This commentary is on the original article by Kuo et al. on pages 65-71 of this issue.
Kuo et al.1 investigate whether diffusion tensor imaging (DTI) can be used to identify preserved corticospinal tract projections from the affected motor cortex in children with unilateral cerebral palsy (CP). Children in whom this projection is partly preserved have in general better hand function than those with a predominant ipsilateral corticospinal tract projection from the unaffected motor cortex.Traditionally, descending motor pathways have been investigated non-invasively in humans using transcranial magnetic stimulation (TMS). This method provides a functional assessment of corticospinal tract connectivity by measuring the motor-evoked response in muscle in response to a brief magnetic pulse from a coil held over the motor cortex. TMS is not always well tolerated though, and some children are considered ineligible because of an active seizure disorder. An alternative non-invasive structural imaging-based method would be valuable; however, non-invasive assessment of the corticospinal tract is not as easy as it sounds. Identification of the corticospinal tract with DTI is a complex reconstruction process involving a mixture of combining neuroanatomical prerequisites with tracking of 'fibres' based on adjacent voxels with properties consistent with a single, coherently oriented bundle of axons with high fractional anisotropy. Areas where fibres cross are difficult to track; errors could lead to tracking pathways which do not exist. In contrast to TMS, which assesses the whole motor pathway from cortex to muscle, DTI only provides information as far as the brainstem. In children with severe unilateral CP, it may be challenging to reconstruct a highly disorganized tract with DTI, leading to false negative results.TMS applied over the motor cortex does not exclusively stimulate the corticospinal tract. Oher pathways such as the bilaterally projecting reticulospinal tract can also be activated. But early latency motor-evoked potentials predominantly reflect the fastest-conducting corticospinal tract fibres. Some participants with unilateral CP may have high thresholds for activation of the affected corticospinal tract, potentially giving false negative results. Also, as the response to TMS is measured using surface electromyography over target muscles, altered peripheral nerve conduction times are a theoretical confounder.Before we rush to undertake both TMS and DTI in our patients with unilateral CP to obtain as clear a picture as possible of their corticospinal tract reorganization, we need to consider how this will help. Can we target children to optimal therapy interventions based on their 'wiring patterns'? In one study using constraint-induced movement therapy (CIMT), quality of movements improved in both groups but speed of performance only improved in children with preserved crossed projections.
2In contrast, Islam et al.3 saw improvements with CIMT, including speed of performance, in children with all forms of cortic...