Hydranencephaly is a rare condition characterized by the absence of the two sides of the cerebrum, or the largest portion of the brain. The hemispheres of the cerebrum are replaced with a membrane sac, containing cerebrospinal fluid. The structure of the skull is typically intact and normal. The cerebellum (the back of the brain) and brainstem are usually normal as well. Hydranencephaly is a distinct diagnosis from extreme hydrocephalus.
Hydranencephaly is typically an isolated diagnosis; however, in some cases, hydranencephaly has been associated with polyhydramnios, renal anomalies, and trisomy 13. Hydranencephaly is typically diagnosed in the first trimester through ultrasound; however, magnetic resonance imaging (MRI) is the most definitive diagnostic test.
The incidence rate of hydranencephaly ranges from 1 in 4,000 and 1 in 10,000 births. Hydranencephaly is associated with extremely poor outcomes, with nearly half of all babies dying within the first month of life. Fewer than 15% of babies diagnosed with hydranencephaly survive the first year. The recurrence risk for hydranencephaly is non-existent.