The aim of this study was to compare, retrospectively, the value of chronic bilateral stimulation of the internal globus pallidus (GPi) and the subthalamic nucleus (STN) in patients with young onset Parkinson's disease. We selected 13 consecutive patients with similar characteristics at the time of surgery: age at onset < 40 years, disabling motor fluctuations (Hoehn and Yahr stage 4 or 5 in off-drug phases) and levodopa-induced dyskinesias (LID). Eight patients were operated on in the STN and five in the GPi. The Unified Parkinson's Disease Rating Scale (UPDRS), timed motor tests and a LID scale were compared in on- and off-drug conditions before surgery and 6 months after surgery on stimulation using the chronic electrical parameters found to improve best the motor state of the individual patient, without adverse effects. In off-drug phases, the motor score of the UPDRS was improved by 71% with STN stimulation and by 39% with GPi stimulation on average. This difference was statistically significant (P < 0.05). Whereas rigidity and tremor showed good improvement in both groups, the decrease in the akinesia score was more pronounced in the STN group. In the STN group, the improvement of all motor symptoms was very close, or equal, to the best levodopa response. Thus the levodopa test was predictive of outcome. The improvement in off-drug period motor handicap allowed a decrease in the levodopa-equivalent dose only in the STN group (-56%). The voltage, frequency and pulse width used for chronic stimulation were lower in the STN group. In the on-drug phases there was a marked improvement in LID in the GPi group, as measured by the dyskinesias score during an acute levodopa test, whereas there was only a small decrease in the STN group (P < 0.05). However, in the long term, the reduction of levodopa dosage in the STN group led to an indirect reduction of LID similar to that in the GPi group during activities of everyday life. In conclusion, the overall results favour the neurosurgical treatment of Parkinson's disease by stimulating the STN rather than the GPi.

The effect of chronic bilateral high-frequency stimulation of the subthalamic nucleus (STN) on levodopa-induced dyskinaesias was investigated in eight patients with fluctuating Parkinson's disease complicated by functionally disabling off-period dystonia. All of the patients also had severe diphasic and peak-dose chorea, so that it was possible to study the effect of high-frequency stimulation on the different types of levodopa-induced dyskinaesias. Off-period fixed dystonia was reduced by 90% and off-period pain by 66%. After acute levodopa challenge, high-frequency stimulation of the STN reduced diphasic mobile dystonia by 50% and peak-dose choreic dyskinaesias by 30%. The effect of bilateral high-frequency stimulation of the STN on the Unified Parkinson's Disease Rating Scale motor score had the same magnitude as the preoperative effect of levodopa. This allowed the levodopa dose to be reduced by 47%. The combination of reduced medication and continuous high-frequency stimulation of the STN reduced the duration of on-period diphasic and peak-dose dyskinaesias by 52% and the intensity by 68%. Acute high-frequency stimulation of the STN mimics an acute levodopa challenge, concerning both parkinsonism and dyskinaesias, and suppresses off-period dystonia. Increasing the voltage can induce repetitive dystonic dyskinaesias, mimicking diphasic levodopa-induced dyskinaesias. A further increase in voltage leads to a shift from a diphasic-pattern dystonia to a peak-dose pattern choreodystonia. Chronic high-frequency stimulation of the STN also mimics the benefit of levodopa on parkinsonism and improves all kinds of levodopa-induced dyskinaesias to varying degrees. Off-period dystonia, associated with neuronal hyperactivity in the STN is directly affected by stimulation and disappears immediately. The effect of chronic high-frequency stimulation of the STN on diphasic and peak-dose dyskinaesias is more complex and is related directly to the functional inhibition of the STN and indirectly to the replacement of the pulsatile dopaminergic stimulation by continuous functional inhibition of the STN. Chronic high-frequency stimulation of the STN allows a very gradual increase in stimulation parameters with increasing beneficial effect on parkinsonism while reducing the threshold for the elicitation of stimulation-induced dyskinaesias. In parallel with improvement of parkinsonism, the levodopa dose can be gradually decreased. As diphasic dystonic dyskinaesias are improved to a greater degree than peak-dose dyskinaesias, both direct and indirect mechanisms may be involved. Peak-dose choreatic dyskinaesias, associated with little evidence of parkinsonism and thus with low neuronal activity in the STN, are improved, mostly indirectly. Fixed off-period dystonia, mobile diphasic dystonia and peak-dose choreodystonia seem to represent a continuous clinical spectrum reflecting a continuous spectrum of underlying activity patterns of STN neurons.

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Patients with Parkinson's disease frequently have mild to moderate depression and exhibit low hedonic tone. The authors investigate the impact of a single L-dopa challenge and the acute effects of electric stimulation of the subthalamic nucleus (STN) on symptoms of depression and hedonic tone. Depressive symptoms improved with L-dopa and STN stimulation to the same extent. However, hedonic tone improved only with L-dopa. Most of the emotional changes did not correlate with changes in motor performance, indicating they were not just reactive but specific to the treatment. These results demonstrate a single dissociation of depressive symptoms and anhedonia in response to an acute L-dopa and STN-stimulation challenge.

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