Hydrocephalus: Etiology, Pathophysiology, and Clinical Features

Definition and Classification

Hydrocephalus: Massively dilated lateral and third ventricles seen on a coronal section (Robbins & Kumar Pathologic Basis of Disease)

1. Causes of Congenital Hydrocephalus

Obstructive (Noncommunicating) Causes

• The most common congenital cause
• The cerebral aqueduct (of Sylvius) is narrow or replaced by several minute channels
• In a few cases, transmitted as an X-linked recessive trait (L1CAM gene mutation)
• Most cases result from fetal viral infection - especially cytomegalovirus (CMV) or Toxoplasma gondii
• Results in dilatation of lateral ventricles and third ventricle, with normal-sized fourth ventricle

• Chiari II malformation: downward displacement of cerebellar structures through the foramen magnum; almost always associated with lumbosacral myelomeningocele. Hydrocephalus typically becomes manifest after surgical repair of the myelomeningocele
• Chiari I malformation: less commonly associated with hydrocephalus

• Congenital failure of opening of the foramina of Luschka and Magendie
• Cystic dilation of the fourth ventricle, hypoplasia/absence of the cerebellar vermis
• Causes noncommunicating hydrocephalus with enlargement of all ventricles

• Congenital midline tumors (e.g., tectal glioma)
• Vein of Galen malformation (arteriovenous malformation causing venous hypertension)
• Holoprosencephaly with single ventricle

Communicating Congenital Causes

• Congenital absence or deficient number of arachnoid villi
• Intrauterine subarachnoid hemorrhage causing basal cisternal fibrosis
• Intraventricular hemorrhage in premature infants: premature infants < 1500 g have a high risk of intraventricular hemorrhage; approximately 25% develop progressive ventricular enlargement, and 5% ultimately require shunting

2. Causes of Acquired Hydrocephalus

Post-Infectious (Postinfective)

• Bacterial meningitis - the most common acquired cause; fibrinous exudate causes scarring at the basal cisterns and foramina of Magendie/Luschka, blocking CSF flow
• Tuberculous meningitis - creates dense basal exudate that obliterates cisterns
• Viral encephalitis - can cause ependymitis and aqueductal obstruction
• Fungal meningitis (e.g., Cryptococcus) - granulomatous occlusion of subarachnoid pathways

Post-Hemorrhagic

• Intraventricular hemorrhage (IVH) - blood in CSF triggers arachnoid fibrosis; the most common acquired cause in neonates (from germinal matrix hemorrhage in prematurity)
• Subarachnoid hemorrhage (SAH) - ruptures into cisterns, fibrin blocks CSF absorption
• Cerebellar hemorrhage or infarct - directly compresses fourth ventricle outlet

Neoplastic

• Posterior fossa tumors are the most common cause in older children: medulloblastoma, ependymoma, cerebellar astrocytoma - all obstruct the fourth ventricular outflow or aqueduct
• Colloid cyst of third ventricle - blocks both foramina of Monro, causing bilateral lateral ventricular dilatation
• Tectal plate glioma - causes aqueductal stenosis
• Choroid plexus papilloma - rare cause by overproduction of CSF (often with concurrent obstruction)

Inflammatory / Other

• Normal pressure hydrocephalus (NPH) - adults; communicating; associated with prior SAH, meningitis, or idiopathic; classic triad: gait apraxia, dementia, urinary incontinence
• Craniosynostosis - premature fusion of sutures restricts brain growth (note: does NOT cause hydrocephalus per se, but can cause raised ICP)

3. Pathophysiology

CSF Dynamics

Mechanism of Ventricular Enlargement

• Aqueductal obstruction → disproportionate enlargement of lateral + third ventricles; fourth ventricle remains normal
• Fourth ventricle obstruction → dilatation includes the aqueduct
• Basal cisternal obstruction → entire ventricular system including fourth ventricle enlarges

Acute Phase

• Ventricular enlargement is rapid: 80% of maximal enlargement within 6 hours of acute noncommunicating obstruction
• Continued CSF production despite raised ICP
• Elevated intracranial pressure (ICP)
• Transependymal (periventricular interstitial) edema - CSF is forced across the ependymal lining into the periventricular white matter, visible on MRI as periventricular T2 signal

Chronic Phase

• After ventricular enlargement stabilizes, CSF production is balanced by transependymal absorption
• ICP may normalize - this is the basis of normal-pressure hydrocephalus
• Long-term monitoring reveals intermittent ICP spikes
• White matter atrophy surrounds the ventricles; gray matter is relatively spared
• Stretching of corticospinal tracts (which descend from motor cortex around the lateral ventricles) → lower limb spasticity

Infant vs. Older Child

• In infants (before suture fusion, typically before age 3): expanding ventricles separate cranial sutures → head enlargement is the primary manifestation; ICP elevation is buffered
• After sutures close: ventricles expand against a rigid skull → raised ICP is the dominant consequence; no head enlargement

4. Clinical Features in Infants (Before Suture Fusion, < 2 Years)

Head and Skull Signs

• Accelerated head growth - head circumference increasing faster than expected for age; the most reliable early sign
• Frontal bossing - prominent, disproportionately large forehead (brachycephalic skull)
• Tense, bulging anterior fontanelle - fontanelle does not pulsate normally; feels tense even when the infant is sitting upright
• Sutural diastasis - palpable widening of sutures, especially the sagittal suture
• Calvarial thinning - may be seen on imaging; "beaten copper" or "digital markings" on skull X-ray from gyral impressions
• Dilated scalp veins - venous collaterals from raised venous pressure

Eye Signs

• "Sunset" sign (Setting-sun sign): forced downward deviation of the eyes with sclera visible above the iris; caused by pressure on the dorsal midbrain (pretectal area) from the dilated third ventricle, producing a Parinaud-like syndrome
• Failure of upward gaze
• Lateral rectus palsy (CN VI palsy) - false localizing sign from raised ICP

Neurological Signs

• Irritability - common early symptom; infant is fretful
• Poor feeding, lethargy
• Spasticity of lower limbs - from stretching of the corticospinal tracts descending around the lateral ventricles
• Nausea and vomiting (in acute enlargement)

Late / Severe Signs (if untreated)

• Abnormal eye movements and optic atrophy (if diagnosis delayed)
• Developmental regression
• In severely advanced cases: loss of white matter, cortical atrophy

5. Clinical Features in Older Children (After Suture Fusion, > 3 Years)

Symptoms

• Headache - classically worse in the morning (positional, from nocturnal ICP surges) and on waking, often frontal or global; may wake the child from sleep
• Nausea and vomiting - often projectile; worse in the morning
• Diplopia - due to CN VI (abducens) palsy; a false localizing sign from raised ICP; occurs on one or both sides
• Blurred vision - from papilloedema and/or impaired accommodation
• Altered level of consciousness - drowsiness, decreased responsiveness in severe cases

Signs

• Papilloedema - on fundoscopy; the hallmark of chronic raised ICP; may be absent in rapidly developing hydrocephalus
• CN VI palsy (lateral rectus palsy, convergent squint)
• Spasticity of lower limbs - broad-based, ataxic gait; from periventricular white matter stretching around the lateral ventricles
• Wide-based ataxic gait - from frontal lobe fiber tract distortion
• Endocrine dysfunction (in long-standing cases): short stature, precocious or delayed puberty, menstrual irregularities, diabetes insipidus - from hypothalamic/pituitary compression
• Cognitive changes and memory impairment - in chronic cases
• Cushing response (bradycardia, hypertension, irregular respiration) - in critically raised ICP

Specific Features by Cause

• Posterior fossa tumor-related hydrocephalus (medulloblastoma, ependymoma): acute onset, headache, vomiting, ataxia, and cranial nerve palsies, particularly CN VI; papilloedema; lower limb spasticity
• Aqueductal stenosis in older children: may present insidiously with slow cognitive decline, gait disturbance, and endocrine problems

6. Differential Diagnosis of Enlarged Head (Macrocephaly)

Key caution: In craniosynostosis, premature suture fusion may cause an asymmetric skull or growth preferentially in one direction, giving a spuriously large head circumference - never diagnose hydrocephalus on circumference alone. Always correlate with imaging.

Summary: Key Distinguishing Features by Age

• Adams and Victor's Principles of Neurology, 12th Ed. - "Syndromes of Hydrocephalus," p. 635
• Bradley and Daroff's Neurology in Clinical Practice - "Hydrocephalus in Children," p. 1854
• Grainger & Allison's Diagnostic Radiology - "HYDROCEPHALUS," p. 2028
• Robbins & Kumar Pathologic Basis of Disease - "Hydrocephalus," p. 1149
• The Developing Human: Clinically Oriented Embryology - "Hydrocephalus," p. 1082