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Comparative Study
, 32 (6), 1032-9

A Paradox: After Stroke, the Non-Lesioned Lower Limb Motor Cortex May Be Maladaptive

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Comparative Study

A Paradox: After Stroke, the Non-Lesioned Lower Limb Motor Cortex May Be Maladaptive

Sangeetha Madhavan et al. Eur J Neurosci.

Abstract

What are the neuroplastic mechanisms that allow some stroke patients to regain high-quality control of their paretic leg, when others do not? One theory implicates ipsilateral corticospinal pathways projecting from the non-lesioned hemisphere. We devised a new transcranial magnetic stimulation protocol to identify ipsilateral corticospinal tract conductivity from the non-lesioned hemisphere to the paretic limb in chronic stroke patients. We also assessed corticospinal tract degeneration by diffusion tensor imaging, and used an ankle tracking task to assess lower limb motor control. We found greater tracking error during antiphase bilateral ankle movement for patients with strong conductivity from the non-lesioned hemisphere to the paretic ankle than for those with weak or no conductivity. These findings suggest that, instead of assisting motor control, contributions to lower limb motor control from the non-lesioned hemisphere of some stroke survivors may be maladaptive.

Figures

Figure-1
Figure-1
Schematic showing examples for the presence and absence of ipsilateral conductivity as determined from our TMS protocol. Panels a and b show recruitment curves when the TMS coil is positioned contralateral (blue) and ipsilateral (red) to the paretic muscle. Ipsilateral conductivity is assumed when the slope of the ipsilateral recruitment curve is steeper than the contralateral slope, thereby generating a negative value for the index of cortical excitability (ICE).
Figure-2
Figure-2
Representative examples from patients VI (top) and VII (bottom). a. Coronal view of diffusion tensors. White matter direction is illustrated with anterior-posterior fibers in green, lateral fibers in red, and superior-inferior fibers in blue (e.g., the corticospinal tracts). Cross hairs are positioned to denote the degeneration of white matter in the posterior limb of internal capsule. Patient VI had a FA asymmetry of 0.17 and patient VII had a FA asymmetry of 0.05. b. Recruitment curves from the paretic TA with the coil positioned contralateral (blue) and ipsilateral (red) to the muscle. The ICE values for patients VI and VII were 0.27 and −0.42 respectively. Note that the slope of the ipsilateral recruitment curve is higher than the contralateral curve on patient VII. c. Non-paretic leg tracking during antiphase pattern. The black line is the target and the blue line is the response. The movement of the paretic ankle is shown in red. Note the degradation in coordination for patient VII.
Figure-3
Figure-3
Tracking accuracy during different conditions for stroke patients. The y axis shows the accuracy index (AI) and the x axis shows the three different tasks – UNI, IP, and AP movement patterns. The black bars represent the paretic ankle during tracking and the white bars represent the non-paretic ankle during tracking. Data represent averages (± SEM) of 10 patients. A significant interaction was examined using t-tests (**, p < 0.01).
Figure-4
Figure-4
Index of corticospinal excitability (ICE). The y axis shows ICE values and x axis represents the paretic and non-paretic tibialis anterior (TA) muscle. Positive ICE values represent no ipsilateral conductivity and negative values represent ipsilateral conductivity. Note the absence of ipsilateral conductivity for the non-paretic TA.
Figure-5
Figure-5
Relationship between ICE calculated from paretic limb recruitment curves when the coil was placed over the ipsilateral (non-lesioned) cortex, and the difference in AI between AP and IP calculated from the non-paretic ankle tracking data. The black bar represents the mean of the tracking error difference in patients (n = 4) who had negative values of ICE (evidence of ipsilateral conductivity to the paretic limb), and the white bar represents the mean in patients (n = 5) who had positive values of ICE (no evidence of ipsilateral conductivity to the paretic limb). More negative values represent greater degradation in AI during AP than IP tracking. Error bars represent 1 SEM. **, p < 0.01.

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