Deep brain stimulation (DBS) is now well established as an adjunct therapy for Parkinson’s disease (PD), especially for patients affected by long-term complications of levodopa therapy such as motor fluctuations and severe dyskinesias. Indeed, chronic DBS targeting the internal globus pallidus (GPi) and the subthalamic nucleus (STN) has been shown to yield a benefit both in motor function and in functional disability.
1–5Currently, the subthalamic nucleus has become the preferred target because the antiakinetic effect seems to be more robust, which allows a greater reduction of antiparkinsonian drug treatment, and requires less stimulation energy.
6–10 However, an increasing number of studies have reported that STN DBS may be associated with neuropsychological, cognitive, and psychiatric dysfunction.
11–13 Among these disturbances, apathy, defined as a lack of feeling, emotion, interest, concern, or motivation,
14 has been largely described as a very common psychiatric STN DBS side effect.
13 The STN is described as being situated in a central position in all five corticobasal ganglia-thalamocortical circuits, which each have specific motor, oculomotor, associative, and limbic functions.
15 Neuroanatonomical and physiological studies in animals have demonstrated that the STN can be functionally divided into sensorimotor (dorsolateral), limbic (medial), and cognitive (ventromedial) regions.
16 Because of the small size of the STN and current diffusion within the structure, STN DBS may act on different functional circuits, either activating or inhibiting different neuronal networks, including emotional and associative circuits. Apathy following STN DBS could be related to the stimulation of the STN associative circuit.
Because of all these reasons, patients with cognitive impairment or severe psychiatric disease are now excluded from subthalamic surgery. For some of them, GPi appears to be the target of choice.
17 Nevertheless, only few studies have investigated the non-motor effects of GPi DBS,
18–21 particularly focusing on apathy known as a common consequence of STN DBS.
13To further characterize the psychiatric symptoms that may occur in GPi DBS, we report a prospective study on the incidence of apathy, mood disorders, and anxiety consecutive to GPi DBS in disabled parkinsonian patients.
Discussion
Few studies have been performed with the objective to evaluate the non-motor effects of GPi DBS in PD. Okun et al.
19 in the COMPARE study, a prospective blinded randomized trial comparing the cognitive and mood effects of unilateral STN DBS versus unilateral GPi DBS in patients with PD, did not find any significant mood differences between STN and GPi DBS groups on the Visual Analog Mood Scale from pre-DBS to post-DBS performance at 7 months. Nevertheless, more patients in the STN group had experienced postsurgical mood and cognitive adverse events (e.g., anxiety, confusion, irritability, aggressiveness, obsessive compulsive or manic symptoms, decreased confidence/motivation) than in the GPi group. Anderson et al,
20 in a randomized, blinded comparison of the safety and efficacy of STN and GPi DBS in patients with advanced PD also reported more cognitive and behavioral changes after STN than GPi implantation, though they did not use any validated psychiatric scale. In the same line but not in PD, Hälbig et al.
18 did not find any change in neuropsychiatric measures of 15 patients with dystonia after GPi DBS. Recently, Kirsch-Darrow et al.
21 assessed apathy prior to DBS and 6 months post-DBS (STN and GPi). They showed an increase of apathy after surgery but did not find any relationship between apathy and DBS site.
Our results are in line with most of these previous studies, showing that there is no depression or anxiety after GPi DBS. Furthermore, we confirm the results of our previous study,
17 indicating that GPi DBS in advanced Parkinsonian patients with contraindications for STN DBS is effective in reducing cardinal and axial motor symptoms in the off-dopa condition and also preserves cognitive functions, even for patients at high risk (i.e., displaying cognitive decline at baseline). When focusing on apathy assessment, we showed that GPi DBS may be safe for parkinsonian patients, which constitutes a main difference with our previous report concerning apathy post-STN DBS,
13 using exactly the same design in the same center. Our results support the idea that there is a clear relationship between apathy and DBS site. Several hypotheses can be made to explain this difference.
We did not find any levodopa reduction before and after surgery, which could explain that there was no postoperative apathy score increase. There are mixed findings regarding whether apathy after DBS is related to reductions in levodopa dosage. Therefore, several studies did not find any relationship between apathy and reduction in levodopa, despite substantial reductions in LED.
21, 37–41 In the light of these results, we can assume that the absence of apathy after GPi DBS is not linked with the stability of LED after surgery.
The territories of basal ganglia have been anatomically divided into sensorimotor, associative, and limbic regions.
42 Because the GPi (∼478 mm
3) is a bigger nucleus than the STN (∼158 mm
3),
42,43 we can assume that either the lesion itself or stimulation effect of STN would be more likely to affect non-motor pathways potentially involved in apathy than would the GPi. Thus, changes in mood, cognitions, and motivation after STN DBS could be the result of the current spread to non-motor portions of the nuclei.
19,42,43At last, cortical-subcortical neuronal pathways involved in GPi DBS may be different from STN DBS. Concerning STN DBS, correlations have been observed between variation in apathy scores and changes in glucose metabolism, using fluorodeoxyglucose-positron emission tomography (FDG-TEP). These correlations were positive in the right frontal middle gyrus [Brodmann area (BA) 10] and right inferior frontal gyrus (BA 46 and BA 47), and negative in the right posterior cingulated gyrus (BA 31) and left medial frontal lobe (BA 9).
44 These results confirmed the role of the subthalamic nucleus in associative and limbic circuitry in humans and suggest that it is a key basal ganglia structure in motivation circuitry. The stability of apathy scores after GPi DBS support the hypothesis that GPi has a lesser influence on the limbic circuit. Further studies using FDG-PET are needed to confirm this hypothesis.
As a conclusion, while evidence is provided for the safety of the pallidal target regarding neuropsychiatric functions, we believe that GPi should be considered as the target of choice for advanced PD patients who are refractory to adjustments in medication and cannot benefit from STN surgery because of axial motor or cognitive contraindications.