Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH
Horseshoe kidney, or sometimes renal fusion, is a congenital disorder where the two kidneys fuse together into one during fetal development, resulting in one large horseshoe-shaped kidney.
Normally during fetal development, the kidneys develop in the pelvis, and progress through a series of phases called pronephros, mesonephros, and metanephros, before finally migrating upward into the abdomen and becoming everyone’s favorite organ duo.
For people with horseshoe kidney, their kidneys fused together at some point during development, and there are two main working theories on how this might happen. The first is mechanical fusion, which happens during the metanephros stage, which is around about the 5th week of gestation. At this point, the two kidneys are still in the pelvis, and are therefore pretty close together, so close that it’s thought that some flexion or growth of the developing spine and pelvic organs essentially pushes them together, causing the lower or inferior poles of the kidneys to touch and fuse together, forming what’s called a fibrous isthmus, fibrous because it’s composed of connective tissue.
The other theory involves a teratogenic event. Teratogenic meaning something that disrupts fetal development in some way. In this case it’s thought that the posterior nephrogenic cells, which are the cells that help to form part of the kidney, migrate and rendezvous in the wrong spot, and therefore again form an isthmus connecting the two kidneys, but this time since the isthmus is composed of kidney cells as opposed to connective tissue, it’s called a parenchymal isthmus.
Whichever one happens, now you’ve got this single, horseshoe-shaped kidney in the pelvis. During the 7th and 8th weeks, it tries to migrate up into the abdomen, but it literally hits a bit of a roadblock and hooks around the inferior mesenteric artery, which keeps it lower in the abdomen than normal.
Horseshoe kidney is actually a relatively common congenital disorder, affecting about 1 in 500 people, and is about twice as common in men. Most people with horseshoe kidney are asymptomatic, although it does predispose someone to certain complications like hydronephrosis, backup of urine due to an obstructed ureter.
Also, having a horseshoe kidney is associated with kidney stones, infection, and chromosomal disorders like Turner syndrome, trisomy 13, 18, and 21. Finally, it’s associated with an increased risk of kidney cancer, especially Wilm’s tumor and carcinoid tumor.
People with horseshoe kidney alone typically don’t need treatment and have a normal lifespan, although it’s important to keep in mind the abnormal structure of the horseshoe kidney if surgical intervention is needed for one of the complications or some other abdominal procedure.
Alright, quick recap, horseshoe kidney’s a congenital disorder where the kidneys fuse sometime during development, and as it migrates it gets hooked around the inferior mesenteric artery, keeping it lower in the abdomen. Thanks for watching, you can help us by donating on Patreon, subscribing to our channel, or telling your friends about us on social media.
Sources edit
First Aid USMLE Step 1 (text)
Pathoma (text/video)
http://emedicine.medscape.com/article/441510-overview#a7
http://emedicine.medscape.com/article/441510-treatment#d11
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1400255/
https://en.wikipedia.org/wiki/Kidney_development