Rare Brain Disorder in Newborns-Hydranencephaly: From Prenatal Infections to Vascular Accidents

Hydranencephaly in Babies: What Parents Need to Know

Of all the conditions we encounter in medicine, those that affect the very structure of the developing brain are among the most profound and challenging. They force us to confront the delicate and complex processes that build a human life, and they call for our deepest compassion and support for the families affected. One such condition is Hydranencephaly, a rare and severe congenital disorder that presents immense challenges from the very beginning.

In this article, we will explore what hydranencephaly is, delving into its causes, the symptoms it presents, how it is diagnosed, and the approaches we take to manage the condition and support the infants and their families.

What Exactly Is Hydranencephaly? A Deeper Look

Hydranencephaly is a rare and severe congenital cephalic disorder where the brain’s cerebral hemispheres—the large, outer parts responsible for higher-level functions like thought, memory, personality, voluntary movement, and sensation—are absent. In their place are sacs filled with cerebrospinal fluid (CSF). Imagine the skull as a vessel that should contain the intricate, folded tissue of the cerebrum; in an infant with hydranencephaly, this vessel is instead primarily filled with fluid, with only the most basic brain structures remaining.

This condition is considered an extreme form of porencephaly, which involves cysts or cavities in the cerebral hemispheres. In hydranencephaly, however, the damage is so extensive that nearly the entire cerebrum is destroyed and reabsorbed by the body during fetal development.

What’s Missing vs. What’s Present

It’s a common misconception that the entire brain is missing. This is not the case, and the distinction is critical to understanding how an infant with this condition can survive birth.

Structures Often Absent or Destroyed:

• Cerebral Hemispheres (Cerebrum): The largest part of the brain, a total absence of this structure means the loss of all associated functions.
• Cerebral Cortex: The outer layer of the cerebrum, which is the seat of consciousness, language, memory, and perception. Without it, there is no capacity for conscious thought, vision, or hearing.

Structures Typically Preserved (Though Often Underdeveloped):

• Brainstem: Connects the brain to the spinal cord and controls the most fundamental autonomic functions.
• Cerebellum: Located at the back of the brain, it plays a role in motor control and balance.
• Thalamus and Basal Ganglia: Deep brain structures involved in relaying sensory information and coordinating movement.

Because these lower brain structures are intact, an infant with hydranencephaly can regulate some of the body’s most basic functions, such as:

• Breathing
• Heart rate and blood pressure
• Digestion
• Temperature regulation
• Startle reflexes
• Sucking and swallowing reflexes

The presence of these structures is why an infant with hydranencephaly can survive birth and, for a time, perform essential life-sustaining functions. However, the profound absence of the cerebral cortex means that the capacity for any meaningful interaction with the world is absent.

Causes and Development

Hydranencephaly is not typically inherited. Instead, it is considered an acquired condition that occurs during pregnancy. It is the result of a catastrophic injury or disruption to the fetal brain after the 12th week of gestation, once the basic brain structures have already formed. The leading theories for its cause include:

• Vascular Occlusion: The most widely accepted cause is a blockage of the carotid arteries, the major blood vessels supplying the brain’s hemispheres. This cuts off oxygen and nutrients, leading to massive tissue death (infarction). The necrotic brain tissue is then liquefied and reabsorbed, leaving a fluid-filled cavity.
• Intrauterine Infections: Certain maternal infections that cross the placenta can cause severe inflammation and destruction of fetal brain tissue. These include TORCH infections (Toxoplasmosis, Rubella, Cytomegalovirus, Herpes).
• Toxic Exposures or Trauma: Though less common, exposure to certain toxins or direct trauma to the fetus could potentially lead to the necessary brain injury.
• Genetic Abnormalities: In very rare cases, a genetic mutation may predispose the fetus to this type of vascular or developmental accident.

In many cases, a specific cause cannot be identified.

Diagnosis and Symptoms

Prenatal Diagnosis: Hydranencephaly is often diagnosed during routine prenatal ultrasounds. A sonographer may observe a cranium that appears largely empty or filled with fluid where the brain’s hemispheres should be. It can sometimes be misdiagnosed as severe hydrocephalus.

