Hydrocephalus

• Average total CSF volume: ~150 mL ^[3]
  • 20% within the ventricular system
  • 80% in the subarachnoid and cisternal spaces
• Production rate: ~450–500 mL/day (i.e. the entire CSF volume is renewed approximately 3 times per day) ^[1][3]
• This means CSF is not a stagnant pool — it is in constant flow. Any obstruction, even partial, can cause rapid accumulation.

• Active secretion primarily by the choroid plexus (~75%) located within the lateral ventricles (most), 3rd ventricle, and 4th ventricle ^[2][3]
• Remaining ~25% from other sources: ependymal wall, brain parenchyma interstitial fluid ^[3]
• Mechanism: choroid plexus epithelial cells actively transport Na⁺ and HCO₃⁻ into the ventricles, with water following osmotically. Carbonic anhydrase is critical in this process — this is why acetazolamide (a carbonic anhydrase inhibitor) can reduce CSF production.

The pathway follows a predictable anatomical route — and each bottleneck is a potential site of obstruction:

• Primary route (~80%): Arachnoid granulations (also called arachnoid villi) — these are finger-like projections of arachnoid membrane that protrude into the dural venous sinuses, predominantly the superior sagittal sinus ^[2][3]
  • Absorption is passive and pressure-dependent: CSF flows into the venous sinus when CSF pressure exceeds venous sinus pressure (i.e. down a pressure gradient)
  • This is why venous sinus thrombosis can cause communicating hydrocephalus — the pressure gradient is reversed
• Secondary route (~20%): Extracranial lymphatics — via the subarachnoid space around exiting cranial nerves (especially the olfactory nerve/cribriform plate) and perivascular (Virchow-Robin) spaces ^[3]

The skull is a rigid structure with constant volume (after fontanelle and suture closure) ^[5]:

• Brain parenchyma: ~80%
• Blood (arterial + venous): ~10%
• CSF: ~10%

An increase in any one compartment must be compensated by a decrease in others, primarily through ^[5]:

1. CSF outflow from cranium to spinal subarachnoid space (slow compensatory mechanism)
2. Venous blood outflow from cranial sinuses (rapid compensatory mechanism)

When these compensatory mechanisms are overwhelmed, intracranial pressure (ICP) rises steeply — this is the exponential portion of the pressure-volume (elastance) curve.

Exception: In infants, the sutures and fontanelles are not yet fused → the cranial cavity is expansile → ICP may not rise significantly, but the head progressively enlarges ^[1][2][5]. This is why infants present with macrocephaly rather than the classic raised ICP signs seen in adults.

Definition: Obstruction of CSF flow within the ventricular system (i.e. before CSF exits into the subarachnoid space).

Key imaging feature: Ventricles dilated PROXIMAL to the obstruction, with normal-sized 4th ventricle (if obstruction is at or proximal to the aqueduct) ^[3]. If the 4th ventricle is the site of obstruction, it may be dilated too.

Why is LP dangerous? Because ventricular CSF does NOT freely communicate with the lumbar subarachnoid space. Removing CSF below the obstruction creates a transtentorial pressure gradient → risk of downward (uncal/tonsillar) herniation → brainstem compression → death ^[1].

"LP is absolutely contraindicated (& lethal)" in non-communicating hydrocephalus ^[1]

Causes of Obstructive Hydrocephalus

Congenital ^[1][2][3]:

• Aqueductal stenosis — the most common cause of congenital hydrocephalus
  • Can be primary (developmental) or secondary (post-infectious gliosis, e.g. after congenital CMV/toxoplasmosis)
  • Some forms are X-linked (L1CAM mutation)
• Arnold-Chiari malformation (Type II) — downward herniation of the cerebellar vermis and brainstem through the foramen magnum, obstructing CSF flow at the posterior fossa outlet
  • Strongly associated with myelomeningocele ^[5]
• Dandy-Walker syndrome — cystic dilatation of the 4th ventricle with agenesis/hypoplasia of the cerebellar vermis → obstruction of 4th ventricle outflow
• Neural tube defect ^[1]
• Congenital infection ^[1]
• Congenital mass lesions ^[1]

Acquired ^[1][2][3]:

• Tumours — the most common acquired cause in adults:
  • Posterior fossa tumours (especially in children): medulloblastoma, ependymoma, pilocytic astrocytoma — compress the 4th ventricle or aqueduct
  • CPA tumours: vestibular schwannoma, meningioma
  • Brain metastasis, gliomas ^[3]
  • Craniopharyngioma / pituitary macroadenoma — can compress 3rd ventricle
  • Colloid cyst of the 3rd ventricle — classically causes intermittent obstruction at the foramen of Monro, leading to episodic severe headaches and even sudden death
  • Pineal region tumours — compress the aqueduct
• Vascular: cerebellar infarct (with swelling), ICH, intraventricular haemorrhage (IVH) ^[3]
• Infections: ventriculitis, post-infective aqueductal stenosis, brain abscess, neurocysticercosis (a trapped cyst can obstruct the 4th ventricle or aqueduct) ^[3]
• Chronic meningitis: TB, Cryptococcus — basal exudates can obstruct foramina ^[5]

Definition: Obstruction of CSF flow outside the ventricular system — i.e. ventricular CSF freely communicates with the subarachnoid space, but absorption is impaired (typically at arachnoid granulations).

Key imaging feature: ALL ventricles are dilated, including the 4th ventricle ^[3]

LP is diagnostic AND therapeutic in communicating hydrocephalus — because there is free communication between ventricles and the lumbar subarachnoid space ^[1].

"LP is diagnostic & therapeutic" in communicating hydrocephalus ^[1]

Causes of Communicating Hydrocephalus [1][2][3]

This is classified separately because of its unique pathophysiology and clinical importance.

• A surgically treatable cause of cognitive decline ^[1]
• Chronic communicating hydrocephalus with intermittently raised ICP (ICP may be normal on single measurement, but shows pathological B-waves on continuous monitoring) ^[5]
• ICP not high despite large ventricles — this apparent paradox is explained by complex pathophysiology of abnormal brain compliance ^[1]

• Complex pathophysiology of abnormal brain compliance ^[1]
• The prevailing theory: early in the disease, there is a period of raised ICP (from the inciting cause, e.g. SAH, meningitis) that expands the ventricles. Over time, a new equilibrium is reached where ICP normalises, but the ventricles remain enlarged
• According to Laplace's law: Force = Pressure × Area. Even at "normal" pressure, the enlarged ventricular surface area means the total outward force on the brain parenchyma remains elevated → ongoing damage
• The expanded ventricles compress the corona radiata fibres (white matter tracts running from cortex to internal capsule) which pass alongside the lateral ventricles ^[5]
• This compression particularly affects:
  • Periventricular motor fibres serving the lower limbs (legs are represented medially in the motor homunculus, closest to the ventricles) → gait disturbance is usually the earliest and most prominent feature
  • Frontal lobe white matter connections → subcortical/frontal-type dementia
  • Fibres from the frontal micturition centres → urge incontinence

• Idiopathic (iNPH): majority of cases — diagnosis of exclusion
• Secondary: previous SAH, meningitis (↓ absorption capacity)

Classic clinical triad ^[1]:

1. Gait disturbance ("gait apraxia")

   • Usually the earliest and most prominent feature — this helps distinguish NPH from Alzheimer's disease (where cognitive decline comes first) ^[5]
   • Described as "glue-footed" — feet seem stuck to the ground, with difficult initiation, short shuffling steps, wide-based, magnetic gait, and instability ^[5]
   • Pathophysiology: compression of periventricular motor fibres (especially those serving lower limbs, which are located medially)
   • Can mimic parkinsonian gait — but there is no true rigidity or tremor

