Upper Limb Immobilisation: A Neural Plasticity Model with Relevance to Poststroke Motor Rehabilitation

Abstract

Advances in our understanding of the neural plasticity that occurs after hemiparetic stroke have contributed to the formulation of theories of poststroke motor recovery. These theories, in turn, have underpinned contemporary motor rehabilitation strategies for treating motor deficits after stroke, such as upper limb hemiparesis. However, a relative drawback has been that, in general, these strategies are most compatible with the recovery profiles of relatively high-functioning stroke survivors and therefore do not easily translate into benefit to those individuals sustaining low-functioning upper limb hemiparesis, who otherwise have poorer residual function. For these individuals, alternative motor rehabilitation strategies are currently needed. In this paper, we will review upper limb immobilisation studies that have been conducted with healthy adult humans and animals. Then, we will discuss how the findings from these studies could inspire the creation of a neural plasticity model that is likely to be of particular relevance to the context of motor rehabilitation after stroke. For instance, as will be elaborated, such model could contribute to the development of alternative motor rehabilitation strategies for treating poststroke upper limb hemiparesis. The implications of the findings from those immobilisation studies for contemporary motor rehabilitation strategies will also be discussed and perspectives for future research in this arena will be provided as well.

Figure 1

This figure illustrates the interaction between two prevailing theories of motor recovery after stroke, named here as the “reactivation” and “rebalancing” theories (a), and some of the contemporary motor rehabilitation strategies for treating poststroke upper limb hemiparesis that have been largely underpinned by these theories (b). Red explosion-like balloon: hemiparetic stroke. Light green circle: depression, that is, decreased size and/or excitability, of residual cortical motor representations in the adjacent, perilesional tissue. Dark green circle: overactivity of homologous cortical motor representations in the opposite, undamaged cerebral hemisphere. Light blue arrow: decreased transcallosal inhibition. Dark blue arrow: increased transcallosal inhibition. Red thin downward arrow: reduced use of the paretic upper limb contralateral to the stroke side. Black thin upward arrow: increased skilled use of the paretic upper limb through physiotherapy in the form of task-specific exercises. Red-yellow bolts: adjunctive therapies, such as excitatory and inhibitory brain stimulation (+BS and −BS, resp.) and peripheral somatosensory stimulation (PSS), to be combined with physiotherapy exercises. White tick upward arrow: increase activity in the ipsilesional motor cortex. White tick downward arrow: decrease activity in the contralesional motor cortex. See text for further details.

Figure 2

(a) highlights the similarity between the maladaptive neural plasticity patterns that often occur after a hemiparetic stroke and that are currently thought to play an important role in mediating the individual's motor deficits (see Figure 1(a) for comparison) and the neural plasticity patterns that are induced by upper limb immobilisation (red crossed circle) in otherwise healthy individuals. Light green circle: depression, that is, decreased size and/or excitability, of the cortical motor representation(s) corresponding to the immobilised body part(s). Dark green circle: overactivity of the homologous cortical motor representation(s) in the opposite cerebral hemisphere. Light blue arrow: decreased transcallosal inhibition. Dark blue arrow: increased transcallosal inhibition. (b) indicates potential interventions that could be delivered during a paradigm of upper limb immobilisation in healthy individuals in order to prevent maladaptive or promote adaptive neural plasticity in the motor system. These interventions might include, for instance, covert motor strategies, such as action observation (AO) and (likely) motor imagery (MI) (red-yellow balloon), and adjunctive therapies, such as excitatory and inhibitory brain stimulation (+BS and −BS, resp.) and peripheral somatosensory stimulation (PSS) (red-yellow bolts). White tick upward arrow: increase activity in the motor cortex contralateral to the immobilisation. White tick downward arrow: decrease activity in the motor cortex ipsilateral to the immobilisation. The idea here is that this might contribute to the development of alternative motor rehabilitation strategies for treating poststroke upper limb hemiparesis. See text for further details.