A horseshoe kidney is a congenital anomaly where the kidneys are fused, forming a single, U-shaped organ instead of two separate ones. This condition is present at birth, resulting from the joining of kidney tissue during early fetal development.
Understanding Horseshoe Kidney
A horseshoe kidney involves the fusion of the lower poles of the kidneys, creating a U-shape. While upper pole fusion is rare, lower pole connection occurs in over 90% of cases. The connecting bridge, known as the isthmus, can consist of functional kidney tissue in about 80% of cases or fibrous tissue.
This fused kidney is positioned lower in the abdomen or pelvis compared to normally developed kidneys. It is the most common form of renal fusion anomaly, affecting approximately 1 in 500 individuals, with a slightly higher prevalence in males. Despite its unique anatomy, a horseshoe kidney can function effectively, and about one-third of individuals experience no symptoms throughout their lives.
How Horseshoe Kidney Develops
Horseshoe kidney formation begins during fetal development, specifically around 4 to 6 weeks of gestation. At this stage, the kidneys start developing in the pelvic region. Normally, these developing kidneys gradually ascend to their final position in the upper abdomen, undergoing a 90-degree medial rotation as they rise.
With a horseshoe kidney, fusion occurs while the kidneys are still in the pelvis, before their full ascent. This early fusion creates the connecting isthmus, which acts as a physical barrier, preventing the fused kidney from fully ascending past the inferior mesenteric artery. This obstruction leads to the kidney’s lower-than-normal positioning. The fusion also interferes with normal rotation, resulting in the kidney’s collecting system facing forward.
Factors Influencing Formation
While the exact cause of horseshoe kidney is often not fully understood and can occur sporadically, it is associated with broader developmental issues. Certain genetic conditions have a known association with its formation. These include chromosomal abnormalities such as Turner syndrome, and trisomies like Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome).
Beyond genetic factors, some environmental influences during pregnancy are explored as potential contributors, though these links require more research. Maternal diabetes, for instance, may play a role in various renal malformations, including horseshoe kidney. Certain medications taken during pregnancy have also been considered, but direct causation is limited. Horseshoe kidney is not inherited in a simple Mendelian pattern but can be part of a complex developmental picture involving multiple factors.