Hydranencephaly is a rare condition in which the brain's cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid. An infant's head size and spontaneous reflexes such as sucking, swallowing, crying, and moving the arms and legs may all seem to be okay. However, after a few weeks the infant usually becomes irritable and has increased muscle tone. After a few months of life, seizures and hydrocephalus (excessive accumulation of cerebrospinal fluid in the brain) may develop. Other symptoms may include:

• Visual impairment
• Lack of growth
• Deafness

• Blindness
• Spastic quadriparesis (paralysis)
• Intellectual deficits

Hydranencephaly is considered to be an extreme form of porencephaly (a rare disorder characterized by a cyst or cavity in the cerebral hemispheres) and may be caused by vascular infections or traumatic disorders after the 12th week of pregnancy.

Diagnosis may be delayed for several months because early behavior appears to be relatively typical. Some infants may have additional abnormalities at birth including seizures, myoclonus (spasm or twitching of a muscle or group of muscles), and respiratory problems.