What causes hydrocephaly to enlarge the brain and head?

Hydrocephalus does not enlarge the brain tissue itself; rather, it causes the head to enlarge due to the accumulation of cerebrospinal fluid (CSF) within the brain's ventricles.21

Normally, CSF circulates and drains into the circulatory system. In hydrocephalus, this drainage is blocked or the fluid is overproduced, leading to a buildup of pressure. In infants and young children, this internal pressure forces the unfused skull bones to bulge outward, resulting in an abnormally large head.2

This condition is often caused by congenital malformations that obstruct the flow of CSF, such as those affecting the aqueduct of Sylvius. While the fluid accumulation causes the skull to expand, the resulting pressure also compresses the brain, which can lead to serious complications like convulsions and intellectual disability. Treatment typically involves the surgical insertion of a shunt to drain the excess fluid and relieve pressure.21