Hydrocephalus is a condition in which the ventricles in the brain are enlarged, also called ventriculomegaly, due to an increased amount of cerebrospinal fluid. Hydrocephalus can occur when functional brain tissue atrophies, when the amount of fluid produced by the ventricles surpasses the absorption rate, or when the flow of fluid is blocked in various parts of the brain.
Approximately one-third of all cases of hydrocephalus are associated with other intracranial (inside the skull) anomalies, and approximately two-thirds of cases are associated with extracranial (outside the skull) anomalies. Hydrocephalus is associated with a range of anomalies such as agenesis of the corpus callosum, Chiari malformation type II, thanatophoric dysplasia, and chromosomal abnormalities like triploidy, trisomy 13, and trisomy 18. Isolated hydrocephalus does occur, predominately in males, and is often linked to a mutation of the L1CAM gene.
Ventriculomegaly and hydrocephalus have an approximate incidence rate somewhere between .5 and 1.5 per 1,000 pregnancies, occurring more frequently in males. Long-term survival is possible in cases of isolated hydrocephalus, but the majority of cases are associated with other diagnoses. The recurrence risk for hydrocephalus depends on the underlying cause, so further genetic testing is indicated following a diagnosis. The recurrence risk for isolated hydrocephalus that is not associated with a mutation of the L1CAM gene is approximately 4%. [32]