A fetal brain cyst is a fluid-filled sac found within the developing brain, often detected during a routine prenatal ultrasound. This finding is relatively common and represents a diverse group of conditions, ranging from temporary fluid collections to structural abnormalities or evidence of prior injury. Understanding the specific type of cyst is necessary because the implications for the fetus can vary widely, from a completely benign finding that resolves on its own to a marker of a more complex underlying issue.
Choroid Plexus Cysts: The Most Common Finding
Choroid plexus cysts (CPCs) are the most frequently identified type of fetal brain cyst, occurring in about 1% to 2% of pregnancies during the second trimester. The choroid plexus is the structure responsible for producing cerebrospinal fluid (CSF), which surrounds and protects the brain and spinal cord. The cysts themselves are collections of CSF trapped within the layers of the choroid plexus tissue, similar to a small blister forming on the skin.
These cysts are generally considered a normal variant of development and are not structural brain abnormalities. They typically resolve spontaneously as the pregnancy progresses, with the vast majority disappearing by the 28th week of gestation. When a CPC is the only finding on a detailed ultrasound, it is termed an “isolated” cyst and is almost always without consequence for the baby’s neurodevelopment.
Historically, CPCs gained attention due to their association with Trisomy 18. Approximately one-third of fetuses with Trisomy 18 also have CPCs. However, the risk of Trisomy 18 is considered very low if the cyst is isolated and no other structural abnormalities are present on the ultrasound. The presence of other markers, such as heart defects or abnormal hand positions, significantly increases the concern for a chromosomal issue.
Cysts Resulting from Developmental Malformation
A different category of cysts arises from structural abnormalities during the complex process of brain formation. These cysts are not transient fluid traps but rather defects in the membranes or drainage systems that organize the developing brain structures. The primary example of this type is the Arachnoid Cyst (AC), which accounts for the majority of non-transient fetal cysts.
Arachnoid cysts form outside of the brain tissue, situated between the arachnoid membrane and the surface of the brain. They are thought to result from an aberrant splitting of the arachnoid membrane during prenatal development, creating a pocket that fills with CSF. These cysts are classified as extra-axial lesions, and they do not typically communicate with the brain’s ventricular system.
While ACs can be found anywhere in the brain, common fetal locations include the midline interhemispheric space. Unlike choroid plexus cysts, arachnoid cysts are usually persistent and do not resolve before birth. A major concern is the potential for the cyst to grow and cause a mass effect. If growth occurs, it can sometimes lead to hydrocephalus, which may require monitoring or intervention after birth to prevent long-term neurological sequelae.
Cysts Caused by Injury or Infection
Acquired cysts form when an event causes damage to previously healthy fetal brain tissue, leading to necrosis or tissue death. The body’s response is to reabsorb the damaged cells and replace the void with cerebrospinal fluid, effectively walling off the injured area. The prognosis for these acquired cysts is tied directly to the extent and location of the initial brain injury, rather than the cyst itself.
One common type is the Porencephalic Cyst, which is a cavity located within the cerebral hemisphere, resulting from a destructive event. The most frequent cause of this tissue destruction is an intrauterine vascular event, such as a fetal stroke or hemorrhage. These cysts often communicate directly with the brain’s ventricles or the subarachnoid space and can vary significantly in size and location.
Infections that cross the placenta can also cause brain injury that leads to cyst formation. Congenital infections, such as Cytomegalovirus (CMV) and Toxoplasmosis, can cause inflammation and destruction of brain cells. These infections are known to cause Subependymal Cysts, which are typically found near the ventricles in the area of the germinal matrix. The presence of these cysts, particularly when accompanied by other findings like calcifications, serves as a marker for the underlying infectious process that caused the tissue damage.
Diagnosis and Post-Detection Monitoring
The initial detection of a fetal brain cyst is typically made during a routine second-trimester anatomy scan. Following this finding, a more specialized assessment is performed. This detailed scan focuses on the precise location, size, and characteristics of the cyst, as well as checking for any other associated structural anomalies in the brain or body.
Fetal Magnetic Resonance Imaging (MRI) may be recommended, particularly if the ultrasound findings are complex or if a developmental anomaly is suspected. MRI provides a clearer visualization of the brain’s soft tissues, helping to confirm the diagnosis and assess the relationship of the cyst to surrounding structures, like the cortex and ventricles. The purpose of monitoring is to track the cyst’s progression, checking for resolution in the case of CPCs or for growth and mass effect in the case of persistent cysts.
If the cyst is complex, large, or accompanied by other anomalies, genetic testing is often offered to the parents. This testing helps determine if the cyst is part of a larger chromosomal syndrome, such as Trisomy 18. The diagnostic process aims to accurately classify the cyst and provide the most accurate information regarding the baby’s potential outcomes.