Hydrocephalus

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Hydrocephalus

Hydrocephalus is a pressure-dependent enlargement of the cerebral ventricles resulting from an imbalance between CSF production and resorption. The term literally means "water on the brain," though it is important to distinguish true hydrocephalus from ventriculomegaly due to brain tissue loss (hydrocephalus ex vacuo). - Bradley and Daroff's Neurology in Clinical Practice, p. 1854

CSF Physiology - The Foundation

CSF is produced by the choroid plexus within the lateral ventricles at a rate of approximately 20 mL/hour (total CSF volume ~150 mL). It flows through a well-defined pathway:

CSF pathways - Bailey and Love's Short Practice of Surgery

Lateral ventricles → Foramen of Monro → Third ventricle → Cerebral aqueduct (of Sylvius) → Fourth ventricle → Foramina of Magendie (midline) and Luschka (lateral) → Subarachnoid space → Absorbed at arachnoid granulations along the superior sagittal sinus.

Hydrocephalus results from obstruction anywhere in this pathway, or from failure of absorption at the arachnoid granulations. - Bailey and Love's, p. 4040

Classification

1. Non-communicating (Obstructive) Hydrocephalus

The blockage is within the ventricular system itself.
CSF cannot flow from the ventricles to the subarachnoid space.
Only the ventricles proximal to the obstruction enlarge.
Common sites: cerebral aqueduct, foramen of Monro, fourth ventricle outflow.
Causes: aqueductal stenosis/gliosis, tumors, hemorrhage, Chiari II malformation, colloid cyst of the 3rd ventricle.

2. Communicating (Extra-ventricular Obstructive) Hydrocephalus

The obstruction is outside the ventricular system - in the subarachnoid space or at the arachnoid granulations.
The entire ventricular system enlarges.
Causes: subarachnoid hemorrhage (post-SAH scarring), meningitis (bacterial, TB, fungal, syphilis), leptomeningeal carcinomatosis, choroid plexus papilloma (rarely - CSF overproduction).

3. Hydrocephalus Ex Vacuo

Compensatory increase in CSF volume secondary to loss of brain parenchyma (infarction, neurodegenerative disease).
Not a true hydrocephalus - no elevation of ICP.
Does not benefit from shunting.
Robbins & Kumar Basic Pathology, p. 821

Causes by Age Group

Neonates and Infants (pre-suture closure, < 2 years)

Congenital aqueductal stenosis/gliosis (most common - often post in utero infection)
Post-hemorrhagic hydrocephalus - intraventricular hemorrhage (IVH) in premature infants < 1500g; ~25% develop progressive ventricular enlargement; 5% ultimately require shunting.
Chiari II malformation - associated with lumbosacral myelomeningocele; fourth ventricular outlet foramina displaced below the foramen magnum.
Dandy-Walker malformation - cystic dilatation of the fourth ventricle.
Post-infective - meningitis causing scarring and closure of the cerebral aqueduct or foramina of Magendie/Luschka.
Vein of Galen malformation (raised intracranial venous pressure).

Older Children (post-suture closure)

Posterior fossa neoplasms (most common) - compress the cerebral aqueduct/fourth ventricle.
Aqueductal stenosis (developmental or acquired).
Meningitis.

Adults

Acute: Cerebellar hemorrhage or infarction compressing the aqueduct; subarachnoid hemorrhage; colloid cyst of third ventricle (ball-valve mechanism causing intermittent symptoms), intraventricular ependymoma, cysticercosis (racemose form).
Chronic/communicating: Post-SAH, post-meningitis, leptomeningeal disease, idiopathic.
Normal Pressure Hydrocephalus (NPH): Idiopathic (or secondary to prior SAH/trauma/meningitis).
Bradley and Daroff's Neurology, pp. 1854-1856; Grainger & Allison's Diagnostic Radiology

Clinical Features

Infants (sutures still open)

The skull can expand, so ICP remains low for longer. Features include:

Feature	Detail
Progressive macrocephaly	Most reliable sign; head circumference crossing centile lines
Frontal bossing	Prominent forehead
Bulging anterior fontanelle	Present even at rest (tense)
Sutural diastasis	Separation of skull sutures
Enlarged scalp veins	From venous obstruction
"Setting-sun" sign	Downward deviation of eyes (Parinaud syndrome); failure of upward gaze
Lateral rectus palsy	6th nerve "false localizing" sign
Leg spasticity	Stretching of corticospinal tracts around enlarged ventricles
McEwen sign	"Cracked-pot" sound on skull percussion
Irritability, poor feeding, lethargy	Non-specific but common

Older Children and Adults (fused sutures)

Signs and symptoms are those of raised intracranial pressure:

Headache - classically early morning, made worse by lying flat/coughing (increased ICP)
Nausea and vomiting - early morning vomiting is characteristic
Papilloedema - on fundoscopy
Diplopia - 6th nerve palsy (false localizing sign)
Altered consciousness - drowsiness, confusion, coma with severe cases
Gait disturbance - wide-based, ataxic
Sudden death may occur with severe acute ICP elevation

Special feature in adults: Akinetic mutism (due to pressure on structures around the third ventricle); temporal lobe seizures; CSF rhinorrhea; endocrine dysfunction (amenorrhea, diabetes insipidus, obesity from third ventricle pressure effects).

