Hydranencephaly is a rare birth defect that affects brain development. A baby with the condition is missing the cerebral hemispheres. Hydranencephaly causes an enlarged head and other serious symptoms. Some families consider therapeutic abortion because hydranencephaly is usually fatal within the first year of life.
Hydranencephaly is a rare birth abnormality that affects the central nervous system. A baby with the condition is missing certain portions of their brain called the cerebral hemispheres. The cerebral hemispheres are two halves of the cerebrum, the front and largest part of their brain.
Instead of cerebral hemispheres, there are sacs filled with cerebrospinal fluid (CSF). CSF is fluid that cushions and protects their brain and spinal cord.
A baby with hydranencephaly eventually develops an enlarged head and other serious symptoms. The condition is usually fatal before a baby is born or shortly afterward.
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Hydranencephaly and hydrocephalus both involve CSF and cause an enlarged head.
Hydrocephalus is fluid buildup in your brain, which leads to pressure inside your skull that may cause brain damage. The condition can be caused by a birth abnormality, or it can be a symptom of injury or illness. Hydrocephalus can even be a symptom of hydranencephaly.
But with hydranencephaly, your brain itself forms abnormally during fetal development.
It’s important to differentiate between the two conditions because treatment and outlook are different. Many babies with hydrocephalus can improve with proper diagnosis and treatment, but babies with hydranencephaly usually don’t.
Holoprosencephaly involves a different brain malformation. With this condition, a baby's forebrain fails to divide into the two cerebral hemispheres. With hydranencephaly, the hemispheres are completely missing.
Depending on the severity of holoprosencephaly, babies born with the condition can survive.
Hydranencephaly is rare. Scientists estimate that it occurs in 1 in 10,000 to 1 in 5,000 pregnancies.
Scientists aren’t sure what causes hydranencephaly. They believe it might be inherited, but they don’t understand how. Some cases have been linked to exposure to toxins during pregnancy.
A baby born with hydranencephaly might appear to be normal at first. Symptoms start to appear in the first few weeks and months of life, such as:
Hydranencephaly may be diagnosed before a baby is born during prenatal ultrasound. The ultrasound uses sound waves to create an image of a developing fetus while still in the womb (uterus). The test may show that their cerebral hemispheres are missing.
If a prenatal ultrasound shows possible hydranencephaly, a healthcare provider might order an MRI for more detailed images.
Sometimes hydranencephaly isn’t detected during pregnancy and a baby is born with it. Diagnosis might take weeks or months because a baby with the condition often looks and acts normally at first.
A healthcare provider can make the diagnosis based on the baby’s symptoms and brain MRI. An MRI can show the difference between hydranencephaly and other conditions such as hydrocephalus and holoprosencephaly.
Other tests might include:
When prenatal testing confirms hydranencephaly, some families choose therapeutic abortion. This ends the pregnancy before birth.
When a baby is born with hydranencephaly, there’s no cure. Treatment aims to ease symptoms and make the baby as comfortable as possible.
Surgery to implant a shunt (tube) can help drain fluid away from the brain and reduce pressure on the brain. It’s a useful treatment for hydrocephalus but has limited use for hydranencephaly. Another possible treatment is endoscopic choroid plexus coagulation (ECPC) to treat a rapidly enlarging head.
A healthcare provider also might recommend:
Scientists don’t understand what causes hydranencephaly, so they don’t know how to prevent it. Some cases have been linked to toxins during pregnancy, such as cocaine, smoking and sodium valproate (an antiseizure medication). Pregnant women should avoid exposure to toxins.
The outlook for children with hydranencephaly is poor. Most babies die before they are born or before they turn one year old. Rarely, a child with hydranencephaly can survive for years with intensive supportive care.
Having a child diagnosed with hydranencephaly is extremely difficult. Families need education to understand how severe the disorder is. They also can benefit from counseling and support groups to connect with other families dealing with similar conditions.
A note from Cleveland Clinic
Hydranencephaly is a rare birth defect that affects brain development. It’s diagnosed before birth or shortly afterward. If your baby has hydranencephaly, your healthcare provider will offer support, education and counseling for you and your family.
Last reviewed by a Cleveland Clinic medical professional on 04/18/2022.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy