23.3 Hydrocephalus and CSF Diversion (03.L)
Key Takeaways
- Obstructive hydrocephalus blocks CSF flow inside the ventricular system; communicating hydrocephalus is an absorption (or extra-ventricular flow) problem with open ventricles in series.
- External ventricular drains treat acute symptomatic hydrocephalus after SAH, IVH, and many posterior-fossa crises, and they give ICP data while they drain.
- Drain height is set in cm H2O above the tragus (foramen of Monro); raising the drain reduces output. Wean by stepwise elevation or a clamp trial with serial exams and CT.
- Ventriculoperitoneal shunt malfunction presents with headache, vomiting, upgaze palsy, or abdominal signs; overdrainage produces low-pressure symptoms and can create subdural hygromas or hematomas.
- Lumbar drains are only for communicating CSF pathways without a crowding mass. Lumbar puncture or lumbar drainage with a posterior-fossa mass risks downward or upward (reverse) herniation.
Hydrocephalus is CSF production outrunning exit. In the neuro ICU that usually means blood, tumor, or swollen cerebellum plugging a bottleneck, or inflamed arachnoid granulations refusing to absorb. Independent OpenExamPrep teaching for outline 03.L is the obstructive-versus-communicating split, when to place an external ventricular drain (EVD), how to set and wean it, when a ventriculoperitoneal (VP) shunt is the chronic answer, and why a lumbar drain (LD) is not a smaller EVD. Device infection returns in the next section and again with perioperative complications in chapter 26; here the physiology comes first.
Adult CSF production is about 20 mL/hour (roughly 500 mL/day) from choroid plexus, independent of intracranial pressure until perfusion collapses. CSF exits the lateral ventricles through the foramina of Monro, the third ventricle, the cerebral aqueduct, the fourth ventricle, and the foramina of Luschka and Magendie into the cisterns, then over the convexities to arachnoid granulations. Block any of those indoor doors and the ventricles upstream balloon. Leave the doors open but clog the granulations (blood, pus, protein) and all ventricles enlarge together.
Obstructive versus communicating
| Obstructive (non-communicating) | Communicating | |
|---|---|---|
| Problem | Block inside the ventricular system | Absorption or extra-ventricular circulation |
| Typical ICU causes | Aqueductal cast from IVH, cerebellar hematoma or infarct closing the fourth ventricle, colloid cyst at the foramen of Monro, pineal mass | Aneurysmal SAH, meningitis, some IVH after the blood has reached the cisterns |
| Imaging clue | Upstream dilatation, downstream ventricle small; a trapped temporal horn; obliterated fourth ventricle with large laterals | All four ventricles large, cisterns may be full of blood |
| Lumbar drain or LP | Dangerous if a pressure gradient remains | May be used when pathways truly communicate and there is no crowding mass |
| Default acute drain | EVD (sometimes bilateral if one foramen of Monro is blocked) | EVD still common if the exam is poor; LD only after communication is certain |
A “small fourth and huge laterals” after a cerebellar burst is obstructive. “Every ventricle big after thick cisternal SAH” is usually communicating, even though blood also transiently obstructs. You do not need a perfect label at 2 a.m. if the patient is herniating — you need a drain that is anatomically legal.
EVD indications that show up on this exam
Place or urgently arrange an EVD when hydrocephalus is symptomatic: falling GCS, new upgaze palsy or “setting sun” eyes, bradycardia with hypertension (Cushing), or ventriculomegaly plus a mass lesion that will not wait. Classic neuro ICU indications:
- Aneurysmal SAH with acute hydrocephalus (chapter 21). The 2023 AHA/ASA aSAH guideline gives Class 1 support to EVD and/or lumbar drainage for acute symptomatic hydrocephalus. EVD is the default when the exam is poor, the fourth ventricle is packed, or you need ICP numbers.
- Intraventricular hemorrhage casting the aqueduct or third ventricle, with declining consciousness (chapter 20). An EVD treats pressure; it is not a clot vacuum unless you are in a selected irrigation/lytics protocol.
- Posterior fossa mass effect (cerebellar hematoma, infarct, tumor) with fourth-ventricle obstruction. Hydrocephalus can kill before the tonsils finish descending. EVD can bridge to suboccipital decompression. Drain modestly: excessive supratentorial unloading with a tight posterior fossa can contribute to upward (reverse) herniation.
- Selected TBI protocols that use an EVD as both monitor and drain (Brain Trauma Foundation context in chapter 16/18), when ventricles are large enough to cannulate.
An EVD is not a treatment for every big ventricle on a chronic shunt CT, and it is not a substitute for reversing herniation with osmotherapy and a scalpel when the problem is a huge temporal hematoma without hydrocephalus.
Height of the drain, output, and weaning
The drip chamber is leveled to an external landmark that approximates the foramen of Monro, usually the external auditory meatus / tragus with the head of bed at the ordered angle. The number on the burette is cm H2O. CSF flows when intracranial pressure exceeds that set point (plus the resistance of the tubing).