Normal Pressure Hydrocephalus (NPH)

The classic Hakim-Adams triad:

Gait apraxia - "magnetic gait," wide-based, shuffling (earliest and most prominent)
Dementia/cognitive impairment - subacute onset
Urinary incontinence

CSF pressure appears normal on LP (though ICP was likely elevated in the past during the enlargement phase). Long-term monitoring reveals intermittent ICP spikes. NPH most often presents in the elderly.

Bradley and Daroff's Neurology, pp. 1855-1857

Pathophysiology of Acute Hydrocephalus

Once outflow is blocked:

80% of maximal ventricular enlargement occurs within the first 6 hours (continued CSF production despite elevated pressure).
A slower phase of enlargement follows.
Fluid accumulates in periventricular white matter → interstitial (transependymal) edema.
As it becomes chronic, CSF pressure may normalize ("normal pressure" phase), but long-term ICP monitoring still shows intermittent elevations.
Long-standing hydrocephalus causes white matter atrophy, periventricular demyelination.

Transependymal flow (CSF diffusing through the ependyma into surrounding brain) is a key imaging finding detectable on CT and MRI.

Hydrocephalus - Dilated Lateral Ventricles (coronal section)

Hydrocephalus: coronal section showing massively dilated lateral ventricles (red boxes = bodies; yellow boxes = posterior horns). - Robbins & Kumar Basic Pathology, Fig. 21.3

Imaging

CT / MRI Findings - Active (Obstructive) Hydrocephalus

Temporal horn dilatation disproportionate to overall lateral ventricular enlargement (most sensitive early sign)
Inferior convexity of the third ventricular floor and enlargement of its anterior and posterior recesses
Transependymal (periventricular interstitial) edema - periventricular hypodensity on CT, T2/FLAIR hyperintensity on MRI
Sulcal effacement - sulci, major fissures, and basal cisterns are small or obliterated (distinguishes true hydrocephalus from ex vacuo)
Bulging of fontanelles in infants

Differentiating Hydrocephalus from Ex Vacuo

Feature	True Hydrocephalus	Ex Vacuo
Sulci/fissures	Effaced, small	Widened, prominent
Basal cisterns	Small	Normal or large
ICP	Elevated	Normal
Transependymal edema	Present	Absent
Head circumference	May increase (infants)	Normal

Aqueductal Stenosis on MRI

Focal narrowing at the superior colliculi or intercollicular sulcus on sagittal MRI.
Lateral and third ventricles dilated; fourth ventricle normal size.
Grainger & Allison's Diagnostic Radiology, pp. 2028-2029

Treatment

Underlying Cause

Remove the obstructive lesion when possible (e.g., resect a posterior fossa tumor, treat meningitis).

CSF Diversion

Procedure	Description
Ventriculoperitoneal (VP) shunt	Gold standard; one-way pressure-sensitive valve; CSF drained into peritoneal cavity
Ventriculoatrial (VA) shunt	Drain into right atrium; used when peritoneal cavity is unsuitable
Endoscopic Third Ventriculostomy (ETV)	Surgical fenestration of the third ventricle floor; CSF drains into suprasellar cistern; preferred in aqueductal stenosis and children > 6 months; avoids a foreign body
External Ventricular Drain (EVD)	Temporary; used in acute hydrocephalus (e.g., post-SAH, cerebellar hemorrhage) to emergently decompress and monitor ICP

Shunt Complications

Shunt obstruction (most common) - choroid plexus or glial tissue blocks the catheter; presents with recurrence of hydrocephalus symptoms.
Shunt infection - presents with fever, meningism; requires shunt removal + antibiotics.
Shunt overdrainage - subdural hygroma/hematoma; slit-ventricle syndrome.
Shunt disconnection - detectable on plain skull-to-abdomen X-ray; calcification may be seen at fracture ends.
Revisions are frequently required in children as they grow.

Shunt malfunction is assessed by comparing current imaging with baseline post-operative scans. Fluid tracking along the shunt tubing on CT/MRI suggests CSF leak or obstruction.

NPH - Shunting

A 2024 Cochrane review (PMID 39105473) examined shunting for idiopathic NPH. The "large-volume tap test" (LP withdrawal of 30-50 mL CSF) is used to predict shunt responsiveness - a 2025 systematic review found evidence for its utility but noted lack of standardization in protocol.

