Gamma knife radiosurgery as a lesioning technique in movement disorder surgery

Magnetic resonance imaging-based stereotactic localization of the globus pallidus and subthalamic nucleus

OBJECTIVE

To optimize the accuracy of initial stereotactic targeting for movement disorders surgery, we performed stereotactic localization of the internal segment of the globus pallidus (GPi) and subthalamic nucleus (STN) using magnetic resonance imaging protocols in which the borders of these nuclei were directly visualized.

METHODS

Fifty-one consecutive cases using the pallidal target and six using the subthalamic target were studied. Localization of these nuclei was performed using the Leksell stereotactic head frame and inversion recovery sequences (GPi) or T2-weighted spin echo sequences (STN). Targeting accuracy and individual variation in the spatial coordinates of these structures were independently measured by identification of nuclear boundaries during multiple microelectrode penetrations.

RESULTS

The lateral and vertical coordinates of an atlas-defined point in the GPi, with respect to the line between the anterior and posterior commissures, was highly variable. Initial targeting the GPi based on direct visualization of the target boundaries (external medullary lamina and optic tract) resulted in greater precision than would be expected using fixed anterior and posterior commissure-based coordinates. Initial targeting the STN using magnetic resonance imaging was sufficiently precise to place the initial microelectrode penetration within STN in all six cases.

CONCLUSION

Magnetic resonance imaging-based initial stereotactic targeting of the GPi, based on direct visualization of the target boundaries, is useful to improve target accuracy over that of purely indirect anterior and posterior commissure-based targeting methods. Initial targeting of the STN was reliably accomplished by direct visualization. However, there remains sufficient variability that the final target location in both GPi and STN required electrophysiological mapping in all cases.

Functional radiosurgery

Although the application of stereotactic radiosurgery for the management of functional brain disorders began in 1951, almost 50 years elapsed before it received appropriate attention. Radiosurgical techniques are used to create image-guided, physiological inactivity or focally destructive brain lesions without neurophysiological guidance. The lack of neurophysiological guidance remains the greatest argument against the use of radiosurgery for selected disorders. Current anatomic targets include the trigeminal nerve (for trigeminal neuralgia), the thalamus (for tremor or pain), the cingulate gyrus or anterior internal capsule (for pain or psychiatric illness), the globus pallidus (for symptoms of Parkinson's disease), and the hippocampus (for epilepsy). The use of radiosurgery as a "lesion generator" is based on extensive animal studies that defined the dose, volume, and temporal response of the irradiated tissue. The usefulness of radiosurgery has been compared with that of microsurgical, percutaneous, and electrode-based techniques used for functional neurological disorders. At present, the long-term results after functional radiosurgery procedures remain to be documented. The current indications and expected outcomes after radiosurgery are discussed.