Electrophysiology of Sensory and Motor Nerve Root Fibers in Selective Dorsal Rhizotomies

Aims: Spasticity remains a major impediment in the treatment of cerebral palsy (CP). The single-level selective dorsal rhizotomy (SDR) is a minimally invasive intervention that reduces spasticity in select patients. We provide a descriptive set of normative data that practitioners can utilize to help guide the single-level SDR procedure, including (1) physiological threshold values used to dissociate ventral from dorsal roots; (2) response characteristics of muscles; (3) descriptions of abnormal physiological responses; and (4) percentage of rootlets transected during surgery. Methods: We examined data from 38 patients with CP who underwent SDR. Dorsal and ventral roots were classified based on the amplitude of electromyographic (EMG) responses, number of muscles activated, and abnormal response characteristics. Results: Ventral roots activated more muscles at significantly lower stimulus thresholds and demonstrated larger EMG responses than did dorsal roots. Of the transections made, 64.72 ± 1.69% of each rootlet was transected. Ventral and dorsal roots can be readily separated based on a few key physiological characteristics including response thresholds and the spread of muscle activation. It was observed that a threshold of approximately 0.4 mA could be used to dissociate ventral and dorsal roots during surgery. Conclusions: These data illustrate the range of physiological variance observed while performing SDR in patients with spastic CP. Notably, we encountered outlier patients whose roots demonstrated aberrant response characteristics and displayed uncharacteristically low dorsal root thresholds or abnormally high ventral root thresholds. Practitioners should be prepared to individualize their threshold criteria and customize treatment on a patient-by-patient basis.