Functional and Stereotactic Neurosurgery
This domain tests target selection, patient selection and the limits of each technique rather than fine surgical detail.
Stereotactic principles
A frame such as Leksell or CRW creates a coordinate system referenced to imaging. Error comes from frame application, image fusion and brain shift. Frameless systems use a reference star, and registration error is the practical limitation. Targets are chosen indirectly from atlases and directly on MRI: subthalamic nucleus and globus pallidus internus on T2, ventral intermediate nucleus on T2 or proton density. Microelectrode recording identifies the target by its firing pattern.
Indications and selection
- Deep brain stimulation is offered for Parkinson disease with motor fluctuations or dyskinesia and a good levodopa response, targeting the subthalamic nucleus or globus pallidus internus; for essential tremor targeting the ventral intermediate nucleus; and for dystonia targeting the globus pallidus internus, where benefit is delayed by weeks to months.
- Destructive procedures such as thalamotomy, pallidotomy and radiosurgical lesioning remain options where stimulation is unavailable or contraindicated; MR-guided focused ultrasound is used for tremor.
- Preoperative assessment is multidisciplinary: cognitive and psychiatric evaluation, MRI, and a levodopa challenge. Cognitive impairment, uncontrolled psychiatric disease and severe gait instability are relative contraindications.
- Trigeminal neuralgia: carbamazepine first. Microvascular decompression is best for classic neurovascular conflict in younger patients. Percutaneous balloon compression, radiofrequency or glycerol suits elderly patients and those with multiple sclerosis. Radiosurgery is a non-invasive option with delayed effect.
- Spasticity: intrathecal baclofen, selective dorsal rhizotomy and tendon lengthening.
Worked example
A 64-year-old with 10 years of Parkinson disease, three hours of on time with severe dyskinesia, a normal MRI and an MMSE of 28 is offered bilateral subthalamic stimulation. Programming is adjusted over weeks, and benefit depends on a preserved levodopa response.
Traps
- Offering stimulation to a patient with a poor levodopa response.
- Expecting immediate benefit in dystonia.
- Failing to confirm the diagnosis and exclude atypical parkinsonism before surgery.
- Overlooking cognitive or psychiatric contraindications.