O130 Motor representation areas of upper limb muscles in healthy children, adolescents and adults – A developmental navigated TMS study

Abstract

Objectives Navigated TMS (nTMS) is commonly used to map motor cortex in adults. However, little is known about the development of the motor representations in younger subjects. We did a nTMS mapping study to assess the feasibility of motor cortex mapping in children, and to examine developmental changes in the locations and extents of distal and proximal upper extremity muscles. The relation to manual dexterity was also studied. Methods Both hemispheres of healthy, right-handed children (6–9 years, n = 10), preadolescents (10–12 years, n = 12), adolescents (15–17 years, n = 12) and adults (22–34 years, n = 12) were examined. The optimal site for abductior pollicis brevis (APB) was localized and resting motor threshold (rMT) determined. Motor mapping was performed at 110% of rMT of the APB was performed while recording EMG of three distal (APB, ADM, FDI) and three proximal (extensor carpi radialis, flexor carpi radialis, biceps brachii) muscles. The representation area was calculated with spline-interpolation and the ratio of hand and arm was correlated with a motor skill task (box and block test, BBT). Results The mapping was feasible in all but two of the youngest children whose rMT exceeded the maximum stimulator output. In younger subjects, the muscles of the forearm were occasionally eliciting larger and more frequent MEPs. The optimal sites of APB were found in anatomically expected locations, either in the precentral gyrus or in central suclus. The hand/arm representations ratio was below 1 in children, and increased approximately linearly with age. This ratio also correlated with BBT test score (rho = 0.456, p = 0.002). Discussion nTMS suits well for studying of development of the motor cortex. It may be worth monitoring several upper limb muscles during motor mapping in children as the excitability of the forearm muscles may be lower than that of small hand muscles. The hand/arm representations ratio increases with development and associates with greater fine motor ability in the right hemisphere. Conclusions Our results demonstrate a developmental course in the organization of the cortex and its associations with motor skills. Significance The results of this study contribute to the elucidation of the development of motor skills and the applicability of nTMS in pediatric populations.