X-linked hydrocephalus is one type of congenital hydrocephalus that results when circulation and absorption of the cerebrospinal fluid within the ventricles are impeded or impaired. The acronym HSAS was coined because stenosis of the aqueduct of Sylvius (a narrow passageway linking the third and fourth ventricles) was originally thought to be the causative agent of the disorder. Edwards (1961) suggested that HSAS was due to an X-linked recessive gene, with primary transmission from mother to son. Poor developmental outcome (mental retardation and severe motor impairment) for shunted HSAS infants provided further support for genetic causation. Recent linkage studies have suggested that X-linked hydrocephalus is associated with mutations in the gene at Xq28. Further studies have identified the mutated gene as a neural cell adhesion molecule LI (L1CAM) that is thought to impair neuronal cell migration, neurite elongation, and fasciculation of axons.