Hydrocephalus

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Hydrocephalus

Definition

Hydrocephalus is a pressure-dependent enlargement of the cerebral ventricles resulting from an imbalance between CSF production and absorption - most commonly due to obstruction of CSF flow or impaired resorption, and rarely due to overproduction. The word literally means "water on the brain," though it is a nonspecific term that must be distinguished from ventriculomegaly caused by brain atrophy (hydrocephalus ex vacuo).

Bradley and Daroff's Neurology in Clinical Practice, p. 1853
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 1149

Pathophysiology and CSF Circulation

CSF is produced by the choroid plexuses within the ventricles. It circulates from the lateral ventricles → third ventricle → cerebral aqueduct of Sylvius → fourth ventricle → exits via the foramina of Magendie (midline) and Luschka (lateral) into the subarachnoid space → bathes the cerebral convexities → absorbed primarily by the arachnoid granulations. Cerebrovascular pulsations also influence net CSF flow. Overproduction (e.g., choroid plexus papilloma) is a rare cause.

When outflow is obstructed, increased CSF volume expands the ventricles and can elevate intracranial pressure (ICP). In acute noncommunicating hydrocephalus, 80% of maximal ventricular enlargement occurs within the first 6 hours. Continued CSF production despite obstruction leads to periventricular interstitial (transependymal) edema as fluid leaches through the ependymal lining into surrounding white matter.

Classification

Type	Mechanism	Site of Obstruction
Noncommunicating (obstructive)	Intraventricular block; ventricles not in continuity with subarachnoid space	Aqueduct of Sylvius, foramina of Monro, 4th ventricle outflow
Communicating	Ventricular system remains patent; block is extraventricular in subarachnoid space	Arachnoid granulations, basal cisterns
Hydrocephalus ex vacuo	Compensatory enlargement due to loss of brain parenchyma (atrophy)	Not a true obstruction
Overproduction (rare)	Excess CSF production	Choroid plexus papilloma/carcinoma

Gross Pathology

The classic appearance is massively dilated lateral ventricles with thinned surrounding white matter:

Hydrocephalus - Dilated lateral ventricles in coronal section through mid-thalamus (Robbins Pathology)

Coronal section showing massively dilated lateral ventricles with thinning of the cerebral white matter - Robbins, Cotran & Kumar Pathologic Basis of Disease

Causes by Age Group

Neonates and Infants

Intrauterine infections causing aqueductal stenosis/gliosis (most common congenital cause)
Chiari II malformation (associated with lumbosacral myelomeningocele)
Dandy-Walker malformation (atresia of foramen of Magendie/Luschka)
Posthemorrhagic hydrocephalus (intraventricular hemorrhage - especially in preterm infants <1500 g; ~25% develop progressive ventricular enlargement)
Post-infective (bacterial meningitis causing meningeal scarring)
Congenital midline tumors; vein of Galen malformations

Older Children

Posterior fossa neoplasms (medulloblastoma, astrocytoma)
Aqueductal stenosis
Chiari I malformation

Adults

Colloid cyst of the third ventricle (ball-valve mechanism causing intermittent acute obstruction)
Cerebellar hemorrhage or infarct with edema compressing the aqueduct
Post-subarachnoid hemorrhage (communicating)
Meningitis (fungal, TB, syphilis, bacterial) causing subarachnoid fibrosis
Intraventricular tumors (ependymoma, intraventricular metastases)
Neurocysticercosis (racemose form)

Clinical Features

Infants (sutures open, before ~2 years)

The skull expands to accommodate raised ICP:

Progressive macrocephaly - head circumference crossing centile lines
Frontal bossing, calvarial thinning
Tense, bulging anterior fontanelle (even when upright and relaxed)
Sutural diastasis, dilated scalp veins
"Sunset sign" (sunsetting eyes) - failure of upward gaze from pressure on the dorsal midbrain
Lateral rectus palsies (CN VI - false localizing sign)
Lower limb spasticity (stretching of corticospinal tracts around dilated ventricles)
Irritability, poor feeding, lethargy
Macewen's "cracked-pot" sign on skull percussion

Children (sutures fused)

Headache (especially morning - worsened by recumbency)
Vomiting (morning, projectile)
Papilledema
Diplopia (CN VI palsy)
Ataxia, wide-based gait
Endocrine dysfunction if chronic (short stature, DI, amenorrhea)

Adults

Headache, papilledema, diplopia
Mental status changes
Sudden death with severe acute rises in ICP
Akinetic mutism (pressure on structures around the 3rd ventricle)
Temporal lobe seizures, CSF rhinorrhea, obesity (3rd ventricular dysfunction)

Normal-Pressure Hydrocephalus (NPH)

NPH is chronic communicating hydrocephalus in adults where lumbar puncture reveals normal or only minimally elevated CSF pressure (though long-term monitoring shows intermittent nocturnal pressure rises).

