| disease | Renal Fusion |
Approximately 1 in 1000 individuals exhibit some form of renal fusion, with horseshoe kidney being the most common. The fused renal mass almost always contains two excretory systems and two ureters. The renal tissue may be evenly divided between the two flanks or entirely on one side. Even in the latter case, the double ureteral orifices open at their normal positions in the bladder.
bubble_chart Etiology
The fusion of the two metanephros occurs early in the embryonic stage when the kidneys are still in the pelvic cavity and positioned very low. As a result, they rarely ascend to the high position where normal kidneys should be, and may even remain within the pelvic cavity. At this time, the kidney can receive blood supply from multiple vessels in this area (such as the main stirred pulse, iliac vessels, etc.). Among patients with both ectopic and fused kidneys, 78% have extra-urinary system malformations, and 65% will exhibit other genitourinary system defects.
bubble_chart Pathological Changes
Due to the early fusion of the renal masses, normal rotation does not occur, so each renal pelvis is located on the anterior surface of the kidney. Consequently, the ureter must cross the isthmus of the horseshoe kidney or traverse the anterior surface of the fused kidney. At this point, one or more ectopic vessels may cause some degree of compression on the ureter, leading to obstruction, hydronephrosis, and the resulting infections. Vesicoureteral reflux is also often associated with renal fusion.
In a horseshoe kidney, the isthmus usually connects each renal root of the nose, and each renal mass is lower than the normal position. The long axis of these renal masses is vertical, whereas the normal renal axis is inclined in a "V" shape relative to the spine due to alignment along the psoas muscle margin. In rare cases, the two renal masses fuse into one, containing two renal pelvises and two ureters. Such a renal mass can be located at the midline, with the ureters opening at fixed positions (combined with crossed fused ectopic kidney).
bubble_chart Clinical Manifestations(1) Symptoms: Most patients with fused kidneys are asymptomatic. However, some may develop ureteral obstruction. Gastrointestinal symptoms resembling peptic ulcer, gallbladder stone, or appendicitis (renal-intestinal reflex) may also occur. If hydronephrosis or stone formation occurs due to ureteral obstruction, infection is more likely.
(2) Signs: Physical examination is usually negative unless an abnormally located renal mass is palpable. In cases of horseshoe kidney, a mass (isthmus) may be palpable anterior to the lower lumbar spine. In cases of crossed ectopia, a mass may be palpable in the flank or lower abdomen.
(3) Laboratory tests: Urinalysis is normal except in the presence of infection. Renal function is also normal unless each fused renal mass is simultaneously affected.
(4) X-ray examination: In horseshoe kidney, if the renal axis is visible on a plain film, it is parallel to the spine, and sometimes the isthmus can be identified. A plain film may also show a large soft tissue mass in one flank with no renal shadow on the other side. If the renal parenchyma is still functional, excretory urography can confirm the diagnosis. The increased density of renal tissue makes the position and shape of the kidney clearer, and urography can also show the renal pelvis and ureter.
① In 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 side of the kidney. For horseshoe kidney, the most diagnostic clue is that the renal calyces at the root of the nose point toward the midline and are closer to the midline than the ureter.
② Crossed ectopic fused kidney shows two renal pelvises and two ureters, one of which must cross the midline to empty into the bladder at its normal position.③ A cake- or block-shaped kidney located in the pelvis (pelvic fused kidney) may show its ureter and renal pelvis, and may compress the top of the bladder.
CT scanning can clearly delineate the outline of the kidney but is not essential. For pelvic fused kidney or those located in the flank, a plain film with a ureteral catheter inserted will provide the first clue to the diagnosis. Retrograde urography will show the position of the renal pelvis and changes due to infection or obstruction. Renal scintigraphy can show the renal mass and its outline, similar to ultrasound imaging.
bubble_chart Treatment Measures
Generally, no treatment is required unless infection or obstruction occurs. Incising the isthmus of a horseshoe kidney can improve its drainage. When one pole of the horseshoe kidney has poor drainage, it can be removed.
The prognosis for most cases is generally favorable. When ureteral obstruction and infection occur, surgical intervention should be employed to improve renal drainage, which also enhances the effectiveness of antimicrobial therapy.
Due to the high incidence of ectopic renal vessels and one or both ureters arching around or crossing the renal mass, fused kidneys are prone to ureteral obstruction. Consequently, hydronephrosis, stones, and infections are more common. A large fused kidney occupying the sacral concavity can also lead to difficult delivery.
A separated kidney that fails to be normally positioned can be confused with a horseshoe kidney. They are arranged along the edge of the psoas muscle, while the horseshoe kidney is parallel to the spine, with its root of the nose located in front of the psoas muscle. The calyces of the isthmus of the horseshoe kidney face the midline and are close to the spine. If there is a significant obstruction in the ureter causing a part of the kidney, renal pelvis, or ureter to not be visualized, it may lead to a diagnosis of fistula disease in fused or lump kidneys during excretory urography. Intravenous urography or retrograde urography can also reveal the excretory pathways within the renal mass.
