Epidural electric stimulation of posterior structures of the human lumbar spinal cord: 1. muscle twitches - a functional method to define the site ofstimulation

Objectives: To describe an electrophysiological method for determining the relation between lumbar cord dorsal roots and cathode of epidural electrode for spinal curd stimulation (SCS). Materials and methods: Data has been collected from 13 subjects who have be en under evaluation of effectiveness of SCS for control of spasticity. Indu ced muscle twitches from both quadriceps (Q), adductors (A), hamstrings (H) , tibial anterior muscles (TA) and triceps surae muscles (TS) were simultan eously recorded with surface-electrode polyelectromyography (pEMG) and anal yzed for amplitudes. latency times and recruitment order. Results: Stimulation of dorsal lumbar cord structures evoked characteristic EMG events during muscle twitch responses. Their amplitudes varied with st imulus strength. Latency times were rather invariable regardless of stimulu s strength. Two distinct recruitment orders were demonstrated depending on whether the stimulating cathode was placed over the upper (= response from quadriceps and/or adductor muscles) or the lower (= response from tibialis anterior and triceps surae) lumbar cord segments. The chances to stimulate upper lumbar cord segments are best around the 12th thoracic vertebra. Conclusions: pEMG recording of muscle twitches enables us to accurately dif ferentiate between upper and lower lumbar cord segments. Furthermore, our f indings regarding amplitude, latency and recruitment order strongly suggest that we stimulate posterior roots not posterior columns of the lumbar spin al cord.