Classic Triad (Hakim-Adams triad):

Gait disturbance - "magnetic gait" / apraxic gait (feet stuck to the floor, shuffling, wide-based; resembles Parkinson disease)
Urinary incontinence (particularly in those with prominent gait disturbance)
Dementia - subcortical type with psychomotor slowing, apathy; cortical functions (language, visuospatial) relatively preserved

Gait is the presenting feature in most cases, and gait disturbance as the dominant symptom predicts better response to treatment.

Etiology: Secondary to prior SAH, trauma, or infection in ~2/3 of patients; idiopathic in ~1/3.

Diagnosis: MRI shows enlarged ventricles + transependymal CSF absorption (periventricular hyperintensity on T2/FLAIR). LP reveals normal/low-normal pressure. A large-volume tap test (removing 30-50 mL CSF) with subsequent clinical improvement in gait is a supportive diagnostic maneuver.

A 2024 Cochrane systematic review on shunting for idiopathic NPH confirms benefit of shunting but highlights limited high-quality RCT data (PMID 39105473).

Imaging

CT findings (noncommunicating/obstructive):

Dilatation of temporal horns disproportionate to lateral ventricular size
Enlargement of anterior and posterior recesses of the 3rd ventricle with inferior bowing of the floor
Periventricular lucency (transependymal edema)
Sulcal spaces, major fissures, and basal cisterns are effaced/small

The CT below shows acute hydrocephalus with transependymal contrast diffusion:

CT showing transependymal flow - contrast diffusing through ependymal wall into surrounding brain parenchyma

Axial CT showing transependymal flow: contrast infused via ventricular shunt catheter has diffused into surrounding periventricular brain - Bradley and Daroff's Neurology

MRI advantages:

Better for identifying the cause (aqueductal stenosis on sagittal, tectal gliomas, posterior fossa masses)
Aqueductal stenosis: lateral and third ventricles dilated, fourth ventricle normal size, focal narrowing of aqueduct on sagittal sequences
Demonstrates transependymal edema as periventricular T2/FLAIR hyperintensity
Cine phase-contrast MRI can assess aqueductal CSF flow dynamics

Key sites of obstruction to identify:

Foramina of Monro - colloid cysts, suprasellar tumors, arachnoid cysts
Cerebral aqueduct - tectal plate gliomas, pineal region tumors, gliosis
Fourth ventricular outflow - Dandy-Walker, posterior fossa tumors, Chiari II

Treatment

Surgical (mainstay)

Procedure	Indication
Ventriculoperitoneal (VP) shunt	Most common; CSF drained from ventricles to peritoneal cavity via one-way pressure valve
Ventriculoatrial (VA) shunt	Alternative if peritoneal cavity unsuitable
External ventricular drain (EVD)	Acute/emergency hydrocephalus (e.g., post-SAH, cerebellar hemorrhage); also allows ICP monitoring
Endoscopic third ventriculostomy (ETV)	Noncommunicating hydrocephalus (especially aqueductal stenosis); creates alternative CSF pathway; avoids implanted hardware

Shunt complications: Malfunction (most common - causes abrupt decompensation resembling original presentation), infection, over-drainage, shunt revision as child grows.

Acute/Emergency situations:

Cerebellar hemorrhage/infarct with acute hydrocephalus: urgent EVD placement + posterior fossa craniectomy
Post-SAH hydrocephalus: EVD for acute phase; ~50% require permanent VP shunt

Medical:

Acetazolamide (carbonic anhydrase inhibitor) and furosemide - used temporarily to reduce CSF production, particularly in post-hemorrhagic hydrocephalus of prematurity
Not a long-term solution

Summary Table

Feature	Noncommunicating	Communicating	NPH
Site of block	Intraventricular	Subarachnoid/arachnoid granulations	Arachnoid/subarachnoid (subtle)
Common causes	Aqueductal stenosis, tumor, Chiari	Post-SAH, meningitis, choroid plexus tumor	Idiopathic, post-SAH, trauma
Ventricles affected	Upstream from block	All (entire system)	All
CSF pressure	Elevated	Elevated or normal	Normal (intermittently elevated)
Treatment	ETV or VP shunt	VP shunt	VP shunt (tap test first)

Sources:

Bradley and Daroff's Neurology in Clinical Practice (9780323642613), pp. 1853-1857
Robbins, Cotran & Kumar Pathologic Basis of Disease (9780443264528), pp. 1148-1149
Grainger & Allison's Diagnostic Radiology (9780323760751), pp. 2027-2036
Goldman-Cecil Medicine (9780323930345)
Pearce et al., Cochrane 2024 - Shunting for idiopathic NPH (PMID 39105473)

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