Conversion that still appears in calculations: 1 mm Hg ≈ 1.36 cm H2O. A drain open at 15 cm H2O therefore opens near 11 mm Hg. Higher set point = less drainage. Lower set point = more drainage and more risk of overdrainage (collapse of ventricles, subdural hygroma, upward herniation in posterior-fossa disease).
Typical adult starting heights after SAH are in the 10–20 cm H2O range, then titrated to exam, ICP waveform, and CT. Bloody CSF clogs; a sudden drop in output with a rising exam is obstruction until you flush per protocol or replace the catheter. Never assume “the brain got better” because the bag is empty.
Wean versus clamp:
| Strategy | What you do | Pass | Fail |
|---|---|---|---|
| Gradual wean | Raise the drain over days (for example 10 → 15 → 20 cm H2O) | Exam stable, output falls, ventricles stable | Headache, decline, ballooning ventricles |
| Clamp trial | Clamp 24 hours (unit protocols vary) with close neuro checks | Stable exam and CT; ICP stays reasonable if monitored | Pressure headache, vomiting, decline, CT enlargement — unclamp and resume drainage |
Failed wean after SAH or IVH means shunt dependence until proven otherwise. Do not discharge a clamp-fail patient on acetazolamide as a personality-based EVD substitute.
VP shunts: indications, malfunction, infection, overdrainage
A ventriculoperitoneal shunt (or, less often, ventriculoatrial or ventriculopleural) is the chronic diversion when the patient cannot live without a drain: persistent communicating hydrocephalus after SAH/IVH, failed EVD wean, or selected obstructive lesions after the obstruction is also addressed (endoscopic third ventriculostomy is an alternative in some aqueductal cases). Indications are clinical plus imaging, not a ventricle size contest.
Malfunction is obstruction (choroid, debris, abdominal loculation), disconnection, fracture of tubing, or valve failure. Adults present with headache, vomiting, lethargy, papilledema, or upgaze restriction. Obtain a head CT and a shunt series. An abdominal ultrasound or CT looks for a CSF pseudocyst when peritoneal signs appear. A shunt tap is a sterile, last-line diagnostic in selected cases — not a casual ICU procedure.
Infection (often coagulase-negative staphylococcus or Cutibacterium acnes) can present as fever, shunt-tract erythema, peritonitis, or ventriculitis without a florid meningismus. CSF from a tap or from an EVD after hardware removal makes the diagnosis. IDSA 2017 healthcare-associated ventriculitis guidance: remove infected shunt hardware, place a temporary EVD, and give intravenous antimicrobials (see 23.4). Leaving an infected shunt in and “covering with antibiotics” is the board wrong answer.
Overdrainage produces low-pressure orthostatic headache, slit ventricles, and subdural hygroma or hematoma, especially in older patients with a large extra-axial space. Treatment ranges from raising the programmable valve setting to draining or evacuating a symptomatic subdural and revising the valve. A new subdural after recent shunt insertion is overdrainage until you prove otherwise — not a spontaneous elderly fall until the valve has been thought about.
Lumbar drain: legal only when the pathway communicates
A lumbar drain samples or diverts CSF from the lumbar sac. It is used for selected communicating hydrocephalus (some aSAH protocols), CSF leaks after transsphenoidal surgery, and spinal-cord perfusion research protocols. It is not a workaround when the aqueduct or fourth ventricle is blocked.
Never perform a large-volume LP or place an LD in the presence of a posterior fossa mass, obstructive hydrocephalus at the fourth ventricle, or an uncal/tonsillar herniation pattern. Removing lumbar CSF increases the cranio-caudal pressure gradient. Tonsils can descend. Conversely, draining lumbar CSF (or overdraining an EVD) while a tight posterior fossa remains can allow the cerebellum to move up through the tentorial notch — reverse (upward) herniation — with sudden coma, bilateral midposition pupils, and disaster. If you need CSF diversion with a crowded posterior fossa, the usual tool is a carefully managed EVD plus surgical decompression, not a lumbar catheter.
Exam traps
Calling all large ventricles “obstructive” so you can put in a lumbar drain. Clamping an EVD overnight without checks because output was “annoying.” Treating shunt overdrainage subdural as a primary trauma hit without checking the valve. LPing a cerebellar infarct with a closed fourth ventricle. Setting the EVD at 0 cm H2O “to dry the ventricles” in a posterior-fossa patient.
In practice
Hunt-Hess 4 aSAH, ventricles large, GCS falling: EVD at a moderate height, nimodipine as in chapter 21, aneurysm secured, then a planned wean. Clamp-fail on day 14: VP shunt, not wishful acetazolamide. Separate patient: cerebellar infarct, tight posterior fossa, hydrocephalus: EVD plus suboccipital decompression, modest drainage, no lumbar drain. Third patient, 80 years old, programmable shunt last month, now a bilateral subdural: raise the setting and call neurosurgery — that is overdrainage physiology.
A patient with a large cerebellar infarct has an obliterated fourth ventricle and declining consciousness. Which CSF diversion plan is appropriate?
An EVD is leveled at the tragus and open at 15 cm H2O. Which statement about height, weaning, and shunt overdrainage is correct?
Which patient is the most appropriate candidate for a lumbar drain rather than an EVD as the first diversion device?