Sometimes during development the lower ends of the foetal kidneys fuse together, resulting in one u-shaped organ, rather than the normal pair of kidneys lying either side of the spine. This is known as a horse-shoe kidney, or ren arcuatus. The fusion usually takes place between weeks 7 and 9 of development. The horse-shoe kidney will be found lower down the trunk than the normal pair of kidneys, because the fused area – known as the isthmus – prevents the ascent of the kidney during development. The base of the right kidney, which is fractionally lower than the left, will normally sit at the level of the third lumbar vertebrae – approximately the height of the elbow when standing. The top of a horse-shoe kidney will usually only rise to this point.
Horse-shoe kidneys are often asymptomatic and cause no problems. However, the abnormal anatomy can often lead to other problems. Normally a tube called the ureter will drain urine from each kidney down to the bladder. The presence of the isthmus which has formed between the two kidneys can lead to partial or complete obstruction of the flow of urine. Sluggish urine flow from the kidneys, either from their aberrant orientation within the body, or via problems with the ureter, can increase the chances of kidney stones forming. Any of these things can result in urine being trapped inside the kidney, causing it to swell. This is known as hydronephrosis, which literally means ‘water inside the kidney’. This itself can lead to an increased risk of kidney infections. The horse-shoe kidney itself cannot be treated, but it is often possible to treat these symptoms. Horse-shoe kidney is also associated with an increased risk of polycystic kidney disease and renal cancer, especially Wilm’s tumour.
Approximately one in 500 children will be born with horse-shoe kidney, and the incidence is greater in males than females. It is associated with certain genetic disorders such as Turner’s syndrome, a condition where a girl is missing one (or part of one) of their two X chromosomes, and Trisomy 18 (also known as Edward’s Syndrome).