Selective dorsal rhizotomy improves function by increasing the rate of motor activation without altering the motor program of spasticity

Selective dorsal rhizotomy (SDR) is a new surgical procedure for treatment of spasticity, primarily used in children with cerebral palsy. Dramatic gains in ambulation occur post-SDR, which can be quantified by gait analysis. We have analyzed the underlying mechanism for this improvement. We studied patients before and 6 weeks after SDR, EMG recordings were made from anterior tibial (AT) and medial gastroenemius (MG) muscles bilaterally, while the patients performed voluntary, self-paced ankle dorsi- and plantar flexion. The duration and activation pattern of rectified EMG bursts were measured in 76 patients. We found that a pattern of agonist-antagonist co-activation persisted unaltered post-SDR in the majority of patients. The mean burst discharge duration decreased from 1579.5 ± 849.6 msec pre-op in AT to 1039.7 ± 595.7 msec post-op and from 1584.5 ± 864.8 msec to 1032.0 ± 585.8 msec in MG. There was a corresponding increase in the discharge rate in each muscle. We conclude that the motor programs used by these spastic patients is unaltered by SDR. The efficiency of its use, however is markedly improved, and this contributes to the functional improvement seen in SDR patients.