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

Subthalamic nucleus, deep brain stimulation, Parkinson's disease.

The value of the apomorphine test as a predictor of the clinical outcome of deep brain stimulation of the subthalamic nucleus (STN) was evaluated in patients with advanced idiopathic Parkinson's disease (IPD) or multiple system atrophy (MSA). Thirteen IPD patients with severe diurnal fluctuations and one MSA patient not responding to dopaminergic drugs were assessed with the Unified Parkinson's Disease Rating Scale (UPDRS) and the timed finger tapping test (FTT), measured preoperatively on and off apomorphine and postoperatively on and off STN stimulation. UPDRS motor items 20-25 were assessed intraoperatively on and off STN stimulation when the clinically effective target was approached. The motor response to immediate intraoperative and long-term STN stimulation was correlated with results of the apomorphine test. The response to immediate intraoperative STN stimulation was accurately predicted by apomorphine challenge in all 13 IPD patients. Clinical outcome following long-term STN stimulation was correlated significantly with preoperative changes due to apomorphine measured with the UPDRS motor scores (r = 0.7125, P < 0.01) and FTT (r = 0.9276, P < 0.001). Moreover, comparison of long-term STN stimulation to preoperative drug treatment displayed a significant reduction in the duration of off-phases and a significant increase in the duration of on-phases. However, in the single patient with MSA no beneficial response was obtained either to apomorphine or to STN stimulation intraoperatively and during the postoperative externalized test period. Our results indicate that the apomorphine test can predict the outcome of immediate and long-term STN stimulation and may help in the selection of candidates for surgery.

The purpose of this study was to evaluate the effect of unilateral stimulation of the nucleus ventralis intermedius (VIM) on parkinsonian signs like postural stability and locomotion with respect to the severity of Parkinson's disease (PD). Seven patients with idiopathic PD were included in the study. Changes in visual cues on postural stability and step initiation were assessed on a fixed platform system with VIM stimulation switched either on (VIM ON) or off (VIM OFF), and compared with a control group of seven age‐matched normal individuals. Sway scores (area and path) were significantly (p <0.05) higher in the parkinsonian patients with VIM OFF than with VIM ON as well as compared with the control subjects. No correlation was obtained between extent of sway scores and severity of contralateral tremor after cessation of VIM stimulation. Locomotion parameters, by contrast, were not influenced by VIM stimulation: latency until step initiation and walking‐cycle time were the same among parkinsonian patients as among normal individuals, both in the presence and in the absence of VIM stimulation. In conclusion, our results indicate that tremor suppression by VIM stimulation improves postural stability.