Hydrocephalus is defined as an abnormal accumulation of cerebrospinal fluid (CSF) within the ventricular system of the brain, resulting from obstruction of CSF flow, impairment of CSF absorption, or, rarely, CSF overproduction. The pathophysiology reflects either proximal stagnation of CSF, failure of reabsorption, or mechanical blockage anywhere between the ventricular system and the arachnoid granulations.

 

Q. What is the epidemiology of hydrocephalus?

 

Hydrocephalus affects approximately 1–1.5% of the general population.

Congenital hydrocephalus has an incidence of 0.9–1.8 per 1000 births, with an overall reported range from 0.2–3.5 per 1000. It may be detected antenatally, at birth, or in the early months of life. Hydrocephalus occurs across all age groups, with congenital and infantile forms being the most common early-life presentations, whereas normal-pressure hydrocephalus predominates in older adults.

 

Q. How do you classify hydrocephalus?

 

Hydrocephalus can be classified broadly into functional, etiologic, pressure-based, and morphologic categories.

 

A. Functional Classification:

 

Obstructive (non-communicating) hydrocephalus: CSF flow is blocked within the ventricular system before reaching the arachnoid granulations. Ventricular enlargement occurs proximal to the obstruction.

 

Communicating (non-obstructive) hydrocephalus: Ventricular pathways remain patent, but CSF absorption at the arachnoid villi is impaired.

 

CSF overproduction: Rare; occurs mainly in choroid plexus tumors. Absorption is usually partially compensatory unless flow is obstructed.

 

B. Etiologic Classification

 

Congenital

Acquired

Post-infectious

Post-hemorrhagic

Neoplastic

Vascular

Genetic (e.g., X-linked hydrocephalus)

 

C. Pressure-Based Classification

 

Normal-pressure hydrocephalus 

Hydrocephalus ex vacuo

Long-standing overt ventriculomegaly in adults (LOVA)

 

D. Morphologic (Radiologic) Classification

 

Univentricular enlargement (e.g. entrapped 4th ventricle)

Biventricular enlargement (e.g. obstruction at foramen of monro)

Triventricular hydrocephalus (e.g. aqueductal stenosis)

Communicating ventriculomegaly (secondary to IVH or meningitis, e.t.c.)

Entrapped fourth ventricle

 

Q. What are the etiologies of hydrocephalus?

 

A. Congenital Causes

 

Chiari II malformation with myelomeningocele

Chiari I malformation

Primary congenital aqueductal stenosis

Secondary aqueductal gliosis from intrauterine events

Dandy–Walker malformation (atresia of Magendie and Luschka)

X-linked hydrocephalus (L1CAM mutation)

Encephaloceles

Other developmental anomalies affecting posterior fossa CSF pathways or foraminal patency

 

B. Acquired Causes

 

Infectious: bacterial meningitis, TB meningitis, cysticercosis

Hemorrhagic: intraventricular hemorrhage (IVH), subarachnoid hemorrhage (SAH)

Neoplastic: tumors obstructing CSF flow (colloid cyst, medulloblastoma, ependymoma, suprasellar tumors)

Postoperative: especially after posterior fossa tumor excision or intraventricular surgery

Traumatic: post-traumatic SAH or IVH; scarring of subarachnoid pathways

Neurosarcoidosis: granulomatous obstruction of CSF pathways

 

C. Vascular Causes

 

Arteriovenous malformations

Vein of Galen aneurysmal malformation (VGAM)

 

D. Functional/Physiological Causes

Constitutional or benign ventriculomegaly

Arrested or compensated hydrocephalus (non-progressive)

 

Q. What are the special forms of hydrocephalus?

 

Special variants include:

Hydrocephalus ex vacuo

LOVA (long-standing overt ventriculomegaly in adults)

External hydrocephalus (benign external hydrocephalus)

Entrapped fourth ventricle

Arrested hydrocephalus

Triventricular hydrocephalus

Normal-pressure hydrocephalus

 

Q. What are the clinical features of hydrocephalus in older children and adults?

 

Older children and adults typically present with signs attributable to increased intracranial pressure and disruption of periventricular white matter tracts.

