The horseshoe kidney is the most common form of renal fusion.
It consists of two distinct functioning kidneys on each side of
the midline, connected at the lower poles by an isthmus of functioning
renal parenchyma or fibrous tissue that crosses the midline of
Horseshoe kidney occurs in a range from 1 out of 400 live births
to 1 out of 800 live births and in over 90% of cases the renal
fusion is at the lower poles.
The most common associated finding in horseshoe kidney is ureteropelvic
junction (UPJ) obstruction, which occurs in up to 35% of patients..
It causes the majority of problems.
The prevalence of stones in the horseshoe kidney ranges from 20-60%.
Stone disease is thought to be due to the associated hydronephrosis
or UPJ obstruction that causes stasis of urine.
Urinary stasis and stone disease also predispose the horseshoe
kidney to infection. Ascending infection from vesicoureteral reflux
is another cause of infection in the horseshoe kidney.
Certain cancers are more frequent in the horseshoe kidney : The
incidence of renal cell cancer in the horseshoe kidney is no different
from that of the normal kidney. But the incidence of Transitional
cell cancer, sarcoma, Wilms tumors, carcinoid tumors also is higher
in the horseshoe kidney.
Nearly one third of patients with a horseshoe kidney remain asymptomatic,
and the horseshoe kidney is an incidental finding during Sonographic
examination. Physical examination may reveal a midline lower-abdominal
Symptoms, when present, usually are due to obstruction, stones,
or infection. In children, urinary tract infection is the most
common presenting symptom.
In the horseshoe kidney, the kidneys lie one on each side of the
abdomen but their lower poles are fused by a connecting band of
renal tissue, or isthmus, which lies anterior to the aorta and
inferior vena cava. The kidneys tend to be rotated and lie with
their lower poles medially.
The isthmus joining the two kidneys is often hidden by bowel gas
and thus diagnosis of horseshoe kidney is easily missed on sonography,
though the kidney may be noted to have more vertical axes than
normal. The isthmus should be suspected when the axis of the kidney
is distorted and the lower poles of the kidneys are hard to image
sonographically. It should also be suspected on longitudinal scans
of the aorta when an oval hypoechoic mass is seen anterior to
the aorta. It appears as a variably thick band of renal tissue
(or rarely as a thin fibrous band) extending from both lower poles
to connect anterior to the aorta below the level of the inferior
The sonographer should be suspicious of a horseshoe kidney when
die lower poles of the kidneys cannot be clearly outlined, particularly
when both kidneys look a little smaller than expected for age.
Fusion can take other forms, including an L shape, where one kidney
lies horizontally across the midline; crossed ectopia, where both
kidneys lie on the same side; H-shaped fusion of the hilar regions;
and complete fusion to form a 'cake'-shaped solitary kidney.
It may be difficult to visualize the isthmus due to bowel gas
anterior to it but a horseshoe kidney should always be suspected
when the operator is unable to identify the lower poles of the
kidneys confidently. When the isthmus can be seen, it is important
not to confuse it with other abdominal masses, such as lymphadenopathy.
CT is occasionally performed because of this but normally clarifies
* Medical therapy: The horseshoe kidney is susceptible to medical
renal disease. These diseases, if present, are treated as indicated.
* Surgical therapy: Surgical treatment is based on the disease
process and standard surgical indications. The anomalous vascular
supply to the kidney should be kept at the forefront of the surgeon's
mind when planning the surgical approach.
Obstruction of the UPJ is usually treated with open pyeloureteroplasty
Kidney stones can be treated with ESWL, endoscopy, or open surgery.
Outcome and prognosis :
The presence of the horseshoe kidney alone does not affect survival.
As mentioned previously, the horseshoe kidney does have a higher
propensity to become diseased. Therefore, survival is dependent
on the disease process that the affected horseshoe kidney may
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