25.3 Stereotactic Procedures, Deep Brain Stimulation & Awake Craniotomy

Why This Topic Matters on the NCE

Stereotactic procedures are a named intracranial sub-topic (IV.A.3.b.v). This category includes deep brain stimulator implantation, stereotactic biopsy, stereotactic radiosurgery, and awake craniotomy for tumor resection near eloquent cortex. The unifying anesthetic problem is that the patient must be awake and cooperative in the middle of a neurosurgical procedure, often with restricted airway access.

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1. The Stereotactic Frame

A rigid head frame (Leksell, Cosman-Roberts-Wells) is pinned to the skull under local anesthesia and provides a coordinate system for the target.

The frame is an airway emergency waiting to happen. Once it is applied:

• Direct laryngoscopy is effectively impossible with the frame in place.
• The frame key or wrench must be immediately available and its use known to the anesthesia team.
• The airway rescue plan is: remove the frame if possible, or use a supraglottic airway or flexible scope through the frame.
• Frameless neuronavigation is increasingly used and removes this constraint, but frame-based systems remain in use.

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2. Deep Brain Stimulation

Deep brain stimulators are implanted for Parkinson disease, essential tremor, and dystonia, targeting the subthalamic nucleus, globus pallidus internus, or ventral intermediate nucleus of the thalamus.

The core anesthetic conflict

Target localization frequently relies on microelectrode recording of characteristic neuronal firing patterns and on clinical testing — asking the patient to move while stimulation parameters are adjusted.

• Propofol suppresses the neuronal firing patterns used for microelectrode recording. If propofol is used for the initial burr hole, it should be stopped at least 15 to 20 minutes before recording begins, and many centers avoid it entirely during this phase.
• Dexmedetomidine at 0.2 to 0.7 mcg/kg/h is the most commonly used sedative during recording because it provides cooperative sedation with minimal respiratory depression and, at low doses, comparatively little interference with microelectrode signals.
• Benzodiazepines suppress tremor and confound clinical assessment; avoid them.

The medication problem

Antiparkinsonian medication is typically withheld the night before so that symptoms are present and testable during target localization. The consequences follow directly from the levodopa half-life of 1 to 3 hours:

• Severe rigidity, including chest wall rigidity impairing ventilation
• Impaired swallowing with aspiration risk and copious secretions
• Off-period dystonia, anxiety, and difficulty lying still
• Autonomic instability and orthostatic hypotension

Restart antiparkinsonian medication as soon as the procedure permits.

The scalp block

A bilateral scalp block provides the analgesia that makes an awake procedure tolerable. Six nerves are blocked on each side:

1. Supraorbital (V1)
2. Supratrochlear (V1)
3. Zygomaticotemporal (V2)
4. Auriculotemporal (V3)
5. Greater occipital (C2)
6. Lesser occipital (C2, C3)

The greater auricular nerve is often added. Because the scalp is highly vascular, calculate the total local anesthetic dose carefully and use an epinephrine-containing solution; local anesthetic systemic toxicity is a real risk with bilateral blocks.

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3. Awake Craniotomy

Performed for tumor resection adjacent to language, motor, or sensory cortex, allowing intraoperative mapping so the surgeon can resect maximally without producing a deficit.

Techniques

Dexmedetomidine and remifentanil, often by target-controlled infusion, are the workhorse agents. Propofol may be used in the asleep phases but must be off well before mapping.

Requirements and pitfalls

• Patient selection and preparation are the largest determinants of success. The patient must understand and tolerate the plan; severe anxiety, claustrophobia, inability to lie still, or a language barrier are relative contraindications.
• No neuromuscular blockade during motor mapping.
• Avoid hypercapnia — oversedation causes hypoventilation, which raises cerebral blood flow and produces a swollen brain in an open skull.
• Intraoperative seizure is common during cortical stimulation. The immediate treatment is irrigation of the cortex with ice-cold saline, then a small propofol bolus and an antiepileptic such as levetiracetam. Have these immediately available on every case.
• Nausea and vomiting in a pinned, awake patient is dangerous; prophylax aggressively.
• Venous air embolism where the head is elevated.
• Have a plan for converting to general anesthesia with the head pinned and access restricted: video laryngoscope, supraglottic airway, and flexible scope immediately available.

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4. Stereotactic Radiosurgery and Biopsy

• Stereotactic radiosurgery (Gamma Knife, linear accelerator based) is painless once the frame is placed, but requires complete immobility and complete isolation of the patient during radiation delivery. Adults typically tolerate it with the frame placed under local anesthesia; children usually require general anesthesia, with remote monitoring by camera and telemetry as for any radiation therapy suite.
• Stereotactic brain biopsy is brief and minimally painful but carries a real risk of hemorrhage into the biopsy tract with sudden neurologic deterioration and intracranial hypertension. The patient must be monitored and reassessed neurologically afterward.

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5. The Implanted Device Afterward

Once a deep brain stimulator is implanted, the patient will return for unrelated surgery, and the device changes your management:

• Turn the stimulator off before electrocautery. Stimulation artifacts and cautery current can reprogram or damage the device and can cause thermal injury at the electrode tip.
• Use bipolar electrocautery at the lowest effective setting; if monopolar is unavoidable, place the return pad as far from the generator as possible so the current path does not cross the device.
• MRI is restricted and requires device-specific conditional protocols.
• Interrogation and reprogramming by the managing team should follow surgery.

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Exam Traps

• The frame key must be at the head of the bed. Losing the airway in a pinned frame is the disaster.
• Propofol abolishes microelectrode recordings — stop it 15 to 20 minutes before mapping; dexmedetomidine is the preferred sedative.
• Antiparkinsonian drugs are held for DBS mapping, which is why the patient arrives rigid and at risk of aspiration.
• A seizure during cortical stimulation is treated first with ice-cold saline irrigation.
• Turn the stimulator off and use bipolar cautery for any later surgery.