Postnatal Diagnosis and Symptoms: An infant may appear normal at birth. However, within a few weeks or months, symptoms become apparent:

• Irritability and Seizures: The most common early signs.
• Increased Muscle Tone (Spasticity): Limbs may be stiff and difficult to move.
• Poor Feeding and Growth: Difficulty with sucking and swallowing can lead to failure to thrive.
• Enlarging Head Size (Macrocephaly): As CSF accumulates without proper drainage, it can cause the skull to expand rapidly.
• Lack of Developmental Progress: The infant will not meet any developmental milestones, such as smiling, tracking objects with their eyes, or responding to sounds.

A definitive postnatal diagnosis is made through neuroimaging, such as a CT scan or MRI, which clearly shows the absence of the cerebral hemispheres. Transillumination—a test where a bright light is held against the infant’s head in a dark room—can be a striking diagnostic clue, as the entire skull will glow because it is filled with fluid rather than dense brain tissue.

Treatment and Prognosis

There is no cure for hydranencephaly and no way to regenerate the lost brain tissue. Treatment is entirely supportive and palliative, focused on providing comfort and managing symptoms.

• Shunt Placement: If the accumulation of CSF causes pressure, discomfort, or an excessively large head, a surgeon may place a ventriculoperitoneal (VP) shunt. This thin tube drains the excess fluid from the skull into the abdominal cavity, where it can be safely reabsorbed by the body.
• Symptom Management: Medications may be prescribed to control seizures and muscle spasticity.
• Supportive Care: This includes ensuring proper nutrition, maintaining body temperature, and providing a comfortable, loving environment.

The prognosis for hydranencephaly is extremely poor. The vast majority of infants do not survive past their first year of life. While a small number have been documented to live for several years, they remain in a vegetative state, with no capacity for cognitive function. The absence of the cerebrum makes consciousness and purposeful living impossible.

What Causes This Devastating Condition? Unraveling the Etiology of Hydranencephaly

Hydranencephaly is a profound and devastating neurological condition in which the cerebral hemispheres are largely or entirely absent, replaced instead by cerebrospinal fluid (CSF). It is crucial to understand that we believe hydranencephaly is not a primary failure of the brain to form during the earliest stages of development. Instead, current understanding points to a more tragic sequence of events: it is largely considered a destructive process that occurs during the second trimester of pregnancy, specifically after the brain’s fundamental structures have already developed. This critical period, roughly between 12 and 24 weeks gestation, is when the delicate brain tissue of the cerebral hemispheres is formed and highly susceptible to damage. The destructive event essentially causes extensive destruction of this newly formed, vulnerable cerebral tissue, which is then reabsorbed by the body and subsequently replaced by CSF, leading to the characteristic presentation of a fluid-filled skull.

While the exact trigger for this destructive event often remains unknown, leaving families with profound questions, medical research has identified several potential causes and associated risk factors that can lead to this devastating outcome:

