Hydranencephaly or hydroanencephaly is a disease that is caused by an interruption of the formation of the central nervous system either in the womb or shortly after birth. According to the Swiss Society of Neonatology, the disease is characterized by the complete absence of the cerebral cortex. Most babies born with this disease die within the first year; however, at least one individual living up to 33 years has been reported.
Definition & Facts
Hydranencephaly occurs when large parts of the brain, including up to both cerebral hemispheres, are missing at birth or shortly thereafter. Only membranous sacs filled with cerebrospinal fluid remain in the cranial cavity.
The fine membrane that normally cover the brain or the meninges, basal ganglia, pons, and cerebellum remain on the brainstem. These structures are all involved in coordinating and controlling muscle activity. Occasionally the falx cerebri, or the thin membrane between the brain's hemispheres, also remains.
Less than one in 10,000 infants worldwide have this disease; only one in 250,000 births in the US are affected. One study showed that a higher percentage of babies born to mothers under the age of 18 had hydranencephaly than a control group of mothers 20 years of age and older.
Hydranencephaly differs from hydrocephalus in that it is not reversible. Hydrocephalus is colloquially known as 'water on the brain' because it is a build up of fluid inside the brain. Unlike hydranencephaly, it can be a congenital disorder or acquired and can be treated.
Symptoms & Complaints
The baby's limbs will become more muscular than appropriate for their age. This is known as hypertonia and it is caused by seizures. The cranium may begin to swell and visual impairment and respiratory problems may become apparent. Other problems such as hearing impairment, seizures, lack of growth, and difficulty swallowing (dysphagia) may also arise.
There are many different symptoms of hydranencephaly because the symptoms often arise not only from the parts of the brain that are missing, but from the connections that are lost from the missing parts of the brain to the parts of the brain that are still intact.
There is no one single cause of hydranencephaly, though a blockage to the blood supply of the brain contributes to the majority of the cases. If the blood supply is blocked while in the womb it could be caused by intrauterine infections, toxin exposure such as radiation exposure, drug use, or twin-to-twin transfusion syndrome.
Twin to twin transfusion syndrome occurs when the blood supply in a shared placenta flows unevenly between the babies. It can also happen if one of the babies dies before birth. Other causes of hydranencephaly may be a blockage of the carotid artery.
One last possible cause of a baby being born with hydranencephaly is the presence of certain genetic mutations. Mutations in the TUBA1A gene can cause many kinds of brain abnormalities including hydranencephaly.
Mutations of the FLVCR2 gene can cause Fowler syndrome which involves hydranencephaly, and these mutations are inherited through an autosomal recessive pattern. In this case, both parents would have to carry one copy of the genetic mutation for the child to inherit hydranencephaly and every child born to them would have a 50 percent chance of being born with the disease.
Diagnosis & Tests
Diagnosing hydranencephaly is difficult since many infants with the disease are born without any symptoms. Once the physical characteristics are identified the attending physician will make a clinical evaluation. The doctor will use advanced imaging techniques such as an angiogram, which is an X-ray in which dye is injected into the blood vessels.
Other techniques will include a computed tomography (CT) scan or transillumination. Transillumination involves shining a light near the infant's skull allowing the doctors to see right through the thinly walled cranium. A fetal ultrasound may be used to diagnose hydranencephaly before birth.
Treatment & Therapy
Symptoms are treated as they appear and treatment focuses on improving the patient's quality of life. Physical therapy may be advised for infants and children, and can improve and maintain limb function and prevent further deformities. Range of motion exercises prevent muscle contracture. Infants and children with swelling of the brain may undergo surgery to have a shunt placed to help drain fluid and decrease the risk of seizures.
Prevention & Prophylaxis
Families with a child that has been diagnosed with hydranencephaly need caring medical staff. Support groups can help to give the parents a chance to talk to others, offer and receive compassion, and obtain practical information on how to raise a child with hydranencephaly.