Headache

Nausea and vomiting

Papilledema

Impaired concentration and cognitive slowing

Gait instability

Parinaud’s syndrome from tectal pressure

Sixth nerve palsy from elevated ICP

Personality changes or irritability

Urinary incontinence (late)

Blurred vision and visual field defects from chronic papilledema

Slow ventricular enlargement may present subtly, with cognitive and gait changes preceding overt ICP symptoms.

 

Q. What are the clinical features of hydrocephalus in infants and young children?

 

Symptoms include:

Irritability

Excessive crying

Poor head control

Feeding difficulties

Vomiting

Failure to thrive

Developmental delay

 

Signs include:

Progressive macrocephaly with OFC crossing percentiles

Bulging or tense anterior fontanelle

Prominent scalp veins from reversed venous flow

Frontal bossing

McEwan’s sign: cracked-pot percussion note

Setting sun sign: downward deviation of eyes from pressure on suprapineal recess

Hyperreflexia

Irregular respirations or apneic spells

Sutural diastasis in chronic cases

 

Q. How do you measure OFC and what findings suggest hydrocephalus?

 

Technique: 

Place a non-stretchable measuring tape over the supraorbital ridge anteriorly and around the most prominent point of the occiput posteriorly. Ensure the tape is snug and hair is flattened. Repeat twice; if measurements differ by more than 2 mm, take a third and average the closest two.

 

Abnormal Findings:

 

OFC growth exceeding 1.25 cm per week

OFC above 2 SD for age

Head circumference rising across percentile curves

Persistent macrocephaly relative to overall growth pattern

OFC out of proportion to facial/cranial growth

 

Q. What are the radiologic features of hydrocephalus? 

 

The radiologic features can be divided into:

 

A. Acute Hydrocephalus:

Reflects sudden obstruction or acute failure of CSF resorption.

 

1. Temporal horn enlargement ≥ 2 mm

Earliest and most sensitive sign. Normally TH is barely visible.
Criteria for acute hydrocephalus:

TH ≥ 2 mm and fissures/sulci not visible

OR TH ≥ 2 mm + FH/ID > 0.5 (not Evans index)

 

2. Ballooning of frontal horns: “Mickey Mouse” ventricles

 

3. Widening of the third ventricle: Should normally be slit-like

 

4. Periventricular low density on CT: Represents transependymal interstitial edema due to stasis of flow around ventricles
 

5. Periventricular T2 hyperintensity on MRI: Represents CSF stasis rather than actual CSF diffusion through ependyma.

 

6. Obstructive patterns: Dilated ventricles proximal to the block, for e.g. colloid cyst can cause biventricular enlargement

 

7. Loss of cortical sulci and cisterns (effacement): Due to acute rise in ICP

 

8. Upward bowing of corpus callosum from ventricular expansion

 

B. Chronic Hydrocephalus – Radiologic Features

 

Chronic hydrocephalus produces characteristic long-standing changes due to sustained ventricular expansion and cranial remodeling:

 

Beaten copper or beaten silver cranium on skull radiographs

Third ventricular downward ballooning into the sella

Erosion or empty sella from chronic pressure

Temporal horns less prominent than in acute states

Macrocrania (>98th percentile OFC)

Corpus callosum thinning and upward bowing

Widened sutures, delayed closure of fontanelles (infants)

Cortical mantle thinning from longstanding ventricular expansion

Smooth, rounded ventricular contours

Often lack of acute transependymal edema

 

Q. What is the Evans Index and how is it interpreted?

 

Evans Index is the ratio of the maximal frontal horn width divided by the maximal biparietal diameter on the same axial CT or MRI slice.

 

Evans Index >0.3 is considered indicating hydrocephalus.

Evans Index <0.3 is usually normal (0.25 is considered a grey zone).

 

Q. What are the limitations of the Evans Index?

 

Evans Index is widely used but has several important limitations:

The measurement varies with axial slice angle.

