Department of Exercise Sciences


Contralesional hemisphere control of the proximal paretic upper limb following stroke

Lynley V. Bradnam, Cathy M. Stinear, P. Alan Barber and Winston D. Byblow
Published in Cerebral Cortex (2012) 22: 2662-2671

Introduction

Following stroke there is often an imbalance in primary motor cortex (M1) excitability, with relative underexcitability in the stroke affected ipsilesional hemisphere and relative overexcitability in the contralesional hemisphere, and worse outcomes for patients with greater imbalance. Cathodal transcranial direct current stimulation (c-tDCS) can reduce excitability of neurons in M1 and may facilitate motor recovery after stroke. However, little is known about the neurophysiological effects of tDCS on proximal upper limb function.

 

Aim

To test the hypothesis that suppression of contralesional M1 (cM1) excitability would produce neurophysiological effects that depended on severity of upper limb impairment.

 

Method

(Figure 1)

  • Twelve patients with varying upper limb impairment after subcortical stroke were assessed on clinical scales (NIHSS, Fugl-Meyer, and modified Ashworth spasticity scales) of upper limb spasticity, impairment, and function. Each of the 12 patients was age and gender matched with a healthy adult.
  • Excitability within paretic M1 biceps brachii representation was determined from motor-evoked potentials (MEPs) using transcranial magnetic stimulation (TMS) during selective isometric tasks (pronation and flexion tasks), after cM1 sham stimulation and after c-tDCS. Selective Ratio (SR), which reflects the ability to suppress BB prior to pronation, was calculated as a ratio of the average BB MEPAREA for the pronation task to that for the flexion task. The difference between sham and c-tDCS SR was also calculated (ΔSR = SRsham – SRc-tDCS). A positive ΔSR indicates selective BB activation was improved by c-tDCS.
  • Magnetic resonance imaging was used to determine lesion size and fractional anisotropy (FA) within the posterior limbs of the internal capsules indicative of corticospinal tract integrity.
  • MEPs in the nonparetic FDI at rest were collected at various time points using TMS in order to confirming the suppression of cM1 by c-tDCS.
Bradnam-2012-Figure-1

 

Results

  • As expected, SR was higher in patients than healthy adults reflecting impaired BB suppression. Furthermore, higher SRs were associated with poorer clinical scores and a reduction in the integrity of ipsilesional white matter.
  • The ΔSR results in Figure 4 indicated that, overall, the direction of the neurophysiological after-effects of c-tDCS was strongly related to upper limb spasticity, impairment, function, and FA asymmetry between the posterior limbs of the internal capsules. Indeed, c-tDCS improved selective proximal upper limb control for mildly impaired patients and worsened it for moderate to severely impaired patients (Figure 4).
  • After c-tDCS, FDI MEP amplitude was reduced compared with baseline at 5 and 30 min post-stimulation, confirming the suppression of cM1 excitability by c-tDCS.
Bradnam-2012-Figure-4

 

Interpretation

There is unlikely to be a one size fits all treatment protocol when using noninvasive brain stimulation in stroke rehabilitation. We have shown that c-tDCS of cM1 may be beneficial for patients with mild impairment and contraindicated for patients with moderate to severe impairment. A possible explanation is that, for patients with major disruption of ipsilesional corticospinal tract, suppression of cM1 may result in down-regulation of important compensatory activity – the greater excitability of the contralesional hemisphere.

An important clinical aspect of the present study is that clinical measures of upper limb function and impairment, and spasticity in the paretic elbow flexors, may be useful for determining whether contralesional c-tDCS is contraindicated for an individual patient. Further experiments may identify how to individually prescribe tDCS as an adjuvant to therapy.

 

Acknowledgements

We would like to thank Suzanne Ackerley for performing the clinical assessments, Frederique Noten for assistance with data collection and preparation of figures, and Shailesh Kantak for insightful comments on the manuscript. We also would like to give special thanks to the patients for participating in the study and their families for supporting their participation.