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- Accumulation of CSF in the ventricles and subarachnoid spaces, leading to their enlargement
- Overall head size may enlarge in response, depending on age and cause.
- Normal pathway of CSF: choroid plexus and interstitial fluid (sources), lateral ventricles, foramina of Monro, 3rd ventricle, aqueduct of Sylvius, 4th ventricle, foramina of Luschka and Magendie, subarachnoid space, arachnoid villi, and venous circulation
- Hydrocephalus results from obstruction to CSF flow, impaired reabsorption, or overproduction of CSF.
- Noncommunicating (obstructive) hydrocephalus results from obstruction within the ventricular system.
- Communicating hydrocephalus usually results from impaired CSF reabsorption or (rarely) overproduction (e.g., due to a choroid plexus papilloma).
- The noncommunicating/communicating distinction has no prognostic significance but has implications for etiology and choice of therapeutic intervention.
- Intraventricular hemorrhage is most commonly due to prematurity but may also occur with trauma. It results in impaired CSF absorption due to meningeal adhesions, granular ependymitis, and clots. Posthemorrhagic hydrocephalus (PHH) occurs in 35% of all neonates surviving intraventricular hemorrhage; its incidence increases with increasing severity of hemorrhage.
- Tumors or cysts near the foramina or the aqueduct or within the ventricular system
- Infection (meningitis, intrauterine infection) can lead to leptomeningeal adhesions and granulations that block reabsorption of CSF.
- Chiari malformation, type II (associated with myelomeningocele, brain migrational disorders, small posterior fossa, inferior displacement of medulla and cerebellar vermis, kinking of the brainstem, aqueductal stenosis, beaking of the tectum)
- Dandy-Walker malformation (absence of cerebellar vermis, small cerebellar hemispheres, enlarged posterior fossa, often with cystic 4th ventricle)
- X-linked and autosomal dominant hydrocephalus; the former is often associated with aqueductal stenosis and mutations in L1CAM on Xq28.
- Sporadic primary aqueductal stenosis
- Dysmorphic syndromes (e.g., Apert syndrome, Cockayne syndrome, Crouzon syndrome, Pfeiffer syndrome, trisomy 13, trisomy 18, trisomy 21, triploidy)
- Alexander disease
- Mucopolysaccharidoses (e.g., type II [Hunter], type VI [Maroteaux-Lamy])
- Migrational disorders/congenital muscular dystrophies (e.g., Miller-Dieker, muscle-eye-brain disease, Fukuyama congenital muscular dystrophy, Walker-Warburg syndrome)
- Neurocutaneous syndromes (e.g., neurofibromatosis type 1, rare)