2. Cognitive decline (subcortical/frontal dementia)

   • Psychomotor slowing, ↓ attention and concentration, impaired executive function, apathy ^[5]
   • Unlike Alzheimer's disease, episodic memory is relatively preserved early on
   • Pathophysiology: compression of frontal white matter connections

3. Urinary incontinence (urge-type)

   • Usually the last component of the triad to appear
   • Pathophysiology: disruption of frontal inhibitory pathways to the pontine micturition centre

"Need to distinguish from other causes of dementia such as AD, which does not respond to shunting" ^[1]

As discussed above under NPH:

• Dementia (subcortical/frontal type) ^[5]
• Gait apraxia ^[5]
• Incontinence (urge-type) ^[5]
• No other S/S of ↑ICP, eg. headache, N/V ^[5]

1. Direct compression: Expanded ventricles compress the surrounding brain parenchyma, especially the periventricular white matter (corona radiata)
2. Periventricular ischaemia: Compression of periventricular capillaries → local ischaemia → periventricular leukomalacia
3. Transependymal CSF absorption: When ICP rises, CSF is forced through the ependymal lining into the periventricular extracellular space → interstitial oedema (periventricular lucencies on CT / hyperintensities on T2-FLAIR MRI) ^[3][5]
4. ↓ Cerebral perfusion: ↑ ICP → ↓ CPP (CPP = MAP − ICP) → global cerebral ischaemia ^[5]
5. Brain herniation: Pressure gradients across dural compartments → herniation syndromes → brainstem compression ^[5]

CPP = MAP − ICP (if ICP > JVP) CPP = MAP − JVP (if JVP > ICP)

This is the driving pressure for cerebral blood flow:

CBF = CPP / CVR = (MAP − ICP) / CVR ^[5]

Where CVR = cerebrovascular resistance.

Cerebral autoregulation maintains constant CBF across a range of MAP (roughly 50–150 mmHg in normotensive individuals). When ICP rises sufficiently to reduce CPP below the lower limit of autoregulation, CBF drops precipitously → cerebral ischaemia.

In chronic hypertension, the autoregulatory curve is shifted rightward — meaning these patients are vulnerable to ischaemia at higher MAPs than normotensive individuals ^[5].

When a patient presents with headache (supine > erect; worse early a.m.), vomiting, blurring of vision, diplopia (CN VI), deterioration in consciousness, and papilloedema ^[1], you must consider all causes of raised ICP — not just hydrocephalus.

Key point from lecture slides: The common causes of raised ICP listed are: space-occupying mass lesion (haematoma, tumour, abscess), hydrocephalus (communicating/non-communicating), brain swelling (focal/diffuse), hyperaemia, and venous congestion ^[1]

This is perhaps the higher-yield differential for clinical exams. NPH must be distinguished from other causes of dementia because it responds well to CSF diversion ^[1][5].

When an infant presents with a progressively enlarging head, consider:

By temporal profile (this helps narrow the differential rapidly):

For acute hydrocephalus, the diagnosis is essentially clinical + imaging:

Evans' index = maximum width of the frontal horns / maximum biparietal diameter of the inner skull, measured on the same axial slice.

• Normal: < 0.3
• Hydrocephalus: Evans' index > 0.3 indicates ventriculomegaly

Why it works: the frontal horns are among the first parts of the lateral ventricles to dilate in hydrocephalus. Comparing them to the biparietal diameter normalises for head size.

A highly specific finding for iNPH on imaging:

• Ventriculomegaly (Evans' index > 0.3) WITH
• Tight high-convexity sulci (effaced at the vertex/medial surface) AND
• Widened Sylvian fissures (laterally)

Why this pattern? In NPH, the enlarged ventricles push brain tissue upward and outward → the superior convexity sulci are compressed against the skull, while the Sylvian fissures (which are more compliant) remain dilated. This "top-tight, bottom-wide" pattern is characteristic and distinguishes NPH from atrophy (where sulci are widened everywhere).

