Functional neurosurgery includes surgery conducted to ablate, augment, or modulate targets that lead to improvement in neurological function or behavior. Surgical approaches for this purpose include destructive lesioning with different mechanical or biologic agents or energy sources, non-destructive electrical modulation, and cellular or chemical augmentation. Our purpose was to review the role of stereotactic radiosurgery used for functional indications and to discuss future applications and potential techniques. Imaging and neurophysiological research will enable surgeons to consider new targets and circuits that may be clinically important. Radiosurgery is one minimal access approach to those targets.

Initially designed for the treatment of functional brain targets, stereotactic radiosurgery (SRS) has achieved an important role in the management of a wide range of neurosurgical pathologies. The interest in the application of the technique for the treatment of pain, and psychiatric and movement disorders has returned in the beginning of the 1990s, stimulated by the advances in neuroimaging, computerized dosimetry, treatment planning software systems, and the outstanding results of radiosurgery in other brain diseases. Since SRS is a neuroimaging-guided procedure, without the possibility of neurophysiological confirmation of the target, deep brain stimulation (DBS) and radiofrequency procedures are considered the best treatment options for movement-related disorders. Therefore, SRS is an option for patients who are not suitable for an open neurosurgical procedure. SRS thalamotomy provided results in tremor control, comparable to radiofrequency and DBS. The occurrence of unpredictable larger lesions than expected with permanent neurological deficits is a limitation of the procedure. Improvements in SRS technique with dose reduction, use of a single isocenter, and smaller collimators were made to reduce the incidence of this serious complication. Pallidotomies performed with radiosurgery did not achieve the same good results. Even though the development of DBS has supplanted lesioning as the first alternative in movement disorder surgery; SRS might still be the only treatment option for selected patients.

Classical trigeminal neuralgia is a chronic pain condition that was clinically recognized centuries ago. Nevertheless, the pathological mechanism(s) involved in the development of classical trigeminal neuralgia is still largely based on the theory of peripheral versus central nervous system origin. Limitations of both hypotheses are discussed. Evidence of radiation effects in the electrical conduction of peripheral nerves is reviewed. Results of experimental studies using modern and current radiosurgery techniques and doses are also brought to discussion in an attempt to elucidate the radiation mechanisms involved in the conduction block of excessive sensory information triggering pain attacks. Clinical features and prognostic factors associated with pain control, recurrence, and facial numbness in patients submitted to surgical procedures for classical trigeminal neuralgia are discussed in the context of the features related to the pathogenesis of this condition. Studies focusing on the electrophysiology properties of partially demyelinated trigeminal nerves submitted to radiosurgery are vital to truly advance our current knowledge in the field.

The success of deep brain stimulation (DBS) surgery in treating medically refractory symptoms of some movement disorders has inspired further investigation into a wide variety of other treatment-resistant conditions. These range from disorders of gait, mood, and memory to problems as diverse as obesity, consciousness, and addiction. We review the emerging indications, rationale, and outcomes for some of the most promising new applications of DBS in the treatment of postural instability associated with Parkinson's disease, depression, obsessive-compulsive disorder, obesity, substance abuse, epilepsy, Alzheimer's-type dementia, and traumatic brain injury. These studies reveal some of the excitement in a field at the edge of a rapidly expanding frontier. Much work still remains to be done on basic mechanism of DBS, optimal target and patient selection, and long-term durability of this technology in treating new indications.

Background: Behavioral surgery (BS) is resurging because of unmet clinical need, advances in basic sciences, neuroimaging, neurostimulation, and stereotaxy. However, there is a danger that BS will fall unless acceptable strategies are adopted by BS providers. Methods: A critical review of conditions leading to rise of psychosurgery (PS) and concerns resulting in its fall was conducted to learn lessons and safeguard BS of the future. Results: PS rose and spread in 1960 like wildfire without adequate preclinical and clinical studies. Hundreds of patients had PS without adequate preoperative diagnosis or assessment, proper consent, and non-objective reporting of outcome. Furthermore, there was public opposition against PS because of its potential abuse to control violent behavior and dissidents. Advances in neurostimulation, neuroimaging, and stereotaxy, and emergence of treatment-resistant mental disorders led to increased interest in BS. Several recent studies have shown BS to be safe and effective. However, concerns related to strength of evidence, safety, efficacy, consent, and objectivity of studies have been raised. Unless clinical and regulatory governance structures are adopted in each jurisdiction, BS will face the same fate as that of PS in the past. Conclusion: The future of BS as a safe and effective therapy is dependent upon adopting clear moral ethical and governance standards on the following lines: Patients must have failed adequate therapies; must be assessed by psychiatrist-led multidisciplinary teams; patients' abilities to give consent and diagnosis must be verified by independent authorities designated for this purpose by the state; and the independent authority must also decide whether the teams were adequately trained to perform BS.

The amygdala was a popular target during the era of psychosurgery, specifically for the treatment of intractable aggression. This mesiotemporal structure was thought to primarily mediate fear and anger. However, recent evidence suggests that the amygdala is part of a complex network that mediates the formation of a larger repertoire of positive and negative emotions. Dysfunctions within the network or the amygdala itself can lead to various mental illnesses. In those cases, deep brain stimulation (DBS) applied focally may treat the symptoms. This review presents data supporting the potential therapeutic role of DBS of the amygdala in the treatment of anxiety disorders, addiction, and mood disorders. The success of DBS for psychiatric conditions will likely depend on our ability to precisely determine the optimal target for a specific case.

Vagus nerve stimulation (VNS) is a unique epilepsy treatment in that a peripheral intervention is used to treat a disease that is entirely related to pathological events occurring within the brain. To understand how stimulation of the vagus nerve can be used to stop seizures, an understanding of the peripheral anatomy and physiology of the vagus nerve is essential. The peripheral aspects of the vagus nerve are discussed in this review, with an explanation of which fibers and branches are involved in producing these antiepileptic effects, along with speculation about the potential for improving the therapy.

Precision is the ultimate aim of stereotactic technique. Demands on stereotactic precision reach a pinnacle in stereotactic functional neurosurgery. Pitfalls are best avoided by possessing in-depth knowledge of the techniques employed and the equipment used. The engineering principles of arc-centered stereotactic frames maximize surgical precision at the target, irrespective of the surgical trajectory, and provide the greatest degree of surgical precision in current clinical practice. Stereotactic magnetic resonance imaging (MRI) provides a method of visualizing intracranial structures and fiducial markers on the same image without introducing significant errors during an image fusion process. Although image distortion may potentially limit the utility of stereotactic MRI, near-complete distortion correction can be reliably achieved with modern machines. Precision is dependent on minimizing errors at every step of the stereotactic procedure. These steps are considered in turn and include frame application, image acquisition, image manipulation, surgical planning of target and trajectory, patient positioning and the surgical procedure itself. Audit is essential to monitor and improve performance in clinical practice. The level of stereotactic precision is best analyzed by routine postoperative stereotactic MRI. This allows the stereotactic and anatomical location of the intervention to be compared with the anatomy and coordinates of the intended target, avoiding significant image fusion errors.