A fetal cystic hygroma (CH) is a congenital malformation of the lymphatic system, characterized by the presence of one or more fluid-filled sacs, or cysts, typically located around the head and neck of the fetus. This condition arises when the lymphatic vessels, which are designed to drain excess fluid (lymph) from tissues, fail to connect properly with the venous system during development. While a diagnosis of cystic hygroma can be concerning for expectant parents, outcomes are highly variable, ranging from spontaneous resolution to severe complications, depending largely on the underlying cause and the size of the malformation.
Defining Fetal Cystic Hygroma
Cystic hygroma is a type of lymphatic malformation where the developing lymphatic system does not establish a normal connection to the major veins in the neck, leading to an abnormal accumulation of lymph fluid. This failure of drainage causes the formation of distended, clear, fluid-filled spaces, which are visible on an ultrasound as cysts. About 75% of these lesions are found in the occipital-cervical region of the neck, but they can also occur in the axilla, groin, or chest.
The malformation is classified based on the size of the fluid-filled cavities. Macrocystic hygromas contain large cysts, while microcystic forms consist of smaller, more numerous cysts. The appearance of internal walls, or septations, is a significant detail. Septated hygromas are associated with a higher likelihood of chromosomal abnormalities and a less favorable prognosis compared to non-septated ones. A cystic hygroma is distinct from a simple increased nuchal translucency measurement, though both are markers for potential issues.
Detection and Underlying Genetic Links
Detection of a fetal cystic hygroma often occurs during routine first-trimester screening, typically between 10 and 14 weeks of gestation, when a detailed ultrasound is performed. The visual confirmation of a septated cystic mass in the nuchal region distinguishes the condition from a simple thickened nuchal fold. Once detected, a detailed or Level II ultrasound is warranted to confirm the diagnosis and assess for any other structural anomalies.
The diagnosis necessitates genetic testing because of the strong association with underlying chromosomal abnormalities, found in approximately 50% to 55% of cases. The specific genetic conditions most commonly linked include Turner Syndrome (monosomy X), which accounts for about 21% of aneuploidies, and the common trisomies, such as Down Syndrome (Trisomy 21) and Trisomy 18. To determine the fetal karyotype, invasive procedures like Chorionic Villus Sampling (CVS) or amniocentesis are offered, depending on the gestational age.
Fetal Monitoring and Prognosis During Pregnancy
Following a diagnosis, fetal monitoring is required, even in cases where genetic testing reveals a normal karyotype. The monitoring protocol involves serial ultrasounds to track the size and characteristics of the hygroma, as well as the overall fetal health. A fetal echocardiogram is also routinely performed, as structural heart defects are common associated malformations, occurring in up to 50% of euploid fetuses with a cystic hygroma.
The course of the condition in utero is highly variable. Approximately 20% to 43% of cystic hygromas, particularly smaller, non-septated lesions in chromosomally normal fetuses, may spontaneously resolve as the pregnancy progresses. This resolution, however, does not guarantee a completely normal outcome, as other subtle abnormalities may still be present. In more severe cases, the hygroma can progress to hydrops fetalis, a generalized accumulation of fluid in at least two different fetal compartments (e.g., skin, abdomen, or around the lungs).
For pregnancies that continue, delivery planning requires a multidisciplinary approach involving maternal-fetal medicine specialists and neonatologists. Delivery should ideally take place in a specialized hospital setting with a Neonatal Intensive Care Unit (NICU) and pediatric surgical team available. A large hygroma that could obstruct the baby’s airway may necessitate a specialized delivery procedure, such as the Ex Utero Intrapartum Treatment (EXIT) procedure, to secure the airway before the umbilical cord is cut.
Post-Natal Treatment and Long-Term Outlook
For infants born with a persistent cystic hygroma, the primary post-natal treatment is surgical excision of the abnormal lymphatic tissue. Complete removal is the goal, as residual tissue can lead to a recurrence rate of around 15%. When surgical removal is not feasible due to the lesion’s size or its proximity to structures like nerves or blood vessels, non-surgical options may be considered.
Sclerotherapy, which involves injecting a solution directly into the cyst to cause inflammation and shrinkage, is used for specific macrocystic lesions. The long-term outlook depends heavily on several factors, primarily whether an underlying genetic syndrome was present and the extent of any associated structural anomalies, such as cardiac defects. If the cystic hygroma was an isolated finding in a fetus with a normal karyotype and did not affect the airway or other vital structures, the short-term pediatric outcome can be favorable, with up to a 95% chance of a normal outcome. A long-term follow-up with pediatric specialists is recommended to monitor for any subtle developmental or structural issues that may appear later in childhood.