Fetal kidney cysts are fluid-filled sacs that can form on a baby’s kidneys while still developing in the womb. Understanding the origins of these cysts is important, as their presence can sometimes indicate underlying conditions affecting the developing urinary system.
Understanding Fetal Kidney Cysts
Fetal kidney cysts are typically identified during routine prenatal ultrasound examinations, often performed around the 20th week of pregnancy. These cysts can vary considerably in size and number, and they may affect one or both kidneys.
While some small cysts may be harmless and resolve on their own before or after birth, others can signal more significant issues. The presence of these cysts can sometimes impair kidney function. If only one kidney is affected, the other kidney often grows larger to compensate and perform the necessary filtration tasks for the body. However, if both kidneys are severely affected, this can lead to serious health complications for the fetus.
Genetic Origins of Fetal Kidney Cysts
Inherited genetic mutations can lead to the formation of kidney cysts in a fetus. These mutations interfere with the normal development and function of kidney cells, leading to cystic structures. Such genetic conditions are passed down through families, with specific inheritance patterns determining the likelihood of a child being affected.
One notable condition is Autosomal Recessive Polycystic Kidney Disease (ARPKD), which is caused by mutations in the PKHD1 gene. This gene produces fibrocystin, a protein involved in kidney tubule and bile duct structure. In ARPKD, a child must inherit two copies of the mutated PKHD1 gene, one from each parent, for the disease to manifest. If both parents are carriers of a single mutated gene, there is a 25% chance with each pregnancy that their child will inherit two affected copies and develop ARPKD.
ARPKD typically affects both kidneys, causing them to become enlarged and filled with small cysts. This can lead to progressive loss of kidney function, often presenting during the perinatal period or infancy. The condition can also affect the liver, causing issues with bile ducts and fibrosis.
The estimated prevalence of ARPKD is approximately 1 in 20,000 live births. Other genetic syndromes, such as certain ciliopathies including Meckel-Gruber syndrome, Joubert syndrome, and Bardet-Biedl syndrome, can also feature kidney cysts as part of a broader set of symptoms.
Developmental Failures and Structural Causes
Kidney cysts can also arise from errors during the physical development of the fetal kidneys and urinary tract, independent of direct genetic inheritance. These developmental abnormalities are often sporadic.
A common condition resulting from such developmental failure is Multicystic Dysplastic Kidney (MCDK). MCDK is a non-hereditary condition where the affected kidney does not form properly and is replaced by multiple fluid-filled cysts. This malformed kidney typically has no function. MCDK most often affects only one kidney, and the affected kidney may gradually shrink over time, sometimes even disappearing completely. The healthy, unaffected kidney can then compensate. However, if both kidneys are affected by MCDK, the condition is often incompatible with life, primarily because the kidneys are unable to produce urine, which is essential for the development of the fetal lungs.
Another structural cause involves blockages within the urinary tract during fetal development, known as obstructive uropathy. If urine flow is obstructed, urine can back up into the kidneys, leading to swelling and the formation of cysts in the kidney tissue. Examples include posterior urethral valves (PUV), which can block the urethra.
This backup can cause damage to the developing kidney, resulting in a condition called renal dysplasia. Reduced urine output due to severe obstruction can also lead to low amniotic fluid (oligohydramnios), which can severely impact lung development. Kidneys affected by obstructive uropathy may appear abnormally bright or contain cysts on prenatal ultrasound.