Overview

Some form of renal fusion occurs in about 1 out of every 1,000 individuals, with the most common being the horseshoe kidney. The fused kidney mass almost always contains two excretory systems and two ureters. The renal tissue may be evenly equidistributed between the two coeliacs or entirely on one side. Even in the latter case, the double ureteral orifices open into the intrinsic position of the bladder.

Aetiology

The fusion of the two posterior kidneys occurs in the early embryonic period, when the kidneys are still in the pelvis in a low position. As a result, they seldom rise to the high position to which normal kidneys should go, and may even remain in the pelvis. At this time, the kidney may be supplied by multiple blood vessels in this region (e.g., aorta, skeletal vessels, etc.). In patients with both ectopic and fused kidneys, 78% have extra-urinary malformations and 65% will show other genitourinary defects.

Pathologic Changes

Because the renal masses have fused early on, they do not rotate normally, so each renal pelvis is located on the anterior surface of the kidney. Consequently, the ureter would have to cross the isthmus of the horseshoe kidney or traverse the anterior surface of the fused kidney. At this point, one or more ectopic vessels can cause some degree of compression on the ureter and cause obstruction, and hydronephrosis and resulting infections are common. Vesicoureteral reflux is also often associated with renal fusion.

In a horseshoe kidney, the isthmus often connects the lower pole of each kidney, and each renal mass is lower than normal. The long axis of these masses is vertical, whereas the normal renal axis is tilted in an "eight" relative to the spine because it is aligned along the margin of the psoas major muscle. Rarely, the two renal masses are fused together and contain two renal pelvises and two ureters. Such a renal mass may be located in the midline, with the ureter opening in a fixed position (crossed ectopic kidneys with fusion).

Clinical manifestations

(1) Symptoms: most patients with fused kidneys are asymptomatic. However, some may develop ureteral obstruction. Gastrointestinal symptoms (renal intestinal reflexes) coolly resembling those of peptic ulcer, cholelithiasis, or appendicitis may also be present. If there is ureteral obstruction causing hydronephrosis or stone formation, the infection is easy to get.

(2) Physical signs: Physical examination is usually negative unless a kidney mass in an abnormal position is palpable. In those with horseshoe kidneys, it is possible to palpate a mass in front of the low lumbar vertebrae (isthmus). In crossed ectopic cases, a mass may be palpable in the coeliac or lower abdomen.

(3) Laboratory tests: Urinalysis is normal except for infection. Renal function is also normal, unless each fused renal mass is simultaneously diseased.

(4) X-ray: In horseshoe kidneys, the renal axes, if seen on plain films, are parallel to the spine, and sometimes the isthmus can be recognized. Plain film can also show a case of coeliac abdomen with a huge soft tissue mass while the other side of the kidney shadow is missing. If the renal parenchyma is still functional, the diagnosis can be made by excretory urography. The increased density of the renal tissue makes the location and shape of the kidney clearer, and the renal pelvis and ureter can be visualized on urography.

① In a horseshoe kidney, the renal pelvis is located on the anterior surface of the renal mass, whereas in a normal kidney, the renal pelvis is located on the medial aspect of the kidney. In horseshoe kidneys, the most diagnostic clue is that the lower pole renal calyces appear to be oriented toward the midline and are closer to the midline than the ureter.

② A crossed ectopic fused kidney shows 2 renal pelvises and 2 ureters, one of which must cross the midline in order to urinate in the intrinsic position of the bladder.

(iii) A pie-shaped or block-shaped kidney is centered in the pelvis (pelvic fusion kidney), but its ureter and renal pelvis can be visualized and can compress the bladder apex.

CT scans can clearly visualize the kidney but are not essential. For pelvic fusion kidneys or those located in the coeliac abdomen, plain radiographs with a ureteral catheter inserted will provide the first clue to the diagnosis. Retrograde imaging will show the location of the renal pelvis and changes due to the presence of infection or obstruction. Renal scintigraphy will show the renal mass and its contours as if it were an ultrasound image.

Complications

This is due to the high incidence of ectopic renal vessels and the arching of one or both of the ureters around or across the renal mass. Fused kidneys are prone to ureteral obstruction. As a result, hydronephrosis, stones and infections are common. A large fused kidney occupying the concave surface of the sacrum can also cause obstructed labor.

Differential Diagnosis

Separated kidneys that do not succeed in normal placement can be confused with horseshoe kidneys, which are aligned along the margins of the psoas major muscle, whereas horseshoe kidneys are aligned parallel to the spine, with their lower poles just anterior to the psoas major muscle, and the calyces of the isthmus of the horseshoe kidneys are oriented toward the midline and close to the spine. If a significant obstruction in one of the ureters prevents visualization of the kidney, renal pelvis, or a portion of the ureter, a fused or block kidney may be missed on excretory urography. Intravenous urography or retrograde urography can also show excretory channels in a renal mass.

Treatment

No treatment is usually necessary unless infection or obstruction occurs, and incision of the isthmus of the horseshoe kidney improves its drainage, and when one of the poles of the horseshoe kidney is not draining well, it may be removed.

Prognosis

The prognosis is good in most cases. When ureteral obstruction and infection occur, surgical means should be used to improve renal drainage, which also makes antimicrobial therapy more effective.