Why CT first? It is fast (< 5 minutes), widely available (even at 3am in any Emergency Department), and excellent at detecting acute hydrocephalus, haemorrhage, and mass lesions. In the acute setting, a non-contrast CT brain is the single most important investigation.

Key findings in hydrocephalus on CT ^[3][5][6]:

Distinguishing from cerebral atrophy (hydrocephalus ex vacuo) ^[3][6]:

• In atrophy: ventriculomegaly is proportional to sulcal/cisternal widening ^[6]
• In true hydrocephalus: ventriculomegaly > sulcal/cisternal widening (disproportionate) ^[6]

Why MRI? Superior soft tissue contrast, multiplanar imaging, and specific sequences provide far more detail than CT. MRI is the imaging modality of choice for:

• Identifying the site and cause of obstruction (especially posterior fossa lesions, aqueductal stenosis, tumours)
• Assessing periventricular oedema (best on T2-weighted FLAIR)
• CSF flow studies (phase-contrast MRI)
• Evaluating NPH (DESH sign, aqueductal flow void)
• Ruling out differentials (e.g., CVST on MR venography)

Key MRI sequences and findings:

LP measures CSF pressure and allows trial drainage — BUT BE CAREFUL!! ^[1]

This is simultaneously one of the most useful and most dangerous investigations in hydrocephalus. The safety depends entirely on whether the hydrocephalus is communicating or non-communicating:

"No LP if raised ICP unless absolutely sure communicating hydrocephalus" ^[1]

LP findings in hydrocephalus (when safe to perform):

Methods ^[1][3]:

• External Ventricular Drain (EVD): gold standard for ICP monitoring. A catheter is placed directly into the lateral ventricle through a burr hole. Allows both continuous ICP measurement and therapeutic CSF drainage
• Intraparenchymal pressure monitor: fibreoptic catheter placed in brain parenchyma. Measures ICP but cannot drain CSF
• LP (in communicating cases only): intermittent measurement of opening pressure

When to use ICP monitoring ^[1]:

• ICP monitoring when GCS unreliable or evolving condition ^[1]
• When clinical examination cannot reliably track neurological status (e.g., intubated, sedated patients)
• Definitely abnormal > 20 cmH₂O ^[1] — this is the threshold to escalate treatment
• Used to guide titration of ICP-lowering therapy

Why USS in infants? The open anterior fontanelle provides an acoustic window that allows direct visualisation of the ventricles without radiation or sedation.

Key findings: dilated lateral ventricles, dilated 3rd ventricle, periventricular echogenicity (oedema)

When? A patient with a known VP/VA shunt presenting with new symptoms of hydrocephalus (suggesting shunt malfunction).

What it involves: plain X-rays of the entire course of the shunt — skull XR, C-spine XR, CXR, AXR ^[3]

Key findings ^[3]:

• Shunt disconnection/fracture: visible break in the catheter line
• Catheter migration/dislodgement: tip not in expected position
• Kinking: catheter looped or angulated

This is a quick first-line investigation to identify mechanical shunt failure before proceeding to CT brain for ventricular assessment.

Why? To detect papilloedema — optic disc swelling caused by raised ICP transmitted along the optic nerve sheath ^[6].

• Acute papilloedema: swollen disc with blurred margins, dilated superficial capillaries, absent spontaneous venous pulsation
• Chronic papilloedema: optic atrophy, constricted visual fields → permanent vision loss if untreated

Key points ^[6]:

• Papilloedema is a late sign ^[1] — its absence does NOT exclude raised ICP
• In infants, papilloedema may not develop (expandable skull decompresses ICP)
• Always perform fundoscopy before LP to help assess for raised ICP

• Age > 60, at least 2 of: gait disturbance, cognitive decline, urinary incontinence
• Gait assessment: timed up-and-go (TUG) test, 10-metre walk test — quantify for pre/post comparison
• Cognitive assessment: MMSE or MoCA (look for subcortical pattern: slow processing, poor attention, impaired executive function)
• Rule out other causes: check B12, TFT, RFT, Ca, glucose (reversible causes of dementia) ^[11]

• MRI preferred over CT ^[5]:
  • Ventricular enlargement >> sulcal effacement ^[5]
  • Evans' index > 0.3
  • DESH sign (tight high-convexity, wide Sylvian fissure)
  • Periventricular radiolucency especially on T2 FLAIR sequence ^[5]
  • Aqueductal flow void present (excludes aqueductal stenosis)
  • Phase-contrast cine MRI: hyperdynamic aqueductal CSF flow (aqueductal stroke volume > 42 μL)

• Trial drainage: external lumbar drainage of CSF (40–100 mL) to test for clinical improvement ^[5]
• Practical protocol: LP, remove 30–50 mL CSF, measure opening pressure
• Assess gait (TUG, 10-metre walk) and cognition at baseline, then at 1 hour, 24 hours, and 72 hours post-LP
• Positive test: ≥ 20% improvement in gait speed or ≥ 3-point improvement in MMSE

• Continuous lumbar CSF drainage via an indwelling lumbar catheter over 72 hours, draining ~150–200 mL total
• Higher sensitivity (~80–90%) than single tap test
• Requires inpatient monitoring (risk of overdrainage, infection, nerve root irritation)

• Phase-contrast cine MRI quantifying aqueductal stroke volume
• Resistance to CSF outflow (Rout) measured via infusion test (research-level; infrequently used in HK clinical practice)

"ABC first" — "ALWAYS resuscitate first" ^[1]

Before doing anything about the ICP, you must ensure the patient is haemodynamically stable, ventilating adequately, and oxygenating well. This is because:

• Hypotension → ↓ MAP → ↓ CPP (CPP = MAP − ICP) → worsening cerebral ischaemia
• Hypoxia → direct neuronal injury + cerebral vasodilation (to compensate) → ↑ cerebral blood volume → ↑ ICP
• Hypercarbia → cerebral vasodilation → ↑ blood volume → ↑ ICP

Practical steps:

• Airway: Intubate if GCS ≤ 8 (cannot protect airway) ^[1]
• Breathing: Target SpO₂ > 97%, PaO₂ > 9 kPa, PaCO₂ 4.5–5 kPa ^[3]
• Circulation: Maintain adequate MAP for CPP (~60–80 mmHg) ^[1]. Avoid hypotension
• Head elevation: 30° — promotes venous drainage from the cranium (reduces intracranial venous blood volume → ↓ ICP) ^[3]
• Loosen neck constraints — tight cervical collars or lines can compress the jugular veins → impede venous outflow → ↑ ICP ^[3]

After any intervention for hydrocephalus:

• Regular follow-up CT to assess adequacy of shunting/ventricular size ^[5]
• Clinical assessment of symptoms at each visit
• Shunt setting check after any MRI (for programmable shunts) ^[1][5]
• Vigilance for shunt complications (see Complications section to follow)
• Serial head circumference in infants

This is the most feared and immediately life-threatening complication of untreated or rapidly progressive hydrocephalus.

When ICP rises, pressure gradients develop across the dural compartments (falx cerebri, tentorium cerebelli). Brain tissue is pushed from high-pressure compartments to low-pressure compartments through rigid openings — this is herniation.

Why is tonsillar herniation so lethal? The medulla contains the cardiovascular and respiratory centres. Compression of these nuclei causes immediate cardiorespiratory arrest — there is essentially no warning and no time to intervene once it occurs.