• Vascular Occlusion (Prenatal Stroke): This is by far the most widely accepted and frequently implicated theory behind hydranencephaly. It involves a severe disruption of blood supply to the developing fetal brain. A blockage, such as a blood clot or thrombus, occurring in the critical internal carotid arteries – the primary arteries responsible for supplying oxygenated blood to the developing cerebral hemispheres – can lead to a massive prenatal stroke. This sudden and severe interruption of blood flow completely cuts off the vital supply of oxygen and essential nutrients to the brain tissue. Without these crucial elements, the delicate brain cells rapidly die (ischemic necrosis), leading to widespread destruction of the cerebral hemispheres. The source of such blood clots can sometimes be traced to placental abnormalities, maternal clotting disorders, or even umbilical cord complications.
• Intrauterine Infections: Certain maternal infections contracted during pregnancy, particularly during the critical second trimester, have the capacity to cross the placental barrier and directly damage the fetal brain. These infections can induce inflammation, directly destroy developing brain cells, or disrupt the intricate vascular network within the fetal brain, indirectly leading to tissue damage. Prominent examples of such neurotropic infections include:
  • Toxoplasmosis: A parasitic infection often contracted from contaminated food or cat feces.
  • Cytomegalovirus (CMV): A common viral infection that can be asymptomatic in the mother but devastating to the developing fetus.
  • Herpes Simplex Virus (HSV): Though less common as a direct cause in utero compared to perinatal transmission, severe disseminated infection can affect the brain.
  • Zika Virus: Explicitly linked to severe congenital brain malformations, including microcephaly and, in some cases, severe cerebral destruction consistent with hydranencephaly.
  • These often fall under the “TORCH” group of infections (Toxoplasmosis, Other [syphilis, varicella-zoster, parvovirus B19], Rubella, Cytomegalovirus, and Herpes simplex), which are known for their potential to cause severe fetal abnormalities, including neurological damage.
• Genetic Abnormalities: While hydranencephaly is generally considered a sporadic event and is not typically inherited in a simple Mendelian (dominant or recessive) pattern, rare genetic mutations or specific genetic syndromes have been linked to some cases. These mutations might affect genes responsible for the development of brain vasculature, the integrity of brain tissue, or pathways involved in inflammatory responses to insult. An example is Fowler syndrome (also known as Cerebrovascular Disruption Syndrome), a rare autosomal recessive condition characterized by multiple profound abnormalities, including hydrops fetalis and significant damage to the developing brain and blood vessels, which can lead to a hydranencephalic presentation.
• Maternal Trauma or Toxin Exposure: Though relatively less common as primary causes, severe external factors can potentially contribute to hydranencephaly. Severe physical trauma to the mother’s abdomen, such as from a serious fall or a car accident, could theoretically lead to placental abruption or direct vascular injury, subsequently disrupting blood flow to the fetal brain. Similarly, exposure to certain environmental toxins or specific teratogenic substances (substances that can cause birth defects) during critical periods of brain development could potentially interfere with vascular development or directly damage brain tissue, leading to a destructive process. However, these are generally considered rare triggers.

It is profoundly important to emphasize that in a significant number of cases, despite extensive medical investigation, a specific and definitive cause for hydranencephaly is never identified. This absence of a clear answer can be incredibly difficult for families, leaving them with lingering questions and a sense of uncertainty. For these families, it is absolutely vital for them to know that this devastating condition is not the result of anything they did or did not do during the pregnancy. The vast majority of cases are tragic, unforeseeable events stemming from complex biological processes that are beyond parental control.

Recognizing the Symptoms of Hydranencephaly: A Comprehensive Overview

While the diagnosis of hydranencephaly points to a profound structural absence of the brain’s cerebral hemispheres, one of the most challenging aspects for parents and clinicians is that an infant may appear deceptively normal at birth. They may have a strong cry, be able to breathe independently, and even startle appropriately. However, this early presentation is often fleeting. Over the first few weeks and months of life, a constellation of symptoms begins to emerge and intensify as the underlying neurological reality becomes apparent.

The primary physical change is often related to the accumulation of cerebrospinal fluid (CSF) in the cranial cavity. Without the cerebral hemispheres to occupy this space, the fluid builds up, exerting pressure from within. This leads to an accelerated rate of head growth, a condition known as macrocephaly. This rapidly enlarging head is often one of the first and most definitive signs that prompts further investigation.

Beyond this, a range of clinical signs and symptoms, driven by the absence of the cerebrum and the reliance on a more primitive brainstem, will become clear.

