Children with secondary dystonia due to cerebral palsy exhibit abnormalities of muscle activation that lead to joint stiffness and limit functional arm movements. Dystonic symptoms can be severe and often interfere with simple daily tasks such as eating and dressing. However, very little is known about the mechanisms of upper extremity impairment, making the slow arm movements in this disorder challenging to treat clinically. The goal of this dissertation is to use peripheral mechanical perturbations as well as transcranial magnetic stimulation to characterize electrophysiological mechanisms of impairment in childhood dystonia due to cerebral palsy.

We discovered that the biceps brachii muscle, which opposes elbow extension, is excessively activated in response to imposed stretch and during voluntary elbow extension in childhood secondary dystonia due to cerebral palsy. Contributors to the excess muscle activity include stretch reflexes, overflow from the triceps brachii muscle, and direct activation of the biceps brachii muscle. Additionally, we showed that reflex responses to muscle stretch from rest and during movement are abnormally elevated at three latencies in dystonia. Long latency reflex responses, which are associated with involvement of primary motor cortex, are especially pronounced. To probe the involvement of motor cortex directly, we used transcranial magnetic stimulation in children without dystonia to show that activation of motor cortex can in fact lead to abnormalities in long latency stretch reflexes.

Experimental results suggest that dystonia due to cerebral palsy involves excessive activation of postural mechanisms, including stretch reflexes, which may interfere with voluntary movement. This result brings to question the commonly accepted notion that coactivation of opposing muscles is the primary cause of hypertonia in dystonia and brings to light the role of stabilizing postural stretch reflexes in the disorder. In addition, the implication of motor cortex in the observed abnormalities in muscle activity indicates the need for future research to address cortical contributions to dystonia due to cerebral palsy. Finally, the careful study of contributors to antagonist muscle activation during movement provides a quantitative basis for the selection of therapies targeting the specific impairments in individual children.