X-linked hydrocephalus/X-linked aqueductal stenosis A form of hydrocephalus caused by narrowing or absence of the opening from the cerebral ventricles to the outside of the brain. It is inherited from the mother by a sex-linked inheritance transmitted on the X-chromosome and only affecting males. Head enlargement is often present at birth. Abnormally bent thumbs have been described. Treatment is with use of a shunt to relieve the pressure but if already severe at birth the outcome is likely to be poor. Other abnormalities of the brain are often present. Spasticity is common and is most marked in the lower limbs. Abnormalities of the kidneys have also been described. Most affected infants are stillborn and if treatment is unsuccessful severe mental handicap and reduced life expectancy is usual.