Bimanual Coordination Impairments
Children with unilateral spastic CP have impairments in bimanual coordination beyond unimanual dexterity deficits.34-36 Similar to motor planning, the relation between type/timing of the lesion and such bimanual coordination impairments is unknown. During symmetrical, bimanual reaching tasks, children with unilateral spastic CP showed the ability to coordinate their bimanual movements by compensating with their non‐involved hand as long as accuracy demands or task complexity were not increased. In a recent study, participants were instructed to hold a grip device in each hand and place one device on top of the other while the grip and load force were recorded simultaneously in both hands (Fig. 3a).37 Children with CP initiated the task by decreasing grip force in the releasing hand before increasing the force in the holding hand during the preparation phase, with the subsequent grip force increase in the holding hand being smaller and occurring later (transition phase) than that of typically developing children (Fig. 3a). The impairments were unrelated to the presence of mirror movements. The impairment was greater when the less‐affected hand served as the holding hand.
In another series of studies,38, 39 participants were asked to open a drawer with one hand and manipulate its contents with the other hand. Children with unilateral spastic CP were less coordinated, with reduced movement overlap of the drawer‐opening hand (solid trace) and manipulating hand (dashed trace) (ii–v) and sequential completion of opening the drawer and manipulating its contents (v, vi) (Fig. 3b).38 Interestingly, bimanual training improved some aspects of this coordination more than constraint‐induced movement therapy.39
Overall, we have described the pathophysiology underlying impaired upper extremity function of unilateral spastic CP, with particular emphasis on the relation between CST damage and hand function. We have described the resulting sensory and motor deficits, with an emphasis on studies of precision grip. These studies show impairments in (1) motor execution; (2) sensorimotor integration; (3) motor planning; and (4) bimanual coordination beyond dexterity impairments. Knowledge about the various forms of unilateral spastic CP is still rather limited. The type/timing of the lesion is fairly predictive of motor execution and sensorimotor integration impairments, although the relation between lesions and motor planning and bimanual coordination is much less understood.
These findings have several clinical implications. First, the finding that precision grip improves only with extensive practice19 emphasizes the importance of intensity of training. In agreement, training protocols that provide such intensity (such as constraint‐induced movement therapy and intensive bimanual training)20, 21 appear to be efficacious in improving hand function, whereas little evidence exists for treatments provided at usual and customary care schedules. Second, the potential interference of the more‐affected hand on the less‐affected hand36 during bimanual activities may partly explain why children with unilateral spastic CP prefer to use only one hand during tasks typically using both hands. Findings that environmental constraints affect performance17 suggest the importance of context, and that environmental constraints could be used in rehabilitation to create variability of practice. Finally, improved performance during simultaneous or sequential (transfer) bimanual actions (e.g. kinematic mirroring) emphasizes the potential contribution that the less‐affected hand could make to rehabilitation of the more‐affected hand.
Although the CST damage is highly predictive of severity of hand impairments, an intriguing possibility is that the specific pathophysiology is predictive of treatment outcomes. For example, it has been suggested that individuals who have undergone reorganization of the CST fare worse after constraint‐induced movement therapy than individuals who maintain contralateral CST innervation.40 This raises the possibility that there is an interaction between the connectivity and integrity of the CST and the efficacy of different training approaches for the upper extremities. Constraining the less‐affected upper extremity may drive decrease activity in the unaffected primary motor cortex and thus decrease the consequent interhemispheric inhibition of that hemisphere over the affected primary motor cortex. This may be suitable for children maintaining contralateral CST innervation; however, in children with ipsilateral CST reorganization, constraining the less‐affected upper extremity may drive down primary motor cortex activity controlling both upper extremities, possibly impeding recovery. One could speculate that bimanual training may be a better approach for children with ipsilateral CST innervation, but the efficacy may depend on the integrity of interhemispheric connections. Thus a ‘one‐treatment fits all approach’ may not be sufficient. We speculate that future rehabilitation efforts will be best guided by testing these possibilities and closely relating treatment efficacy with specific pathophysiology on an individual‐by‐individual basis.