Can we enhance motor recovery after stroke?
Stroke is the leading worldwide cause of adult disability, leaving approximately two thirds of affected patients unable to incorporate the paretic arm into everyday use. Significant spontaneous recovery is commonly observed however, arising from a combination of several factors. It is now recognised that the reorganisation of cortical networks controlling movement plays a role in such improvement, providing a potential substrate for therapeutic interventions aiming to enhance the recovery process. Functional imaging studies have revealed that patients with stroke recruit a wider than normal cortical network when moving the paretic hand, and that the premotor cortex of the intact hemisphere may assume a positive role in more affected patients. Transcranial magnetic stimulation (TMS) was employed in a novel combination with functional MRI to test the interactions of this brain region with remote cortical areas. A state-dependent interhemispheric interaction with the primary motor cortex was revealed, suggesting that the contralesional premotor cortex supports recovered motor function via a transcallosal interaction with the affected hemisphere. In a longitudinal physiological study TMS was used to assess corticospinal and intracortical excitability in a group of patients with first ischaemic stroke. Abnormalities of excitability were observed in both hemispheres correlating with measures of upper limb function in a time-dependent manner. The pattern observed suggests a shift with time from early dependence on the residual corticospinal projection (in the acute phase) to a reliance on intracortical disinhibition in more affected patients (by 3 months), providing access via cortico-cortical projections to remote cortical regions. A number of electrical and magnetic brain stimulation techniques are now available which can induce transient changes in the activity of stimulated cortical regions. Short-lived behavioural improvements can be induced in stroke patients by such means but a more promising therapeutic approach is to ‘prime’ the brain to enhance the response to motor training during physiotherapy. Several aspects of how best to achieve this are at present unclear. Theta burst stimulation is a low intensity repetitive TMS protocol which transiently alters cortical excitability, and which can enhance motor task acquisition in healthy subjects. Recent experiments combining brain stimulation with dopaminergic medication in a group of stroke patients are presented, and wider issues surrounding the future development of such a combined restorative approach are discussed.