Horseshoe kidney is a relatively common (1 in 400 live births) congenital malformation of the kidney in which the kidneys are fused together. 90% of the time the fusion occurs at the lower poles (upper left photo) and the isthmus (the tissue connecting the two poles) is comprised of either renal parencyhmal tissue or fibrous tissue. In humans, renal development begins during the 4th gestational week, and presumably errors during this process can lead to horseshoe kidney. In the majority of cases, patients are asymptomatic; however, the anatomic abnormalities may be associated with a more complex developmental syndrome OR may result in urologic abnormalities (e.g. reflux nephropathy) that causes CKD. In addition, there is some evidence that some renal tumors (e.g., Wilms' Tumor) may occur with increased frequency in patients with these congenital abnormalities.
In contrast, crossed fused ectopia involves both fused kidneys which lie on one side of the spine, and the ureter of the crossed kidney crosses the midline in order to enter the bladder (lower left photo).