Introduction: Evidence of plasticity in neuronal systems, specifically in the motor system is emerging. For example, when the motor cortex is diseased or removed, its action is taken over by residual tissue, and nearby premotor/sensory regions (1). Using transcranial magnetic stimulation (TMS), we investigated the mechanism of functionality in muscles contralateral to the lesion even when the entire motor network (primary motor cortex, premotor cortex, and pyramidal tracts) in the hemisphere has been removed due to injury or surgery.
Methods: Five patients (3F, 11.5±6.4 years) who had undergone complete functional hemispherectomy (H group) for treatment of refractory epilepsy, and five patients (3F, 9.8±3.8 years) with history of epilepsy due to unilateral brain lesions who had also undergone surgical resection (C group) were studied. The demographics and nature of lesions in the patients are detailed in Table 1. Navigated TMS was delivered using a ‘figure 8’ coil (Nexstim Inc.). In both groups, left and right primary hand motor cortices (M1hand) were targeted and single pulses of TMS were delivered while monitoring electromyography in bilateral hand and forearm muscles. Motor evoked potentials (MEP) following TMS were recorded from these muscles and the motor hotspot, resting motor threshold (rMT), and the corticomotor conduction times (CMCT) were determined. Spinal stimulation was performed to examine peripheral conduction. DTO was acquired in three patients in the H group to confirm complete hemispherectomy.
Results: TMS applied to the precentral gyrus of the healthy hemisphere of H group resulted in MEP in both hand muscles in all participants, as shown in Figures 2 and 3. TMS stimulation of the diseased hemisphere did not elicit MEPS. TMS applied to bilateral precentral gyri elicited MEPs with normal corticomotor transit times only in the respective contralateral hand muscles in C group (Figure 1). In three participants in the H group, DTI confirmed complete hemispherectomy with no evidence of contiguous commisural fibers traversing the midline through the corpus callosum. There was no difference in rMT between the groups, or the hemisphere removed/affected.
Conclusions: These data indicate that the motor function is retained in the diseased hemisphere in patients with lesions in the vicinity of the motor cortex, when the rest of the motor network (pyramidal tracts and motor association areas) is intact (as seen in Figure 1). However, in the patients in the H group, the bilateral innervations of the motor system originated from the healthy hemisphere (Figure 2). We hypothesize that this motor plasticity is mediated via the uncrossed corticospinal tracts arising from the healthy primary motor cortex. Normally, at birth the corticospinal tracts project bilaterally at the spinal motor neuron level that becomes contralateral by 2 year of age (2). However, damage to unilateral corticospinal tracts during this period, results in enlargement of the ipsilateral terminal axonal arborization both within spinal gray matter and motoneuronal poos (2), and such plasticity can support functionality in the ipsilateral muscles. MEPS in the hand muscles contralateral to the diseased cortex were elicited by stimulating the spinal nerves ipsilateral pathways. Therefore early and intensive physical therapy can improve functional outcome following hemispherectomy by invoking the mechanism of plasticity.
Publication: Abstract from International Conference on Functional Mapping of the Human Brain, June 2013, Seattle, WA