• 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


Pediatric prevalence estimated at 1:1,000 children

Risk Factors

  • Intraventricular hemorrhage (IVH)
  • Maternal medications (isotretinoin)
  • Megalencephaly
  • VACTERL association


X-linked mutation in L1CAM gene most common genetic cause, accounting for 10% males with idiopathic isolated hydrocephalus, may also be associated with congenital muscular dystrophies/neuronal migrational defects (Walker-Warburg and muscle-eye-brain phenotypes)


  • 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 to 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, less time sensitive
    • May be secondary to arachnoid pathology or impaired elasticity of spinal dura


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—most 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)
    • 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, FGF mutations
    • Other associated syndromes—that is, trisomies 13, 18, 21, mucopolysaccharidoses (type II [Hunter], type VI [Maroteaux-Lamy]), Apert and Crouzon syndromes, NF1
  • Acquired
    • 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.

There's more to see -- the rest of this topic is available only to subscribers.