Special Situations

Post-SAH Hydrocephalus

Acute obstructive or communicating hydrocephalus can develop rapidly after subarachnoid hemorrhage and requires emergency EVD placement for CSF removal and ICP monitoring. Many patients subsequently develop chronic hydrocephalus requiring a permanent VP shunt. - Goldman-Cecil Medicine, p. 3954

Choroid Plexus Papilloma

A rare cause of hydrocephalus via: (a) CSF overproduction, (b) obstructive mass effect, or (c) hemorrhage within the ventricular system causing communicating hydrocephalus.

Raised Intracranial Venous Pressure

An under-recognized mechanism - seen with craniosynostosis (stenosis of jugular foramina), dural arteriovenous fistulas, and venous sinus thrombosis.

Occult Spinal Lesions

In unexplained hydrocephalus, spinal imaging is recommended to exclude occult intraspinal tumors, which can cause hydrocephalus via proteinaceous exudates or pial dissemination impairing CSF absorption. - Grainger & Allison's, p. 2028

Summary Table

Feature	Non-communicating	Communicating	NPH
Obstruction site	Within ventricles	Subarachnoid space	Arachnoid granulations (impaired absorption)
Ventricles affected	Partial (proximal only)	All ventricles	All ventricles
Common causes	Aqueductal stenosis, tumors, Chiari II	SAH, meningitis	Idiopathic (elderly)
Presentation	Acute ICP features	Subacute ICP features	Hakim triad (gait, dementia, incontinence)
Treatment	ETV or VP shunt	Treat cause + VP shunt	VP shunt (if tap test positive)

Sources: Bradley and Daroff's Neurology in Clinical Practice | Robbins & Kumar Basic Pathology | Grainger & Allison's Diagnostic Radiology | Bailey and Love's Short Practice of Surgery, 28th Ed. | Goldman-Cecil Medicine

Make a diagram showing obstructive vs communicating hydrocephalus pathways

Generating Image

Medical educational diagram comparing obstructive (non-communicating) vs communicating hydrocephalus CSF pathways. Split into two side-by-side panels on a clean white background. LEFT PANEL - "Obstructive (Non-Communicating) Hydrocephalus": - Labeled sagittal brain cross-section showing the ventricular system - Lateral ventricle and third ventricle shown enlarged/dilated (colored in blue, darker shade) - Cerebral aqueduct blocked with a red X or red barrier symbol - Fourth ventricle shown as normal size (lighter blue) - Subarachnoid space shown as normal - Arrow showing CSF flow blocked at aqueduct - Text labels: "Lateral ventricle (DILATED)", "Third ventricle (DILATED)", "Cerebral aqueduct (BLOCKED)", "Fourth ventricle (normal)", "Subarachnoid space (normal)" - Common causes listed below: "Causes: Aqueductal stenosis, Posterior fossa tumor, Colloid cyst, Chiari II malformation" RIGHT PANEL - "Communicating Hydrocephalus": - Labeled sagittal brain cross-section showing the ventricular system - All four ventricles shown enlarged/dilated (all in blue, darker shade) - Cerebral aqueduct shown as patent/open (no blockage) - Subarachnoid space shown as thickened/blocked with red X at the arachnoid granulations along superior sagittal sinus - Arrow showing CSF flows freely through ventricles but is blocked at absorption point - Text labels: "Lateral ventricle (DILATED)", "Third ventricle (DILATED)", "Cerebral aqueduct (PATENT)", "Fourth ventricle (DILATED)", "Arachnoid granulations (IMPAIRED ABSORPTION)" with red X - Common causes listed below: "Causes: Subarachnoid hemorrhage, Meningitis, Leptomeningeal carcinomatosis" Shared CSF pathway legend at the bottom: blue arrows showing normal flow direction from lateral ventricles → foramen of Monro → third ventricle → cerebral aqueduct → fourth ventricle → foramina of Magendie & Luschka → subarachnoid space → arachnoid granulations → superior sagittal sinus Clean medical textbook illustration style, clear anatomical labels, professional and educational

Obstructive vs Communicating Hydrocephalus

Here's a summary of what the diagram shows:

Left panel - Obstructive (Non-communicating):

Only the ventricles proximal to the block are enlarged (lateral + third ventricles)
Blockage at the cerebral aqueduct (most common site)
Fourth ventricle and subarachnoid space remain normal
CSF cannot exit the ventricular system

Right panel - Communicating:

All four ventricles are dilated because CSF flows freely through them
The block is at the level of the arachnoid granulations (impaired absorption)
Causes like SAH and meningitis leave fibrous scarring in the subarachnoid space, preventing reabsorption into the superior sagittal sinus

The key clinical distinction: if the fourth ventricle is dilated, the block is outside the ventricles (communicating). If the fourth ventricle is normal, the block is inside (obstructive).

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