Maximal biparietal diameter may not lie on the same slice as maximal frontal horn width.

Underestimates pediatric hydrocephalus because occipital horns often enlarge more than frontal horns.

Variability in the ratio may exceed actual structural differences.

Thus, Evans Index alone should never be used as a sole diagnostic criterion.

 

Q. What is McEwan’s sign?

 

McEwan’s sign is a cracked-pot percussion sound produced when tapping the skull of an infant (mostly at anterior fontanelle) with marked hydrocephalus. It is caused by:

Separation of sutures

Thinning of cranial bones

Increased resonance due to enlarged ventricles

It is a classic sign of chronic infantile hydrocephalus.

 

Q. What is external hydrocephalus, and how do you differentiate it from true hydrocephalus?

External hydrocephalus, also known as benign external hydrocephalus (BESS), is a condition in which CSF accumulates in the subarachnoid spaces, predominantly over the frontal and frontoparietal convexities, due to delayed maturation of arachnoid villi. It is not obstructive or communicating hydrocephalus, and the ventricles maintain normal or only mildly increased diameter.

 

Pathophysiology:

Arachnoid granulations are immature in early infancy. CSF production is normal but absorption is temporarily reduced, thus CSF preferentially expands subarachnoid spaces instead of ventricles. As arachnoid villi mature (by 18–24 months), spaces normalize.
 

Clinical Features:

Occurs between 3 and 18 months of age.

Macrocephaly with OFC typically above the 97th percentile.

Development is normal or near normal.

No symptoms of raised intracranial pressure.

Positive family history of macrocephaly in some cases.

 

Radiologic Features

Enlarged frontal subarachnoid spaces, symmetrical.

Interhemispheric fissure widened.

Ventricles normal or minimally enlarged, not disproportionate.

Cortical mantle thickness normal, which is a major point of differentiation.

Cortical vein sign: visible cortical veins crossing the widened subarachnoid spaces, helping differentiate from subdural effusion.

 

Differentiation from True Hydrocephalus

External hydrocephalus:

Subarachnoid enlargement > ventricular enlargement

Normal cortex thickness

No sulcal effacement

Normal intracranial pressure

No transependymal edema

Stable OFC growth pattern

Resolves spontaneously

 

True hydrocephalus:

Ventricular dilation > subarachnoid space dilation

Sulcal effacement

Periventricular edema

Signs of raised ICP

Progressive symptoms

Requires CSF diversion

Course and Prognosis

Peaks around 6–12 months

Resolves spontaneously by age 2

No long-term neurological deficit expected

 

Management:

Observation only

Serial OFC and developmental monitoring

No role for surgical intervention

Imaging only if symptoms change

 

Q. What is arrested hydrocephalus? 

Arrested hydrocephalus refers to a non-progressive, compensated form of ventriculomegaly in which CSF dynamics reach a steady state. The ventricles are enlarged, but CSF pressure is stable, and the patient has no clinical or radiologic evidence of progression.

 

Pathophysiology:

Initial hydrocephalus (congenital, post-infectious, post-hemorrhagic) occurs.

CSF absorption pathways partially or completely recover.

Ventricular enlargement remains but does not progress.

Brain adapts via decreased CSF production, enhanced alternative absorption, and tissue compliance adjustments.

 

Clinical Features:

Asymptomatic or minimally symptomatic

Normal OFC trajectory in infants

No vomiting, irritability, papilledema, or headache

Mild developmental delays may persist from initial insult, but no ongoing decline

No gait instability or cognitive worsening in older children

Radiologic Features

Stable ventricular size on sequential CT/MRI

No transependymal edema

Normal periventricular signal

Stable corpus callosum morphology

No progressive skull changes

In infants: fontanelle and sutures may have adapted and remain static

 

Diagnosis: Requires three components.

