), however, this correlation was not systematically investigated so far. While not all patients that show some degree of motor recovery can be identified by previous detection of MEP [3], a better understanding of the relatedness between CST integrity and motor recovery in chronic stroke patients might lead to better individual treatment plans. Here we tested the hypothesis that presence of MEP in the affected contra-lesional upper limb muscles of severely affected chronic stroke patients predicts motor recovery after a 4-week daily goal-directed physical therapy intervention. Methods: 34 chronic stroke patients (21 male, mean age 54.8±11.59 years; time since stroke: 66.82±56.2 months) unable to extend their affected fingers, underwent a daily 4-week physical therapy intervention. Presence of MEP in the affected upper limb (first dorsal interosseous muscle, FDI, and musculus biceps brachii, BB) was assessed 8 weeks as well as one day before (pre1, pre2) and after the intervention (post1). A forth assessment was completed 6 months after the intervention (post2, Fig.1). Motor function was determined on the same days using the Fugl-Meyer Motor Assessment (FMA) [4]. For detection of FDI and BB MEP-hotspots, subjects were seated in an upright position and asked to keep their affected upper limb at rest. Single pulse TMS (Magstim Company Ltd., Whitland, U.K) using a figure-of-eight coil (60mm diameter) was applied to the ipsilesional brain hemisphere in ascending stimulation intensities until a MEP of >50µV peak-to-peak amplitude was detected. If no MEP could be elicited at 100% maximum stimulator output (MSO), presence of MEP at rest (MEPr) was negated. In this case, patients were asked in a second condition to engage the BB and FDI from their neutral (rest) position upon an auditory command by pressing against a steady resistance. 500ms after the auditory command, a TMS pulse at 100% MSO was applied to the ipsilesional brain hemisphere, changing the coil position for each TMS pulse systematically according to a 4x4cm grid overlying the M1 area. If no MEP of >50 µV peak-to-peak amplitude was elicited at any of the stimulated grid-points, presence of MEP at intended active muscle contraction (MEPa) became negated (fig.2). If a MEP was detected, the corresponding motor threshold (MT) was determined according to the ‘5/10’ method [5]. To test whether presence of MEPr or MEPa before the intervention predicts improvements in FMA values after the intervention, an independent-samples t-test comparing changes in FMA values was used. Results: 14 patients presented MEPa and 6 patients also presented MEPr. An one-way Anova showed no main effect in FMA before intervention, F(2, 32) = 0.69, p = .51. Post-hoc analyses using Tukey’s HSD indicated also no significance between MEPr and MEPa (p = .62), MEPr and no MEP (p = .49), MEPa and no MEP (p = .99). An independent-samples t-test indicated that FMA scores were significantly higher for patients with MEPr (M = 5.50, SD = 2.66) than for patients without MEPr (M = 1.54 , SD = 4.49), t(32) = 2.07, p = .047, d = 1.07. When comparing FMA changes in patients with MEPa (M = 3.57, SD = 3.03) with those without MEP (M = 1.30, SD = 5.11), no significant difference between the groups could be found, t(32) = 1.49, p = .147, d = 0.54. Conclusion: Presence of MEP at rest reflecting CST integrity [6] seems to correlate with gains in FMA values following a 4-week daily physical therapy intervention, even in severely affected chronic stroke patients.