Detailed Clinical Signs and Symptoms

• Severe Irritability and Discomfort An infant with hydranencephaly often experiences profound irritability. This manifests as excessive, inconsolable crying that can last for hours. The cry may be high-pitched and sound different from that of a typical infant. This distress is not behavioral but a direct result of neurological dysfunction, potentially caused by discomfort from internal pressure, seizure activity, or a general state of neurological disorganization.
• Pervasive Seizure Activity Seizures are a central and very common feature of hydranencephaly. They can begin within days of birth and are often frequent and difficult to control, even with multiple anti-epileptic medications. These seizures can present in various ways, from dramatic, full-body convulsions (tonic-clonic seizures) to more subtle events like muscle spasms (myoclonic jerks), eye fluttering, lip-smacking, or brief periods of apnea (pauses in breathing). Managing these seizures is a primary focus of medical care.
• Profoundly Abnormal Muscle Tone The cerebrum is responsible for coordinating voluntary movement and regulating muscle tone. Without it, infants display significant abnormalities. This can manifest in two opposing ways:
  • Spasticity (Hypertonia): The infant’s muscles are extremely stiff and rigid. Their arms and legs may be held in tight, flexed positions, with clenched fists and pointed toes. Movement can appear jerky and uncontrolled.
  • Hypotonia: The infant is “floppy,” with very little muscle tone, similar to a rag doll. They have poor head control and feel limp when held. Some infants may exhibit a mix of these states, with stiffness in their limbs but a weak or floppy trunk.
• Significant Feeding and Swallowing Difficulties The complex, coordinated reflex of sucking, swallowing, and breathing is often severely impaired. The infant may have a weak or non-existent suck reflex, making it impossible to breastfeed or bottle-feed effectively. This lack of coordination also poses a high risk of aspiration, where milk or saliva enters the lungs, leading to pneumonia and other respiratory complications. Consequently, most infants with hydranencephaly require feeding support, such as a nasogastric (NG) tube for short-term use or a surgically placed gastrostomy (G-tube) for long-term nutrition.
• A Lack of Purposeful Development An infant with hydranencephaly will not achieve typical developmental milestones. While they may exhibit basic reflexes controlled by the brainstem (like the startle reflex or grasping), they do not develop purposeful, voluntary actions. They will not learn to smile socially, track objects or faces with their eyes, coo or babble, or reach for toys. Their interactions with the world are reflexive rather than cognitive.
• Cortical Blindness and Visual Impairment The occipital lobes, located in the rear of the cerebrum, function as the brain’s vision processing centers. Because these lobes are absent in hydranencephaly, infants are considered cortically blind. Their eyes and optic nerves may be perfectly healthy and structurally intact—they can physically perceive light, and their pupils may react. However, their brain has no capacity to interpret these light signals into meaningful images. They cannot “see” in the way we understand it.
• Impaired Temperature Regulation The hypothalamus, a small but vital structure often damaged or malformed in hydranencephaly, acts as the body’s thermostat. When it is impaired, the infant cannot effectively regulate their core body temperature. They can easily become too cold (hypothermic) or too hot (hyperthermic), even in a stable room temperature. This requires constant monitoring and management with appropriate clothing, blankets, and environmental control to keep them safe and comfortable.

The Diagnostic Journey: Uncovering Hydranencephaly

The diagnosis of hydranencephaly is a profound moment for any family, marking the beginning of a complex and emotional journey. The process to confirm the condition can happen at two distinct stages: during pregnancy (prenatally) or after the baby is born (postnatally). Each path involves specific medical evaluations designed to build a clear picture of the infant’s brain structure.

1. Prenatal Diagnosis: Detecting Signs Before Birth

For many families, the first indication of a problem arises during routine prenatal care. The diagnostic process typically unfolds as follows:

• Routine Prenatal Ultrasound: The journey often begins during a standard second-trimester anatomy scan. A sonographer may observe significant and concerning abnormalities. Instead of seeing the developing folds and structures of the cerebral hemispheres, they may find a large, fluid-filled space. The fetal head might also measure larger than expected for its gestational age (a condition called macrocephaly), a direct result of the cerebrospinal fluid (CSF) filling the cranial cavity where the brain should be.
• Follow-Up and Fetal MRI: If an ultrasound raises suspicion of a severe brain anomaly like hydranencephaly, the next critical step is a fetal MRI. While ultrasound is excellent for initial screening, its sound waves can be limited in detail. A fetal MRI uses powerful magnetic fields to generate highly detailed, cross-sectional images of the fetal brain. This advanced imaging provides irrefutable clarity, allowing doctors to:
  • Confirm the Absence of Hemispheres: The MRI can definitively show the near-total absence of the cerebral hemispheres.
  • Visualize Remaining Structures: It clearly delineates the structures that are typically preserved in hydranencephaly, such as the brainstem, cerebellum, and thalamus.
  • Rule Out Other Conditions: It helps distinguish hydranencephaly from other serious conditions like severe hydrocephalus (where the brain is present but compressed by excess fluid) or alobar holoprosencephaly (where the hemispheres fail to divide). This accurate confirmation is vital for counseling parents on the prognosis and preparing for the infant’s needs after birth.