What is Hydranencephaly

TL;DR: Hydranencephaly is a rare, severe neurological condition where the brain's cerebral hemispheres are absent or replaced by fluid-filled sacs, while the brainstem and cerebellum remain largely intact. It is primarily a sporadic, non-inherited developmental disorder that typically presents at birth or during infancy, requiring specialized, supportive, and multidisciplinary medical care.

What is the underlying cause of Hydranencephaly?

Hydranencephaly is characterized by the near-total absence of the cerebral hemispheres. In most cases, the brain tissue is replaced by cerebrospinal fluid (CSF) contained within the meninges. The condition is generally considered a result of a vascular disruption event during fetal development, such as a blockage in the internal carotid arteries. Because the brainstem and cerebellum are usually preserved, the autonomic functions of the body—such as breathing and blood pressure regulation—often continue to function, although cognitive and motor development are profoundly affected by the loss of the higher brain centers in Hydranencephaly.

How common is Hydranencephaly and who is affected?

Hydranencephaly is an extremely rare condition with an estimated incidence of approximately 1 in 10,000 births, though accurate global statistics are difficult to confirm due to the high rate of fetal loss. It affects both males and females equally, and there is no known geographic, racial, or ethnic predisposition. Unlike many genetic syndromes, Hydranencephaly is usually a sporadic event, meaning it is rarely inherited and typically does not recur in future pregnancies for the same parents. Currently, 37 individuals and their families affected by Hydranencephaly have joined the DiseaseMaps.org community to share their experiences and support one another.

What are the primary clinical features of Hydranencephaly?

While symptoms can vary depending on the extent of the brain tissue loss, infants with Hydranencephaly often present with a range of neurological and physical signs. Because the cerebral cortex is absent, the infant lacks the structures responsible for conscious thought, voluntary movement, and sensory processing. Common clinical observations include:

• Increased head size: Due to the accumulation of cerebrospinal fluid, the head circumference may be larger than average (macrocephaly).


• Seizures: Most infants experience some form of seizure activity due to abnormal electrical discharges in the remaining brain tissue.


• Motor impairment: Infants typically exhibit varying degrees of spasticity, paralysis, or limited motor response.


• Autonomic stability: The infant may maintain a regular heart rate and respiratory rhythm, often surprising parents who are told their child has no conscious brain function.


• Visual and auditory deficits: Reactions to light and sound are often absent or significantly diminished.

How does Hydranencephaly differ from other conditions?

It is common to confuse Hydranencephaly with other conditions like hydrocephalus or anencephaly, but they are clinically distinct. In hydrocephalus, the brain is present but compressed by excess fluid. In anencephaly, the skull and brain fail to develop entirely, usually involving the absence of the brainstem as well. Hydranencephaly is unique because the presence of the brainstem allows for the maintenance of basic life-sustaining functions, which distinguishes it from conditions where brainstem function is absent from birth.

Next steps

• Consult a pediatric neurologist or a neurosurgeon to confirm the diagnosis and discuss the specific anatomy of the brain.


• Work with a multidisciplinary care team, including palliative care specialists, to focus on the child’s comfort and quality of life.


• Connect with the 37 members of the DiseaseMaps.org community who have navigated the challenges of a Hydranencephaly diagnosis.


• Seek genetic counseling to discuss the low probability of recurrence in future pregnancies.

Medical disclaimer: This information is for educational purposes only and does not constitute medical advice, diagnosis, or treatment; always seek the advice of your physician or other qualified health provider with any questions regarding a medical condition.

References

• NIH Genetic and Rare Diseases Information Center (GARD): Hydranencephaly Overview.


• Orphanet: Rare Disease Database (ORPHA:399).


• OMIM (Online Mendelian Inheritance in Man): Entry #236600.


• National Institute of Neurological Disorders and Stroke (NINDS): Hydranencephaly Information Page.