Clinical stability

Radiological stability on serial imaging

Absence of raised ICP features

Triggers for Reactivation

 

Even stable hydrocephalus can “reactivate”:

Meningitis

Trauma (sometimes even mild)

New hemorrhage

Pubertal brain growth

Sudden changes in CSF absorption

 

Management:

Observation only if fully compensated

Serial OFC in infants

Neurological and developmental monitoring

Imaging every 6–12 months initially

Intervention only if reactivation occurs

 

Treatment of Reactivation:

ETV if obstructive pattern develops

VP shunt if communicating or absorption failure recurs

Arrested hydrocephalus should never be shunted unless clearly decompensating, as unnecessary shunting may precipitate subdural hematomas and overdrainage-related complications.

 

Q. What is an entrapped fourth ventricle?

 

Entrapped fourth ventricle is a condition where the fourth ventricle becomes isolated due to obstruction of both its inlet (aqueduct) and outlets (foramina of Magendie and Luschka), leading to progressive dilation of the fourth ventricle and compression of the brainstem and cerebellum.

 

Pathophysiology:

Two simultaneous blocks:

1. Aqueductal obstruction:

Prevents CSF flow from third to fourth ventricle.

Often secondary to gliosis, hemorrhage, or infection.

 

2. Outlet obstruction

Scarring of Magendie/Luschka caused by meningitis, arachnoiditis, or postoperative scarring.

CSF continues to be produced by the choroid plexus inside the fourth ventricle, causing a closed compartment with progressive dilation.

 

Etiology:

Post-intraventricular hemorrhage

Post-meningitic or TB meningitis scarring

Postoperative scarring after posterior fossa surgery

Aqueductal gliosis

Arachnoiditis

VP shunting of supratentorial ventricles, leading to collapse above and pressure buildup below

 

Clinical Features:

Symptoms are due to brainstem and cerebellar compression:

Ataxia and truncal imbalance

Dysarthria and dysphagia

Horizontal or vertical gaze palsies

Nystagmus

Irritability or lethargy in infants

Respiratory irregularity or apneic spells

New neurological deficits after VP shunt insertion (classic presentation)

Head circumference may be normal if supratentorial ventricles are decompressed by a shunt

 

Radiological Features:

Isolated dilation of the fourth ventricle

Supratentorial ventricles normal or small (post-shunting)

Fourth ventricle ballooned posteriorly and inferiorly

Brainstem flattened and compressed anteriorly

Vermis displaced posteriorly

Sagittal MRI best demonstrates outlet and aqueductal obstruction

No transependymal edema unless chronic

 

Management Options:

 

1. Endoscopic Aqueductoplasty

Removes glial scar tissue from aqueduct.

Balloon dilation performed after perforation.

Useful when aqueduct obstruction is the primary problem.

 

2. Aqueductal Stenting

Prevents restenosis of newly opened aqueduct.

Required in cases of thick gliosis or scarring.

Provides long-lasting patency.

 

3. Fourth Ventricular Shunting

Fourth ventriculoperitoneal shunt

Fourth ventriculopleural shunt

Indicated when outlet obstruction is absolute or endoscopy is not feasible.

Access via midline suboccipital route or transforaminal approach.

 

4. Endoscopic Third Ventriculostomy (ETV)

Useful only if aqueduct is obstructed but outlets are patent after surgical restoration.

Provides shunt-free diversion of CSF through the third ventricular floor.

 

5. Posterior Fossa Surgery: Rarely used and reserved for dense arachnoiditis not amenable to endoscopy or shunting.

 

Follow-up:

MRI to confirm reduction in fourth ventricular size

Neurological monitoring

Shunt surveillance if implanted

 

Q. What is X-linked hydrocephalus? 

 

X-linked hydrocephalus is a genetic, congenital, severe obstructive hydrocephalus caused by L1CAM gene mutations on the X chromosome. It is inherited in an X-linked recessive pattern, primarily affecting males.

Genetic Basis

L1CAM encodes an adhesion molecule critical for neuron migration and axon pathfinding.
 