Untreated hydrocephalus causes progressive, ultimately irreversible brain injury through several mechanisms:

• Periventricular white matter ischaemia: Expanded ventricles compress periventricular capillaries → chronic ischaemia → demyelination and gliosis
• Cortical compression: Especially in infants where the skull expands but the brain is thinned against the inner table
• Developmental delay and intellectual disability (in children): Chronic compression during critical periods of brain development → mental retardation, cerebral palsy ^[5][7]
• Visual loss: Chronic papilloedema → optic atrophy → permanent blindness ^[6]. Also, direct compression of the optic chiasm by a dilated 3rd ventricle ^[7]

Occur in ~10% of patients with hydrocephalus. Mechanisms include:

• Cortical irritation from periventricular oedema spreading to the cortex
• Cortical ischaemia from impaired perfusion
• Co-existing pathology (e.g., meningitis, tumour, haemorrhage)

Compression of the hypothalamus by a dilated 3rd ventricle can disrupt:

• Growth hormone axis → growth disturbance ^[2]
• GnRH pulsatility → accelerated pubertal development ^[2]
• ADH regulation → SIADH or diabetes insipidus

• Infection (1° or 2°)
• Blockage → hydrocephalus
• Dislodgement/Fracture → hydrocephalus
• Over-shunting → CSDH
• Abdominal pseudocyst
• Slit ventricle syndrome
• Nephritis (VA)
• Bowel perforation (VP) …

Let's break each one down from first principles.

"Infection (1° or 2°)" ^[1]

The lecture slides highlight a common scenario: "Fever + abdominal pain → Shunt infection causing peritonitis? Peritonitis causing shunt infection? → Externalise shunt + antibiotics" ^[1]

Why is infection so feared? Because:

1. Ventriculitis (infection of the ventricular lining) is extremely difficult to treat — the blood-brain barrier limits antibiotic penetration
2. Biofilms on shunt hardware are inherently resistant to antibiotics (bacteria in biofilms are ~1000× more resistant than planktonic bacteria)
3. Repeated infections and revisions damage the brain and reduce future treatment options

"Blockage → hydrocephalus" ^[1]

Important clinical pearl on testing shunt function ^[5]:

• Valve pumping is NOT accurate, not sensitive, not specific ^[5]
• Do NOT pump (will over-shunt CSF) ^[5]
• Can press once to differentiate between distal and proximal obstruction ^[5]: The shunt reservoir sits between two one-way valves. If you press the reservoir and it does not refill → proximal blockage (nothing coming from the ventricle). If you press and it does not empty (remains full) → distal blockage (nothing draining downstream)
• Shuntogram (inject radiopaque contrast into the shunt) can confirm the site of blockage ^[5]

"Dislodgement/Fracture → hydrocephalus" ^[1]

"Over-shunting → CSDH" ^[1]

This is one of the most important and commonly tested complications.

Why does this happen? From first principles: CSF provides buoyancy to the brain (the brain weighs ~1500g in air but only ~50g when floating in CSF). Over-shunting removes this buoyancy → the brain drops downward → stretches the bridging veins that tether it to the skull → these veins have thin walls and no valves → they tear, creating a slow subdural bleed.

"Raised ICP symptom +/- focal deficit → CSDH? (e.g., elderly on aspirin)" ^[1] — This is a common clinical scenario from the lecture slides. Elderly patients on antiplatelet/anticoagulant therapy are at particularly high risk because their coagulation is impaired, amplifying any bridging vein tear.

"Abdominal pseudocyst" ^[1]

"Slit ventricle syndrome" ^[1]

"Nephritis (VA)" ^[1]

"Bowel perforation (VP)" ^[1]

• Inguinal hernia: Increased intraperitoneal pressure from CSF fluid within the peritoneal cavity → weakens the inguinal canal → hernia. More common in infants (patent processus vaginalis)
• CSF ascites: Excessive CSF drainage overwhelms peritoneal absorptive capacity
• Tumour seeding: In patients with CNS malignancies (medulloblastoma, ependymoma, choroid plexus carcinoma), the shunt can act as a conduit for tumour cells to metastasise from the ventricle to the peritoneum ^[3] — so-called "shunt metastasis." This is rare but important in paediatric neuro-oncology