The term horseshoe kidney refers to the appearance of the fused kidney, which results from fusion at one pole. In more than 90% of cases, fusion occurs along the lower pole. Technically, the term horseshoe kidney is reserved for cases in which most of each kidney lies on one side of the spine. It includes symmetric horseshoe kidney (midline fusion) or asymmetric horseshoe kidney (L-shaped kidney). In the latter, the fused part, or isthmus, lies slightly lateral to the midline (lateral fusion). Horseshoe kidney is generally differentiated from crossed fused ectopia, in which both fused kidneys lie on one side of the spine, and the ureter of the crossed kidney crosses the midline to enter the bladder.
Horseshoe kidney is the most common renal fusion anomaly, the incidence being about 1 case per 400 persons worldwide. As reported, it can occur in a single member of a set of identical twins and in both members. At present, no clear evidence of a hereditary trait exists.
Complications of horseshoe kidney include the following:
- Ureteropelvic junction (UPJ) obstruction is a common complication, possibly because of the high insertion of the ureter.
- Recurrent infections occur because of urine stasis and associated vesicoureteric reflux.
- Recurrent stone formation related to UPJ obstruction or infection may occur.
- An increased risk of trauma to the isthmus exists because of its position anterior to the spine.
- Horseshoe kidney may pose problems for surgeons during abdominal surgery for other abdominal problems.
- Evidence indicates that an increased incidence of certain renal tumors is associated with horseshoe kidney.