Hydrocephalus due to congenital stenosis of aqueduct of sylvius (HSAS) is a form of L1 syndrome, which is an inherited disorder that primarily affects the nervous system. Males with HSAS are typically born with severe hydrocephalus and adducted thumbs (bent towards the palm). Other sign and symptoms of the condition include severe intellectual disability and spasticity. HSAS, like all forms of L1 syndrome, is caused by changes in the L1CAM gene and is inherited in an X-linked recessive manner.