Hydrocephalus is a condition involving the abnormal buildup of cerebrospinal fluid (CSF) within the deep cavities of the brain called ventricles. This accumulation causes the ventricles to widen, which then places harmful pressure on the surrounding brain tissues. Understanding how a baby develops hydrocephalus requires first examining the body’s normal system for managing this fluid.
Understanding Cerebrospinal Fluid Dynamics
Cerebrospinal fluid (CSF) bathes the brain and spinal cord, acting as a protective cushion and a transport system for nutrients and waste removal. This fluid is primarily manufactured by specialized tissue called the choroid plexus, which is located inside the brain’s four ventricles. The choroid plexus continuously produces about 500 milliliters of CSF each day.
The CSF follows a specific path through interconnected ventricles and narrow passages. It flows from the two large lateral ventricles into the third ventricle, and then passes through the narrow cerebral aqueduct into the fourth ventricle. From the fourth ventricle, the fluid exits into the subarachnoid space, which surrounds the brain and spinal cord. The final step is the reabsorption of the fluid back into the bloodstream, primarily through structures called arachnoid villi.
Three Ways Hydrocephalus Develops
Hydrocephalus occurs when there is an imbalance in the system of CSF production, flow, and absorption. The condition can be categorized into three physiological mechanisms that lead to fluid accumulation and increased pressure. The most frequent mechanism is the obstruction of CSF flow, known as non-communicating or obstructive hydrocephalus. This happens when a physical blockage, such as a mass or a narrow passageway, prevents the fluid from moving freely between the ventricles, causing it to back up upstream of the block.
The second mechanism is impaired absorption, which is referred to as communicating hydrocephalus. In this scenario, the fluid is able to flow out of the ventricles and into the subarachnoid space. However, the arachnoid villi, which are responsible for draining the fluid into the bloodstream, are unable to reabsorb the CSF effectively, leading to a buildup.
The rarest mechanism is the overproduction of cerebrospinal fluid, where the choroid plexus manufactures more fluid than the system can absorb. An uncommon type of tumor called a choroid plexus papilloma can cause this excessive production. In all three cases, the result is the same: excess fluid accumulates, the ventricles enlarge, and pressure is exerted on the developing brain tissue.
Causes Present at Birth
Hydrocephalus that is present at birth, or congenital hydrocephalus, is typically the result of genetic factors or developmental issues during fetal growth. The most common structural cause is aqueductal stenosis, which involves a narrowing of the cerebral aqueduct, the small channel connecting the third and fourth ventricles. This constriction creates a bottleneck that obstructs the flow of CSF, leading to a severe backup of fluid in the upper ventricles.
Neural tube defects, such as spina bifida, are also strongly linked to congenital hydrocephalus. Babies born with spina bifida often have an associated condition called a Chiari malformation, where the lower part of the brain is pulled down into the spinal canal. This anatomical displacement can physically block the normal exit points for the CSF from the fourth ventricle.
Genetic syndromes can also predispose an infant to the condition, sometimes caused by mutations in specific genes like L1CAM. Infections contracted by the mother during pregnancy, such as rubella, can cause inflammation in the fetal brain tissue. This inflammation can later result in scarring or damage that blocks the CSF pathways.
Causes Acquired After Birth
Acquired hydrocephalus develops after a baby is born, often as a result of an injury, illness, or other neurological event. Intraventricular hemorrhage (IVH), or bleeding within the ventricles, is a significant cause, particularly in premature infants. The blood from the hemorrhage can clot and physically block the narrow CSF pathways, causing an obstructive hydrocephalus. Alternatively, the blood can irritate and damage the arachnoid villi, preventing the reabsorption of fluid and leading to a communicating hydrocephalus.
Infections of the central nervous system, such as bacterial meningitis, are another major acquired cause. Meningitis causes inflammation of the meninges, the membranes surrounding the brain and spinal cord. This inflammation and subsequent scarring can obstruct the flow of CSF or damage the absorption sites.
Physical head trauma, which may occur during a difficult birth or from an injury after delivery, can cause bleeding or swelling that disrupts the fluid dynamics. Mass lesions, such as brain tumors, can also cause acquired hydrocephalus by physically growing into or compressing a ventricle. The tumor acts as a physical barrier, blocking the passage of CSF and causing fluid accumulation behind the obstruction.