Does subthalamic nucleus stimulation induce apathy in Parkinson's disease?

Background. Subthalamic Nucleus Deep Brain Stimulation (STN-DBS) has been shown to significantly improve motor symptoms in advanced Parkinson's disease (PD). Only few studies, however, have focused on the non-motor effects of DBS. Methods. A consecutive series of 15 patients was assessed three months before (M-3), then three months (M3) and six months (M6) after surgery. Mean (+/- SD) age at surgery was 59.7 (7.6). Mean disease duration at surgery was 12.2 (2.8) years. The Mini International Neuropsychiatric Inventory was used to assess psychiatric disorders three months before surgery. Depression was evaluated using Montgomery and Asberg Rating Scale (MADRS). Anxiety was evaluated using the AMDP system (Association for Methodology and Documentation in Psychiatry). Apathy was particularly evaluated using the Apathy Evaluation Scale (AES) and the Starkstein Scale. All these scales were performed at every evaluation. Results. Apathy worsened at M3 and M6 after STN-DBS in comparison with the preoperative evaluation: the AES mean score was significantly impaired between the preoperative (38.4 +/- 7.1) and both the postoperative M3 (44.6 +/- 9.5, p = 0.003) and M6 scores (46.0 +/- 10.9, p = 0.013). Significant worsening of apathy was confirmed using the Starkstein scale. There was no evidence of depression: the mean MADRS score did not differ before surgery (9.1 +/- 7.4) and at both M3 (8.6 +/- 8.2) and M6 (9.9 +/- 7.7) after STN-DBS. The anxiety level did not change between preoperative (9.4 +/- 9.2) and both M3 (5.5 +/- 4.5) and M6 (6.6 +/- 4.6) postoperative states. Conclusion. Although STN-DBS constitutes a therapeutic advance for severely disabled patients with Parkinson's disease, we should keep in mind that this surgical procedure may contribute to the inducing of apathy. Our observation raises the issue of the direct influence of STN-DBS on the limbic system by diffusion of stimulus to the medial limbic compartment of STN.