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

Abstract

Objectives: To describe an electrophysiological method for determining the relation between lumbar cord dorsal roots and cathode of epidural electrode for spinal cord stimulation (SCS). Materials and methods: Data has been collected from 13 subjects who have been under evaluation of eectiveness of SCS for control of spasticity. Induced muscle twitches from both quadriceps (Q), adductors (A), hamstrings (H), tibial anterior muscles (TA) and triceps surae muscles (TS) were simultaneously recorded with surface-electrode polyelectromyography (pEMG) and analyzed 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 stimulus strength. Latency times were rather invariable regardless of stimulus 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 dierentiate between upper and lower lumbar cord segments. Furthermore, our ®ndings regarding amplitude, latency and recruitment order strongly suggest that we stimulate posterior roots not posterior columns of the lumbar spinal cord. Spinal Cord (2000) 38, 394 ± 402

Epidural electrical stimulation of posterior structures of the human lumbosacral cord: 3. Control of spasticity

“…A detailed description of how the correct site of upper lumbar stimulation can be located is given in the ®rst paper of this series. 22 All other stimulation parameters are variable. Frequencies and amplitudes must be adapted on a case-by-case basis, regardless of whether the electrode is operated in unipolar or bipolar mode.…”

“…The de®nitive location of the electrode was determined based on muscle twitch responses by the quadriceps and adductor muscles elicited at an amplitude of 1 ± 4.5 V and a pulse width of 210 ms. For a detailed description of this procedure, the reader is referred to the ®rst paper of this series. 22 For neurosurgical intervention we followed the description given by Campus et al…”

“…Interestingly, all electrodes were found to be located between the bottom segment of TV 11 and the top segment of LV 1. What is more, the site of stimulation along the spinal cord, as determined by muscle twitches patterns, 22 was such that the active cathode invariably stimulated the upper and middle portion of the lumbar cord. Two patients (#4 and #6) showed low-amplitude responses for tibial anterior and triceps surae in addition to high- The decisive role of correct placement becomes apparent when we use the same stimulation parameters on dierent stimulation sites.…”

Epidural electrical stimulation of posterior structures of the human lumbosacral cord: 2. quantitative analysis by computer modeling

“…2 However we have to simplify the situation in order to illuminate the relevant relations in the epidural electrical stimulation of the lumbosacral cord. We analyze which nerve structures become stimulated and the resulting sequence of excitation when the voltage of a stimulating dipole is increased.…”

“…2 The muscle twitches are measured at stimulating frequencies of 5 Hz. At such a low frequency each arti®cially generated action potential causes a single twitch.…”