Both pallidotomy and deep brain stimulation (DBS) have been proven to be effective in suppressing levodopa-induced dyskinesias (LID). The effects of pallidal stereotactic surgery on parkinsonian features have, however, been less consistent.
To supplement existing knowledge of globus pallidus (GP) pathophysiology, we took advantage of the fact that DBS is reversible and that implanted electrodes contained four discrete stimulation sites (contacts 0, 1, 2, and 3) to study the effect of high-frequency stimulation (1 30 Hz) in the GP of parkinsonian patients being treated with bilateral DBS for severe LID and motor fluctuations. We found marked differences in effect between stimulation at two distinct sites in the GP.' Stimulation applied in the dorsal part of the GP (that is, through the contact 3 ["upper"] of the quadripolar electrode) significantly improved all parkinsonian features but generated dyskinesias, even in the "off' drug condition. Stimulation applied in the posteroventral GP (that is, through the contact 0 ["lower"] of the electrode) suppressed LID and reduced rigidity but significantly worsened gait and akinesia in both the "off" and "on" drug conditions.
The videotape illustrates the persisting differential effect of stimulation in the dorsal and posteroventral parts of the GP 6 months after neurosurgical implantation of the electrodes and after chronic continuous bilateral stimulation.
The evidence clearly demonstrating two distinct sites in the GP could explain why pallidotomy results in variable effects on parkinsonian motor features despite a constant effect on LID abolitiom2 Pallidotomy invariably includes a lesion of the posteroventral part of the GP, explaining the constant effect on dyskinesias, but one might suppose that, depending on the size of the lesion, the dorsal GP is variably affected leading to contrasting effects on parkinsonian motor signs. In DBS treatment, the existence of at least two distinct sites in the GP needs to be taken into account when adjusting the therapeutic parameters of pallidal stimulation. The pathophysiological basis for these differential effects is still lacking, and further anatomic studies will be needed because these observations do not fit the classical anatomo-functional model of the basal ganglia.3
Legends to the Videotape
The video shows the effect of DBS applied in the posteroventral globus pallidus (patient on or off levodopa) and in the A videotape accompanies this article.