Mutations lead to:

Defective neuronal migration

Abnormal axonal guidance

Midline structural abnormalities

Aqueductal stenosis development

 

Structural Abnormalities:

Aqueductal stenosis (congenital or acquired from gliosis)

Corpus callosum agenesis or hypoplasia

Hydrocephalus with massive ventriculomegaly

Corticospinal tract malformations

Optic nerve hypoplasia in some cases

Periventricular heterotopias may occur

 

Clinical Features:

Severe hydrocephalus detected antenatally or at birth

Macrocephaly

Adducted thumbs (classic hallmark of L1 syndrome)

Spasticity

Developmental delay

Seizures may occur

Feeding difficulties

Variable cognitive impairment depending on structural severity
 

Radiologic Features:

Marked triventricular obstructive hydrocephalus

Narrowed or absent aqueduct

Enlarged lateral and third ventricles

Thin cortical mantle if severe

Corpus callosum agenesis or hypoplasia

Midbrain structural deformities

Possible periventricular leukomalacia from chronic stretch

 

Diagnosis:

MRI brain

Genetic testing confirming L1CAM mutation

Family history analysis

 

Management:

1. Ventriculoperitoneal (VP) Shunt: Mainstay of treatment. Required early in life to prevent brain injury

 

2. ETV: Often has low success because of:

Concomitant cisternal abnormalities

Poor subarachnoid compliance

Complex aqueductal malformations

 

3. Supportive Management: Physiotherapy for spasticity, seizure management, and developmental therapies

 

Prognosis: Depends on severity of structural abnormalities, timing of intervention, and presence of associated neurological deficits.

 

Q. What are the complications of untreated hydrocephalus?

 

Neurological Complications:

Developmental delay

Cognitive impairment

Learning difficulties

Behavioral deterioration

Motor dysfunction (spasticity, hemiparesis, gait abnormalities)

Visual impairment due to chronic papilledema

Seizures

Structural Complications

Severe macrocephaly (infants)

Skull deformities

Cortical thinning and permanent loss of cortical mantle

Herniation syndromes in acute obstructive hydrocephalus

 

Systemic Complications:

Feeding difficulties

Respiratory instability

Apneic spells

End-stage: cardiorespiratory arrest

 

Untreated hydrocephalus leads to permanent brain injury and is fatal.

 

Q. How do you manage hydrocephalus?

 

A. Temporizing Measures

 

External Ventricular Drain (EVD):
Immediate CSF diversion in acute obstructive hydrocephalus (IVH, SAH, tumors, postoperative).
Used to stabilize ICP before definitive management.

 

B. Definitive Surgical Procedures

 

1. Ventriculoperitoneal (VP) Shunt

Most common definitive treatment

Indications: communicating hydrocephalus, post-infectious hydrocephalus, post-hemorrhagic hydrocephalus, failed ETV, NPH

Programmable valves used to reduce overdrainage risks

 

2. Endoscopic Third Ventriculostomy (ETV)

Now taken as First-line for obstructive hydrocephalus

Indications: congenital aqueductal stenosis, triventricular obstruction, Chiari-related hydrocephalus

Avoids shunt dependence

Success dependent on age, etiology, cisternal anatomy

 

3. ETV with Choroid Plexus Cauterization (ETVCPC)

Effective in infants

Increases ETV success by reducing CSF production

Especially beneficial in spina bifida and complex obstructive etiologies

 

4. Fourth Ventricular Shunting

Indicated for entrapped fourth ventricle

Options: VP or VPL shunts with catheter positioned into the fourth ventricle

 

5. Lumboperitoneal Shunt

Used in communicating hydrocephalus with small ventricles

Useful in NPH when ventricles are not dilated enough for ventricular catheter placement

 

C. Principles of Management

Treat underlying cause (e.g., tumor, infection)

Avoid rapid decompression after SAH/IVH to prevent rebleeding

Ensure sterile technique to prevent shunt infections

Careful valve selection and postoperative monitoring

 

D. Postoperative Follow-up

Shunt patency assessment

Serial imaging

Monitoring for overdrainage (subdural hematoma), underdrainage, infection

Neurodevelopmental assessment in children

Regular ETV success evaluation (ETVSS considerations)