25.4 CSF Diversion, Burr Holes, Ventriculoperitoneal Shunts & Decompressive Craniectomy
Key Takeaways
- The intracranial pressure-volume curve is exponential, so two patients with identical intracranial pressures can have very different compliance and one may decompensate within minutes.
- An acute epidural hematoma is biconvex, does not cross suture lines, arises from the middle meningeal artery, and classically follows a lucid interval, whereas a subdural hematoma is crescentic and crosses suture lines.
- Programmable ventriculoperitoneal shunt valves are magnetically adjustable and can be inadvertently reprogrammed by MRI, so the setting must be verified and reset afterward.
- Every shunt malfunction is treated as both a full stomach and raised intracranial pressure, and nitrous oxide is avoided whenever intracranial air may be present.
- Hypotonic and glucose-containing fluids are avoided in raised intracranial pressure because they worsen cerebral edema and ischemic injury, and hypotension is more damaging than hypertension in traumatic brain injury.
Why This Topic Matters on the NCE
Domain IV.A.3.b.i names decompression (burr holes, ventriculoperitoneal shunt) as its own intracranial sub-topic. These are the bread-and-butter neurosurgical emergencies: the elderly patient with a chronic subdural hematoma, the child with a blocked shunt, the trauma patient with an epidural hematoma. Each has a distinctive physiology that is directly testable.
1. The Framework: Monro-Kellie and the Pressure-Volume Curve
The cranium is a rigid box containing brain (about 80 percent), cerebrospinal fluid (about 10 percent), and blood (about 10 percent). Adding volume must displace one of the others. Compensation is initially effective — CSF is displaced into the spinal subarachnoid space and venous blood is expelled — and then abruptly fails.
The intracranial pressure-volume curve is exponential: at the flat portion, large volume additions produce small pressure changes; past the elbow, a small additional volume causes a steep rise. Two patients with identical intracranial pressures can therefore have completely different compliance, which is why a decompensating patient can deteriorate within minutes.
- Normal intracranial pressure: 5 to 15 mmHg; sustained pressure above 20 mmHg defines intracranial hypertension.
- Cerebral perfusion pressure = MAP minus ICP (or minus central venous pressure, whichever is higher). Target 60 to 70 mmHg in neurologic injury.
- Cushing triad — hypertension, bradycardia, and irregular respiration — is a late sign of impending herniation, not an early warning.
2. Hydrocephalus and CSF Diversion
| Type | Mechanism |
|---|---|
| Communicating | CSF circulates but absorption at the arachnoid granulations is impaired (post-hemorrhagic, post-meningitic) |
| Noncommunicating (obstructive) | Physical obstruction within the ventricular system (aqueductal stenosis, tumor, colloid cyst) |
| Normal pressure hydrocephalus | Elderly triad of gait apraxia, urinary incontinence, and dementia; treated with shunting |
Ventriculoperitoneal shunt
A ventricular catheter, a valve, and distal tubing tunneled to the peritoneum (or atrium or pleura).
- Valve types: fixed differential pressure valves and programmable valves. The exam point is that programmable valves are magnetically adjustable and can be inadvertently reprogrammed by MRI, so the setting must be verified and reset after any MRI.
- Shunt malfunction presents as acute intracranial hypertension: headache, vomiting, decreased consciousness, and in infants a bulging fontanelle, sunsetting eyes, and increasing head circumference. Treat every shunt malfunction as a full stomach with raised intracranial pressure.
- Shunt infection is common in the first months after placement.
- Distal tunneling and peritoneal insertion are surprisingly stimulating and often the most painful part of the case.
- Avoid nitrous oxide where intracranial air may be present.
Endoscopic third ventriculostomy
An alternative to shunting for obstructive hydrocephalus. Two specific hazards:
- Irrigation fluid can acutely raise intracranial pressure if outflow is obstructed, causing sudden hypertension and bradycardia.
- Manipulation of the floor of the third ventricle near the hypothalamus and basilar artery produces bradycardia and hemodynamic instability, and rarely catastrophic basilar artery injury.
3. Chronic Subdural Hematoma and Burr Hole Drainage
The classic patient is elderly, on antiplatelet or anticoagulant therapy, presenting with gradual cognitive decline, headache, or a fluctuating focal deficit weeks after a trivial or forgotten head injury. Cortical atrophy stretches the bridging veins, making them easy to tear and providing space for a large collection before symptoms appear.
- Burr hole drainage can often be performed under local anesthesia with light sedation, which is attractive in a frail, comorbid patient — but only if the patient can lie still and protect the airway.
