Fluid on the brain, scientifically known as hydrocephalus, is a condition where an excessive amount of cerebrospinal fluid (CSF) accumulates within the brain’s cavities, called ventricles, of an unborn baby. This buildup causes the ventricles to enlarge, placing pressure on the developing brain tissue. Congenital hydrocephalus refers specifically to cases present before birth. The causes of this fluid accumulation involve disruptions in the normal fluid flow system, structural defects, external factors like infection, and underlying genetic predispositions.
Understanding Cerebrospinal Fluid Dynamics
The brain and spinal cord are constantly bathed in cerebrospinal fluid (CSF), a clear substance that provides physical cushioning, delivers nutrients, and removes waste products. This fluid is continuously produced by specialized tissue called the choroid plexus, which lines the ventricles deep within the brain. CSF must maintain a balance between production and absorption to keep pressure stable. The fluid circulates through interconnected ventricles and narrow passages before being absorbed into the bloodstream. Hydrocephalus occurs when this system is disrupted, leading to fluid accumulation. This disruption typically happens in one of three ways: overproduction of CSF, failure of absorption mechanisms, or, most commonly in congenital cases, a blockage in the pathways of flow.
Congenital Structural Malformations
Structural malformations are frequently identified as the cause of prenatal hydrocephalus because they create physical obstructions that prevent CSF from circulating freely. These defects are developmental in origin and involve the narrowing or misformation of the natural CSF pathways.
Aqueductal Stenosis
One of the most common forms of obstructive hydrocephalus is Aqueductal Stenosis. Here, the narrow passage connecting the third and fourth ventricles, known as the cerebral aqueduct, is constricted or closed. This blockage causes the fluid to back up into the lateral and third ventricles, leading to their enlargement.
Chiari Malformations
Chiari Malformations are structural defects in the cerebellum. In this condition, brain tissue extends downward into the spinal canal, physically impeding the flow of CSF out of the fourth ventricle.
Dandy-Walker Malformation
The Dandy-Walker Malformation affects the cerebellum and surrounding fluid spaces. It is characterized by an enlarged fourth ventricle and abnormal development of the cerebellar vermis, which disrupts the drainage points for CSF.
Structural issues may also include neural tube defects, such as spina bifida, where the incomplete closure of the spinal column often causes a secondary obstruction to CSF flow. Additionally, arachnoid cysts, which are fluid-filled sacs located between the brain membranes, can compress pathways and block normal circulation.
Infectious Causes Contracted In Utero
Infections acquired by the mother and transmitted to the fetus (in utero infections) can trigger hydrocephalus by causing inflammation and damage to the developing brain. The pathogens cross the placenta, enter the fetal circulation, and target the central nervous system.
The most commonly implicated pathogens are grouped under the acronym TORCH, which includes Cytomegalovirus (CMV), Toxoplasmosis, and Rubella. These infections cause an inflammatory response within the ventricles, often scarring the delicate lining called the ependyma. This scarring leads to the closure of small passages and the formation of adhesions, creating an acquired obstruction to CSF flow.
In the case of Toxoplasmosis, the resulting inflammation and damage can also cause calcifications in the brain, which further disrupt tissue and fluid dynamics. Cytomegalovirus, the most frequent congenital infection, causes extensive tissue destruction and inflammation, leading to brain abnormalities including hydrocephalus.
Inherited and Chromosomal Factors
In some cases, the cause of hydrocephalus is rooted in the fetus’s genetic code, independent of structural defects or external infections. Genetic factors account for a portion of congenital hydrocephalus, with specific gene mutations affecting brain development or CSF regulation mechanisms.
The most recognized genetic cause is X-linked hydrocephalus, which is caused by a mutation in the L1CAM gene located on the X chromosome. This condition is far more prevalent in male fetuses and results in severe aqueductal stenosis, blocking the CSF pathway. The L1CAM gene provides instructions for a protein involved in the formation of cell adhesion molecules necessary for normal brain structure, and a defect compromises the development of the fluid-carrying structures. Mothers carrying the mutation have a 50% chance of passing the gene to their child, though females are usually unaffected carriers due to having a second X chromosome.
Hydrocephalus can also be associated with broader chromosomal disorders, where the baby has an abnormal number of chromosomes. Conditions such as Trisomy 13 (Patau syndrome) and Trisomy 18 (Edwards syndrome) frequently include hydrocephalus as one of many associated developmental abnormalities.