Hydranencephaly is a rare congenital condition in which most or all of the brain’s cerebral hemispheres are destroyed during fetal development and replaced by sacs filled with cerebrospinal fluid. The brainstem, cerebellum, and several deeper brain structures remain intact, which means a newborn with hydranencephaly may appear normal at birth. But the parts of the brain responsible for conscious thought, vision, voluntary movement, and language are largely or entirely absent.
What Happens in the Brain
Hydranencephaly is classified as a disruption rather than a malformation. The brain initially begins to form normally, but sometime during the second trimester, blood flow through the internal carotid arteries is interrupted. These two arteries supply the cerebral hemispheres, the large, folded outer portions of the brain that handle everything from processing sight and sound to forming memories and controlling deliberate movement. When blood flow stops, the tissue those arteries feed is destroyed.
The structures fed by the brain’s rear blood supply survive. These include the brainstem (which controls breathing, heart rate, and basic reflexes), the cerebellum (which coordinates balance and movement), the thalamus, and the basal ganglia. The membrane dividing the brain’s two sides, called the falx, also typically remains. Essentially, the skull is structurally intact and the deeper, more primitive brain regions still function, but the cerebral cortex is gone, replaced by fluid-filled sacs surrounded by a thin membrane.
Causes
The underlying cause is a loss of blood flow (ischemia) to the developing cerebral hemispheres during pregnancy. What triggers that loss of blood flow varies. Bilateral occlusion of the internal carotid arteries is the most commonly cited mechanism. Intrauterine infections, particularly toxoplasmosis and certain viral infections, can damage the blood vessels supplying the brain. Rare genetic factors and vascular accidents during fetal development have also been implicated. In many cases, the exact trigger is never identified.
Signs and Symptoms at Birth
One of the most unsettling aspects of hydranencephaly is that affected infants often appear and act normal in the first days of life. Because the brainstem is intact, newborns can breathe on their own, suck, swallow, cry, and exhibit basic reflexes. Head size may be within the normal range initially.
Within a few weeks, signs typically begin to emerge. Irritability and increased muscle tone (hypertonia) are often the first changes parents notice. Over the following weeks and months, the picture becomes clearer: seizures, visual impairment or blindness, deafness, lack of growth, and severe intellectual disability. Global developmental delay becomes apparent as the infant misses milestones like tracking objects with their eyes, reaching for things, or responding to voices. Some infants develop hydrocephalus, a buildup of fluid pressure that causes the head to enlarge progressively. Others may have microcephaly, a smaller than expected head circumference present at birth.
How It Is Diagnosed
Hydranencephaly can sometimes be detected before birth on routine prenatal ultrasound, which may show the absence of cerebral tissue and the presence of fluid-filled spaces within the skull. When the diagnosis is not made prenatally, it is often suspected in the first weeks of life as neurological symptoms develop.
One classic bedside test involves holding a bright light against the infant’s skull in a dark room. In hydranencephaly, light passes through the fluid-filled cranium much more readily than it would through a brain with intact cerebral tissue, causing the head to glow. This transillumination test is suggestive but not definitive. MRI is the gold standard, clearly showing the absence of cerebral hemispheres and the preservation of brainstem and cerebellar structures.
Distinguishing It From Similar Conditions
Hydranencephaly is sometimes confused with two other conditions. In severe hydrocephalus, fluid accumulates and compresses the brain, but a thin rim of cortical tissue is still present around the edges, something absent in hydranencephaly. In alobar holoprosencephaly, the brain fails to divide into two hemispheres during early development. That condition is associated with missing midline structures (including the falx) and often comes with distinctive facial abnormalities, neither of which is typical of hydranencephaly. These distinctions matter because they carry different prognoses and genetic implications for families.
Life Expectancy and Survival
Most infants with hydranencephaly survive only weeks to months. The brainstem can sustain basic life functions for a time, but without a functioning cortex, the body’s ability to regulate itself eventually fails. Seizures, infections, and respiratory problems are common complications that shorten survival.
Longer survival does occur, though it is the exception. A review in the journal Pediatric Neurology documented survival up to 19 years in rare cases, and reported two patients who lived 24 and 66 months respectively. These children survived even with flat (isoelectric) brain wave readings and absent responses on tests that measure the brain’s reaction to stimuli. This suggests the brainstem alone can sustain life far longer than once assumed, though these children had profound disabilities throughout.
Management and Treatment
There is no way to restore the missing cerebral tissue. Treatment focuses on comfort, managing complications, and controlling head size if fluid continues to accumulate.
When the head grows excessively due to fluid buildup, the most common intervention is placement of a shunt, a thin tube that drains excess fluid from the skull into the abdomen where the body absorbs it. A retrospective study of 52 infants found that head size decreased by at least 1 centimeter in roughly 62% to 71% of infants treated with a shunt or with procedures that reduce fluid production by deactivating the tissue that makes cerebrospinal fluid. Infants who received only palliative care saw continued head growth. Controlling head size can make daily care easier and reduce discomfort, even though it does not change the underlying neurological picture.
Seizure management is another major component of care. Comfort-focused interventions, including positioning, temperature regulation, and feeding support, make up the day-to-day reality for families. Many families work with palliative care teams to navigate decisions about the level of medical intervention that aligns with their goals and values.