DESCRIPTION

• Active distension of the ventricular system resulting from inadequate passage of cerebral spinal fluid (CSF) from its point of production to point of absorption
• CSF compartment progressively enlarged at the expense of brain and/or spinal tissue

EPIDEMIOLOGY

Pediatric prevalence estimated at 1:1,000 children

ETIOLOGY

Hydrocephalus is a heterogeneous condition with varying classification systems, organized here as “congenital,” present at birth, or “acquired,” occurring after birth, although some conditions may fit into both groups.

• Congenital hydrocephalus
  • Aqueductal stenosis—a common cause of congenital hydrocephalus (~10% cases); stenosis of aqueduct of Sylvius (passageway between 3rd and 4th ventricles)
  • Neural tube defect–associated—that is, myelomeningoceles and Chiari II malformation (cerebellum and brainstem extend into foramen magnum); also a common cause
  • Arachnoid cysts—most often located in posterior fossa, 3rd ventricle/suprasellar
  • Dandy-Walker malformation—4th ventricle enlarged/cystic as either outlet obstruction or failure of cerebellar development
  • Craniosynostosis/skeletal dysplasias—that is, achondroplasia, fibroblast growth factor (FGF) mutations
  • Other associated syndromes such as trisomies 13, 18, 21, mucopolysaccharidoses (type II [Hunter syndrome], type VI [Maroteaux-Lamy syndrome]), Apert and Crouzon syndromes, NF1
• Acquired
  • Intraventricular hemorrhage (IVH)—most in setting of prematurity; posthemorrhagic hydrocephalus develops secondary to meningeal adhesions, clots, and granular ependymitis in 35% of all neonates surviving IVH; worse prognosis when associated with moderate/severe ventriculomegaly, echolucencies, or white matter injury
  • Infection (meningitis) can lead to leptomeningeal adhesions and granulations that block reabsorption of CSF (high-risk infections include enterovirus, CMV, lymphocytic choriomeningitis virus [LCMV], toxoplasmosis; may also occur intrauterine).
  • Brain tumors—in children, most often posterior fossa tumors compressing 4th ventricle
  • Head injury—blood from ruptured vessels may cause inflammation, scarring of meninges, or clot.

RISK FACTORS

• IVH
• Maternal medications (isotretinoin)
• Megencephaly
• VACTERL (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies and limb anomalies) association

PATHOPHYSIOLOGY

• CSF originates from choroid plexus and interstitial fluid of brain/spinal cord and travels via both unidirectional bulk flow and pulsatile flow from cardiac cycle; starts in lateral ventricles → foramina of Monro → 3rd ventricle → aqueduct of Sylvius → 4th ventricle → foramina of Luschka and Magendie → subarachnoid space → into either arachnoid villi to venous system or to lymphatic sinuses
• Hydrocephalus results from obstruction to flow, impaired reabsorption, or rarely overproduction of CSF.
• Obstructive hydrocephalus: blockage of flow or impaired absorption, can necessitate urgent evaluation
  • Acute (hours) (i.e., direct ventricular obstruction) → surgical emergency
  • Subacute/progressive (weeks-months) → increased ventricle size/interstitial edema = if not treated can cause brain herniation/arrest
  • Chronic (years) (i.e., late-onset aqueductal stenosis) → ventricles and skull slowly increase in size = slow loss of brain volume
• Communicating hydrocephalus—no morphologic obstruction to CSF
  • May be secondary to arachnoid pathology or impaired elasticity of spinal dura