Hydrocephalus

NORMAL CSF CIRCULATION & AQUEDUCTAL STENOSIS

A. Normal CSF Circulation in Newborns

In newborns, cerebrospinal fluid (CSF) dynamics differ slightly from adults due to open fontanelles and immature absorption pathways.

1. Production

• Site: Primarily by the Choroid Plexus (tufts of capillaries covered by specialized ependymal cells) located in the Lateral, 3rd, and 4th ventricles.
• Rate: Approx 20–25 ml/kg/day (roughly 0.35 ml/min).
• Total Volume: Newborn CSF volume is small (~50 ml) compared to adults (~150 ml).

2. Circulation Pathway (The "Third Circulation")
Flows via bulk flow along a pressure gradient (pulsatile flow driven by arterial, respiratory, and venous phases).

flowchart TD
    A[Lateral Ventricles] --> B[Foramen of Monro
Interventricular Foramen]
    B --> C[Third Ventricle]
    C --> D[Aqueduct of Sylvius
Midbrain]
    D --> E[Fourth Ventricle]
    E --> F[Foramina of Luschka
Lateral]
    E --> G[Foramen of Magendie
Medial]
    F --> H[Subarachnoid Space
Cisterna Magna & Basal Cisterns]
    G --> H
    H --> I[Circulates over cerebral convexities
and spinal cord]

3. Absorption

• Primary Mechanism: Absorption into the Venous Sinuses (primarily Superior Sagittal Sinus) via Arachnoid Granulations (villi).
• Newborn Specifics: Arachnoid granulations are rudimentary at birth. Significant absorption occurs via:
  • Extracellular spaces of nerve sheaths.
  • Cerebral capillaries.
  • Lymphatic channels (extracranial).

B. Changes in Aqueductal Stenosis

Aqueductal stenosis (AS) is the narrowing of the Aqueduct of Sylvius, the critical bottleneck between the 3rd and 4th ventricles.

1. Anatomical Disruption

• The aqueduct is the narrowest point of the ventricular system (approx 1 mm diameter).
• Stenosis creates a mechanical block preventing CSF exit from the 3rd ventricle.

2. Hydrodynamic Changes (Upstream vs. Downstream)

• Upstream Dilation:
  • Lateral Ventricles: Massive dilation due to accumulating CSF.
  • 3rd Ventricle: Becomes ballooned/dilated.
  • Result: "Bi-ventricular" or "Tri-ventricular" hydrocephalus (depending on nomenclature, strictly it affects Lat + 3rd).
• Downstream Spared:
  • 4th Ventricle: Remains normal or small size (isolated from the pressure).
  • Posterior Fossa: Normal size (unlike Dandy-Walker where it is large).

3. Parenchymal Effects

• Cortical Mantle: Thins rapidly due to expansion of lateral ventricles against the skull.
• Corpus Callosum: Stretched and thinned.
• Tectal Plate Compression: Dilated suprapineal recess pushes on the midbrain tectum $\to$ Parinaud’s phenomenon (Sunsetting eyes - inability to gaze upward).

ETIOLOGY & PATHOPHYSIOLOGY OF HYDROCEPHALUS

A. Etiology of Hydrocephalus

Classified broadly into Non-Communicating (Obstructive) and Communicating (Non-obstructive).

1. Non-Communicating (Obstructive)

Blockage within the ventricular system prevents CSF from reaching the Subarachnoid Space.

• Congenital:

  • Aqueductal Stenosis:
    • X-linked Hydrocephalus (L1CAM mutation): Stenosis + adducted thumbs.
    • Congenital infection (TORCH): Toxoplasmosis/CMV causing ependymitis/gliosis.
  • Dandy-Walker Malformation: Cystic dilation of 4th ventricle + Vermian agenesis + Large posterior fossa.
  • Chiari II Malformation: Associated with Myelomeningocele; displacement of cerebellar tonsils blocks CSF exit at foramen magnum/4th ventricle.
  • Neural Tube Defects: Encephalocele.

• Acquired:

  • Tumors: Supratentorial (compressing 3rd vent) or Infratentorial (Medulloblastoma, Ependymoma, Astrocytoma compressing 4th vent).
  • Intraventricular Hemorrhage (IVH): Acute clot obstructing the aqueduct.
  • Ventriculitis: Pyogenic pus organizing and blocking foramina.

2. Communicating (Non-Obstructive)

CSF exits ventricles but absorption is blocked at arachnoid granulations or flow blocked in SAS.

• Post-Infectious: Tuberculous Meningitis (basal arachnoiditis), bacterial meningitis (fibrosis of granulations).
• Post-Hemorrhagic: IVH (Preterms), Subarachnoid Hemorrhage. Blood products clog arachnoid villi and cause inflammatory fibrosis.
• Hyper-secretory (Rare): Choroid Plexus Papilloma (Production exceeds maximal absorption capacity).

B. Pathophysiology of Hydrocephalus

1. Fundamental Imbalance
Hydrocephalus results from a disturbance in CSF dynamics where:

(Except in rare hydrocephalus ex-vacuo which is compensatory to brain atrophy).

2. Phase I: Acute Compensation

• CSF Displacement: CSF is shunted to the spinal subarachnoid space (if communicating).
• Venous Compression: Intracranial pressure (ICP) rises $\to$ venous sinuses compress $\to$ attempt to increase absorption.
• Open Sutures: In infants, cranial sutures split, and fontanelles bulge to increase intracranial volume and buffer pressure.

3. Phase II: Parenchymal Injury (The Vicious Cycle)

• Transependymal Flow: As ventricular pressure rises, the ependymal lining stretches and tears. CSF is forced into the periventricular white matter (Interstial Edema).
• White Matter Damage:
  • Edema compresses periventricular capillaries.
  • Result: Ischemia and oligemia of white matter.
  • Axonal stretching and myelin disruption.
• Grey Matter Spared: Cortical grey matter is relatively preserved until late stages.

4. Law of Laplace

(Wall Tension = Pressure $\times$ Radius)

• As ventricles dilate (Radius increases), the tension on the ventricular wall increases even if pressure normalizes slightly.
• This tension maintains the dilation (Normal Pressure Hydrocephalus mechanism).

5. Clinical Pathophysiology Correlations

• Macewen Sign: Cracked pot sound on percussion due to separated sutures.
• Sunsetting Sign: Pressure on the midbrain tectum (superior colliculus).
• Pyramidal Signs: Stretching of corticospinal tracts (fibers to legs run closest to ventricles) $\to$ Spasticity/Brisk reflexes, primarily in lower limbs.