- Rapid decompression can cause hypotension and bradycardia and, rarely, re-expansion phenomena or new hemorrhage from a re-perfused, previously compressed cortex.
- Reverse anticoagulation preoperatively as clinically appropriate.
- Recurrence is common; middle meningeal artery embolization is an increasingly used adjunct.
4. Acute Epidural and Subdural Hematoma
| Acute epidural hematoma | Acute subdural hematoma | |
|---|---|---|
| Source | Middle meningeal artery, usually with a temporal skull fracture | Bridging veins |
| CT appearance | Biconvex / lentiform, does not cross suture lines | Crescentic, crosses suture lines |
| Classic history | Lucid interval then rapid deterioration | Often immediate depressed consciousness |
| Underlying brain | Frequently normal — outcome is excellent if evacuated fast | Often severely injured — worse prognosis |
| Urgency | Extreme; time to evacuation drives outcome | Urgent |
An epidural hematoma with a lucid interval is the archetypal "get to the operating room now" case. The anesthetic is a rapid sequence induction with in-line cervical stabilization in a presumed full-stomach trauma patient, maintaining cerebral perfusion pressure while avoiding hypertensive surges, with immediate availability of blood.
5. Decompressive Craniectomy
Removal of a large bone flap with duraplasty for refractory intracranial hypertension, most commonly after malignant middle cerebral artery infarction or severe traumatic brain injury.
- Removal of the flap can produce an abrupt fall in intracranial pressure with hypotension, and reperfusion of ischemic tissue with hyperemia and swelling through the defect.
- Massive brain swelling through the craniectomy is a recognized intraoperative event; maintain the standard brain relaxation armamentarium.
- Afterward, the Monro-Kellie doctrine no longer applies in the usual way — the compartment is open, so intracranial pressure tracks atmospheric pressure and position.
- Syndrome of the trephined (sinking skin flap syndrome) is delayed neurologic deterioration attributable to the missing flap and atmospheric pressure on the brain, corrected by cranioplasty.
- Patients returning for cranioplasty have an unprotected brain beneath the scalp; positioning and head handling must account for it.
6. General Anesthetic Principles for Raised Intracranial Pressure
| Goal | How |
|---|---|
| Avoid ICP spikes | Smooth induction; lidocaine 1 to 1.5 mg/kg and adequate opioid before laryngoscopy; avoid coughing and bucking |
| Optimize venous drainage | Head up 15 to 30 degrees, neck neutral, avoid tight tube ties and jugular compression, avoid excessive PEEP |
| Ventilation | Normocapnia to mild hypocapnia (PaCO2 30 to 35 mmHg) as a temporizing measure; avoid prolonged aggressive hypocapnia |
| Maintain CPP 60 to 70 mmHg | Treat hypotension aggressively; hypotension is more damaging than hypertension in traumatic brain injury |
| Fluids | Isotonic crystalloid; avoid hypotonic fluids and avoid glucose-containing solutions, which worsen cerebral edema and ischemic injury |
| Avoid nitrous oxide | Increases cerebral blood flow and expands intracranial air |
| Ketamine | Historic prohibition has been substantially revised; it is acceptable in ventilated patients with controlled carbon dioxide, but volatile-free or low-MAC technique with propofol remains standard |
| Venous air embolism watch | Any head-elevated position with open venous sinuses |
Exam Traps
- Cushing triad is a late sign. Do not wait for it.
- A biconvex lesion that does not cross suture lines is an epidural hematoma from the middle meningeal artery.
- Programmable shunt valves are reset by MRI and must be re-verified afterward.
- Never give hypotonic or glucose-containing fluids to a patient with raised intracranial pressure.
- Shunt malfunction equals full stomach plus raised ICP.
A 19-year-old struck in the temple briefly lost consciousness, awoke and conversed normally for 45 minutes, and is now rapidly deteriorating with an ipsilateral dilated pupil. CT shows a biconvex hyperdense collection that does not cross the coronal suture. What is the source of the bleeding?
A child with a programmable ventriculoperitoneal shunt undergoes MRI of the brain. What follow-up step is essential?
During resuscitation of a patient with severe traumatic brain injury and an intracranial pressure of 25 mmHg, which fluid choice is most appropriate?
A 4-year-old with a ventriculoperitoneal shunt presents with 12 hours of vomiting, headache, and increasing lethargy. Shunt malfunction is suspected. Which induction plan is most appropriate?