Abstract
Deep brain stimulation (DBS) has provided remarkable benefits for people with a variety of neurologic conditions. Stimulation of the ventral intermediate nucleus of the thalamus can dramatically relieve tremor associated with essential tremor or Parkinson disease (PD). Similarly, stimulation of the subthalamic nucleus or the internal segment of the globus pallidus can substantially reduce bradykinesia, rigidity, tremor, and gait difficulties in people with PD. Multiple groups are attempting to extend this mode of treatment to other conditions. Yet, the precise mechanism of action of DBS remains uncertain. Such studies have importance that extends beyond clinical therapeutics. Investigations of the mechanisms of action of DBS have the potential to clarify fundamental issues such as the functional anatomy of selected brain circuits and the relationship between activity in those circuits and behavior. Although we review relevant clinical issues, we emphasize the importance of current and future investigations on these topics.
Keywords: movement disorders, Parkinson disease, dystonia, tremor, physiology
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INTRODUCTION
Deep brain stimulation (DBS) has provided dramatic clinical benefit for people with essential tremor (ET) and Parkinson disease (PD). Placement of high frequency stimulating electrodes in the region of the ventral intermediate nucleus of the thalamus (VIM) can markedly reduce tremor in these conditions, and stimulation of either the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) may not only reduce tremor, but also decrease bradykinesia, rigidity, and gait impairment that plague people with PD. Furthermore, many have touted the potential benefit of DBS of selected brain regions for other movement disorders such as dystonia or Tourette syndrome, as well as a variety of disorders such as pain, depression, and obsessive compulsive disorder (OCD). Despite these realized and potential advances in treatment, controversy swirls around a number of clinically relevant and basic mechanistic issues. What conditions are amenable to treatment by DBS? What are the mechanisms of action of DBS? What effect does DBS have on the function of brain circuits? We address these controversial issues and emphasize the need for future investigations. To set the stage, however, we first review the history of the development of DBS as a therapeutic tool.
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HISTORY OF DEEP BRAIN STIMULATION
Ever since Fritsch & Hitzig’s (1870) classical demonstration of the localized electrical excitability of the motor cortex, electrical stimulation of the brain has played a major role in investigations of brain function. The first report of human cortical stimulation appeared four years later (Bartholow 1874). Although electrical stimulation was used to map cortical function in the 1930s (Penfield & Boldrey 1937), it was not until human stereotaxic devices were developed that neurosurgeons could begin to investigate the effects of stimulating deeper structures (Spiegel et al. 1947). By the early 1950s, intraoperative stimulation was used to identify deep structures such as the corticospinal tract prior to lesioning the globus pallidus or thalamus (Spiegel & Wycis 1952). Most reports in the 1950s focused on positive phenomena that were elicited by stimulation. In the early 1960s, it was reported that high-frequency (100-Hz) stimulation of the ventrolateral thalamus could diminish tremor (Hassler et al. 1960, Ohye et al. 1964).
The idea of treating neurologic disorders with chronic stimulation began to emerge in the 1960s, but stimulation was largely used for targeting surgical lesions (Bergstrom et al. 1966). Sem-Jacobsen (1966) developed a method of implanting a bundle of multiple electrode wires deep in the brain and leaving them in place for weeks, during which stimulation could be delivered. The goal of the stimulation was to delineate the “best” target for a subsequent lesion. With the implanted wires, a lesion could be made in small steps over a span of days to weeks to try to achieve maximum benefit without untoward effects. Although the goal was still lesion guidance, this is perhaps the earliest report of stimulation through chronically implanted electrodes.
In the early 1970s, reports of using chronic stimulation therapeutically emerged for treating pain (Hosobuchi et al. 1973), movement disorders, or epilepsy (Cooper 1973). Cooper et al. (1976) published the first large series of chronic cerebellar stimulation studies for cerebral palsy. In those cases, stimulation was delivered transcutaneously through inductive coupling devices to electrodes implanted on the surface of the cerebellar cortex. Benefit was said to occur in 49 of 50 patients. However, cerebellar stimulation in cerebral palsy eventually fell out of favor when blinded studies failed to show consistent benefits (Penn 1982). By 1980, other reports of treating movement disorders with chronic stimulation had appeared