The term ‘Horseshoe Kidney’ is the most common variant of a group of fusion abnormalities involving the urogenital system with incidence being 1 in 400 live births [1]. The fusion occurs in utero, around the 4th week of gestation and in more than 90% percent of the cases occurs along the lower pole, and is more common in males at a ratio of 2:1. The fused kidney tends to be in an anatomically lower position than normal due to hindrance of ascend by the inferior mesenteric artery. About one-third of the cases are asymptomatic with recurrent urinary tract infection and stones being the most common causes of morbidity in symptomatic patients.The horseshoe kidney can be associated with a variety of other congenital anomalies including vertebral, anorectal, tracheal, and esophageal malformations. Other fusion anomalies may be encountered including a wide variety of crossed-fused ectopic kidneys, the disc (pancake) kidney, and pelvic lump (cake) kidney. Patients with these types of kidneys suffer similar complications [1].

Complications:

• Ureteropelvic junction (UPJ) obstruction due to the high insertion of the ureter is commonly encountered.
• Recurrent infection.
• Recurrent stone formation, due to the aforementioned factors (eg. Obstruction and infection)
• Increased risk of certain neoplasm’s
• Increased risk of trauma to the isthmus secondary to the position anterior to the spine
• Difficulty for surgeon during abdominal surgery

Associated Anomalies:

Horseshoe kidney can be isolated or associated with other congenital anomalies of the urogenital, gastrointestinal, neurologic, and skeletal systems.

Radiologic Diagnosis:

Intravenous urography (IVU), ultrasound, magnetic resonance imaging (MRI), computed IVU usually reveals the classic findings associated with horseshoe kidney. Findings on the initial tomogram may be deceptive because of the exclusion of the anteriorly lying isthmus. Renal axis abnormalities are confirmed, as seen on the plain radiographs. In midline fusion, the kidneys are symmetrical, with the lower pole calyces lying closer to or actually overlying the spine. The lower calyces are usually medially rotated, and they may actually lie medial to the ureters. Some degree of malrotation of the kidneys is usually present. Associated UPJ obstruction may be present because of the higher ureteric insertion point, which leads to delayed pelvic emptying. Ureters may have the so-called flower-vase appearance, in which the upper ureters diverge laterally over the isthmus and then converge inferiorly.[2]Contrast-enhanced CT scanning has a high degree of accuracy in defining the structural abnormalities of horseshoe kidney, including the degree and site of fusion, the degree of malrotation, associated renal parenchymal changes (eg, scarring, cystic disease), and collecting system abnormalities (eg, duplex system, hydronephrosis). It can also be used to differentiate a parenchymal isthmus from a fibrous isthmus and to show the relation of the isthmus to surrounding structures.

Although routine CT scanning may show the variant arterial supply, this is better defined with CT angiography scanning with 3-dimensional reconstruction and volume rendering. In cases of neoplasm associated with horseshoe kidney, the use of 3-dimensional, multisection helical CT scanning also has been advocated, because it further clarifies the structural details. CT angiography is useful for pre-surgical planning to know the vascular anatomy. Since horseshoe kidneys are frequently the site of obstruction, infection and calculus formation, it is important to know the vascular anatomy of horseshoe kidneys along with isthmus [3].

MR: MRI has an advantage in depicting structural details because of its ability to permit multiplanar imaging, but it is more costly than other examinations. However, an added advantage may be obtained by using MR angiography to delineate the vascular anatomy. MRI is probably the best modality to use in evaluating the extent of renal tumors associated with horseshoe kidney. However, associated small stones may be missed on MRIs.

Ultrasonography

To establish the diagnosis, the most important ultrasonographic findings are the presence of the isthmus and its continuity with the lower poles. Other features, such as malrotation and an altered renal axis, may be difficult to assess with ultrasonography. In cases in which the isthmus is composed of only a thin, fibrous band, this midline soft tissue may not be seen. Other associated findings, such as stones, hydronephrosis, and cortical scarring, are reliably depicted on sonograms. [4]

Nuclear Imaging

Scintigraphy best demonstrates the fusion if the isthmus consists of functioning parenchymal tissue, because this imaging modality depends not only on the structure of the tissue but also on the function of the tissue. Technetium-99m (99m Tc)–labeled dimercaptosuccinic acid (DMSA) can be used to define the fused segments, as well as the altered axis of both kidneys.

Angiography

Angiography is not normally performed to diagnose horseshoe kidney, but it is performed to evaluate the vascular anatomy and its variations in a presurgical setting. It is rarely used these days due to availability of CT angiography.