Hydrocephalus in babies involves an excessive accumulation of cerebrospinal fluid (CSF) within the brain’s ventricles, which are interconnected cavities. This fluid buildup can lead to increased pressure inside the skull. This article explores the different causes of hydrocephalus in infants, from those present at birth to those that develop afterward.
How Hydrocephalus Develops
The brain and spinal cord are surrounded by cerebrospinal fluid (CSF), which cushions them, delivers nutrients, and removes waste products. Specialized tissues called choroid plexuses, located primarily within the brain’s ventricles, continuously produce this fluid. CSF circulates through the ventricular system, from the lateral ventricles to the third, then to the fourth ventricle, before flowing into the subarachnoid space surrounding the brain and spinal cord. The fluid is then reabsorbed into the bloodstream, mainly through arachnoid granulations.
Hydrocephalus occurs when there is an imbalance in CSF production, circulation, and absorption. The most common reason for this imbalance is an obstruction within the CSF pathways, preventing its normal flow. Less frequently, hydrocephalus can result from impaired absorption of CSF back into the bloodstream, often due to issues with the arachnoid granulations. In very rare instances, the brain may produce an excessive amount of CSF, overwhelming the absorption capacity.
Causes Originating Before or During Birth
Hydrocephalus can be present at birth, termed congenital hydrocephalus, resulting from events during fetal development or genetic factors. One common congenital cause is aqueductal stenosis, a narrowing of the aqueduct of Sylvius. This small passage connects the third and fourth ventricles, and its constriction blocks CSF flow, causing fluid to accumulate in the upstream ventricles.
Neural tube defects, such as spina bifida (particularly myelomeningocele), are another significant cause. These conditions often lead to a Chiari II malformation, where parts of the brain, including the cerebellum, extend into the spinal canal. This displacement can obstruct the normal outflow of CSF from the fourth ventricle, leading to hydrocephalus. Genetic factors can also predispose an infant to hydrocephalus, with some cases being inherited or resulting from new gene mutations. For example, X-linked hydrocephalus is a rare genetic disorder caused by mutations in the L1CAM gene.
Maternal infections during pregnancy can contribute to congenital hydrocephalus. Infections such as toxoplasmosis, cytomegalovirus (CMV), and rubella can cause inflammation or damage to the developing fetal brain tissue. This damage may disrupt CSF pathways or impair absorption, leading to fluid accumulation within the brain. These infections can impact the delicate structures responsible for CSF regulation, leading to developmental abnormalities.
Causes Developing After Birth
Hydrocephalus can also develop after a baby is born, a condition known as acquired hydrocephalus. Intraventricular hemorrhage (IVH), or bleeding within the brain’s ventricles, is a frequent cause, especially in premature babies. Fragile blood vessels in premature infants can rupture, leading to bleeding. The blood and blood clots can then block the CSF pathways or damage the tissues responsible for CSF absorption, resulting in hydrocephalus.
Severe infections of the brain or its surrounding membranes, such as meningitis or encephalitis, can also lead to hydrocephalus. These infections cause inflammation that can scar the delicate pathways through which CSF circulates or the surfaces where it is absorbed. This scarring can create blockages or impair the absorption process, causing CSF to build up. The inflammatory exudates themselves can also physically obstruct CSF flow.
Brain tumors, even those that are not cancerous, are another cause of acquired hydrocephalus in infants. Tumors can grow within or near the ventricular system, physically obstructing the normal flow of CSF. The tumor’s location determines how it impacts CSF circulation; for instance, tumors in the posterior fossa are frequently associated with hydrocephalus. Severe head trauma can also initiate hydrocephalus by causing bleeding, swelling, or scarring within the brain. Such injuries can disrupt the delicate balance of CSF dynamics, leading to its accumulation.
When the Cause is Undetermined
In some instances, despite thorough medical evaluations and diagnostic tests, the specific cause of hydrocephalus in a baby cannot be definitively identified. These cases are often referred to as “idiopathic” hydrocephalus.
Medical professionals focus on managing the fluid buildup and its effects on the brain, regardless of whether a specific origin is pinpointed. Ongoing research continues to explore the complex factors that contribute to these unexplained cases.