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Deep brain stimulation (DBS) is a minimally invasive targeted neurosurgical intervention that enables structures deep in the brain to be stimulated electrically by an implanted pacemaker. It has become the treatment of choice in Parkinson’s disease patients with either disabling medication side effects, on-off fluctuations, or tremor. Its efficacy in Parkinson’s disease has been demonstrated robustly by randomized controlled clinical trials, with numerous brain areas having been targeted in the last two decades for indications including dystonia and tremor in movement disorders; depression, obsessive-compulsive disorder, and Tourette’s syndrome in psychiatry; epilepsy, cluster headache, and chronic pain. The principles of framed stereotactic surgery enabling DBS electrode placement are reviewed here, before describing the putative mechanisms underlying DBS and its action upon individual neurons. Recent and future developments in DBS technology such as segmented, directional leads, and smart, adaptive stimulation are then discussed alongside related technological advances such as diffusion tensor imaging, robotic surgery, and molecular and cellular therapies. The role of stereotactic lesions in the era of DBS and the different modalities of lesioning are then summarized.

This scientific commentary refers to ‘Directional deep brain stimulation: an intraoperative double-blind pilot study', by Pollo et al ., (doi:10.1093/brain/awu102). Over the last two decades, deep brain stimulation has become widely accepted as a treatment for Parkinson’s disease, dystonia and tremor, and its use is increasing in disorders such as obsessive-compulsive disorder, depression and pain. The primary objective is always to achieve symptom suppression with few or no side effects; in other words, to attain as wide a therapeutic window as possible. In the past, efforts to achieve this were focused largely on ensuring accurate electrode placement, be it with the help of imaging and/or with microelectrode recordings. Excellent results can be obtained in this way, but unwanted side effects still occur in ∼13% of patients (Deuschl et al. , 2006). Today, a number of new technologies are under development to address this problem. In this issue of Brain , Pollo et al. present data from a randomized clinical trial in which one of these approaches—directional stimulation—produces a wider therapeutic window than traditional methods (Pollo et al. , 2014). To put this result into context, it is worth reviewing, albeit briefly, some of the other strategies that are under development with the aim of maximizing therapeutic benefit while minimizing side effects. ### Demand pacemakers Currently, most pacemakers deliver stimulation continuously at a fixed rate, in what is a rather simple but nevertheless effective therapy. However, constant stimulation can incur unwanted side effects by spreading to other fibre tracts or by disrupting normal brain signals embedded in pathological activity. Alteration of such ‘normal’ signals may affect emotion or cognition, for example. By targeting stimulation specifically …

Neurosurgery encompasses brain, spine, and peripheral nerve disorders that may benefit from operative management. These include congenital neurological diseases, epilepsy, stroke, neurological infection, movement disorders, spinal cord disease, neurological tumours, and pituitary disorders. This chapter focuses on head injury and subarachnoid haemorrhage, two common neurosurgical disorders not detailed elsewhere in this book.