Radiography
Findings
Plain radiographs may show low-lying renal outlines with an altered renal axis. Usually, the kidneys follow the axis of the psoas muscles, with the lower poles lying at a more lateral position than the upper poles. In horseshoe kidney, this axis is reversed, with the lower poles lying closer to the spine. With plain radiographs alone, however, the degree of confidence in this finding is usually low.
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 symmetric, 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. A renal pelvis is often extrarenal and large.
The degree of malrotation has been associated with the degree of fusion. If the isthmus is narrow, the kidneys are usually less malrotated, with the pelvis lying anteromedially in its near-normal position. In cases of a wide isthmus, the renal pelves lie anteriorly or laterally. 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.
The L-shaped kidney's lateral fusion anomaly can also be readily appreciated on IVUs. In this anomaly, one kidney has a relatively vertical position, while the other kidney is relatively horizontal.
Degree of Confidence
The degree of confidence associated with IVU for the diagnosis of horseshoe kidney is high in most cases. Sometimes, ascertaining the diagnosis on the basis of IVU alone is difficult, and further workup — using CT scanning, MRI, or scintigraphy — may be necessary. IVU is also a good modality for discovering associated findings, such as the presence of stones, scarring, and duplex collecting systems.
False Positives/Negatives
Sometimes, malrotated kidneys can be mistaken for horseshoe kidneys. IVU does not aid in the differentiation of a parenchymal isthmus from a fibrous isthmus.
Computed Tomography
Findings
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.
Degree of Confidence
The degree of confidence associated with the use of CT scanning for diagnosing horseshoe kidney is high. However, associated stone disease may be missed with contrast-enhanced CT scanning alone, and nonenhanced CT scanning, ultrasonography, or plain radiography may be required.
Magnetic Resonance Imaging
Findings
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.
Degree of Confidence
The degree of confidence connected with the use of MRI for the diagnosis of horseshoe kidney, as well as for the purpose of defining associated structural findings, is high. However, associated small stones may be missed on MRIs.
Ultrasonography
Findings
Ultrasonography can be useful for diagnosing horseshoe kidney. 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.
Various findings, such as a curved configuration of the lower poles, elongation of the lower poles, and poorly defined lower poles, suggest the presence of horseshoe kidney. Other associated findings, such as stones, hydronephrosis, and cortical scarring, are reliably depicted on sonograms. Ultrasonography has also been useful in the diagnosis of horseshoe kidney in utero.
Degree of Confidence
The degree of confidence depends on the visualization of the isthmus and the proof of its continuity with the lower poles. In many patients, especially those with a large body habitus, overlying bowel gas makes the acquisition of adequate scans difficult for technical reasons. In cases in which the continuity of the poles with the isthmus cannot be clearly demonstrated, the degree of confidence is low.
False Positives/Negatives
Occasionally, a midline soft-tissue mass over the spine (eg, a lymphomatous mass) may be mistaken for an isthmus, especially if it extends laterally to the kidneys. If a horseshoe kidney has a thin, fibrous isthmus, ultrasonography may produce a false-negative result.
Nuclear Imaging
Findings
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.
Many reports of horseshoe kidney exist. This condition is incidentally diagnosed on bone scans,99m Tc-labeled red blood cell studies, or other nuclear medicine studies obtained for reasons other than the evaluation of horseshoe kidney. The use of mercaptoacetyltriglycine (MAG-3) with diuresis is helpful in differentiating nonobstructed parts from obstructed parts of the collecting systems.
Degree of Confidence
Horseshoe kidney can be confidently diagnosed with scintigraphy, which reveals the functioning parenchymal isthmus.
False Positives/Negatives
Occasionally, the hot isthmus of a horseshoe kidney can be mistaken for spinal metastatic disease.
Angiography
Findings
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. Angiograms may show 1, 2, or 3 renal arteries in either of the fused kidneys and can reveal large variations in the blood supply of the isthmus, as discussed in Anatomy. In cases of associated renal tumors, angiography is used to evaluate tumor vascularity. Angiography is occasionally performed to check renal artery stenosis in hypertensive patients who have horseshoe kidney.
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| Overview: Horseshoe Kidney |
 Imaging: Horseshoe Kidney |
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References
Banerjee B, Brett I. Ultrasound diagnosis of horseshoe kidney. Br J Radiol. Oct 1991;64(766):898-900. [Medline].
Bauer SB, Perlmutter AD, Retik AB. Anomalies of the upper urinary tract. In: Walsh, PC, ed. Campbell's Urology. 6th ed. Philadelphia, Pa: WB Saunders; 1992:1357-1401.
Boatman DL, Cornell SH, Kolln CP. The arterial supply of horseshoe kidneys. Am J Roentgenol Radium Ther Nucl Med. Nov 1971;113(3):447-51. [Medline].
Boatman DL, Kolln CP, Flocks RH. Congenital anomalies associated with horseshoe kidney. J Urol. Feb 1972;107(2):205-7. [Medline].
Cook WA, Stephens FD. Fused kidneys: morphologic study and theory of embryogenesis. Birth Defects Orig Artic Ser. 1977;13(5):327-40. [Medline].
Decter RM. Renal duplication and fusion anomalies. Pediatr Clin North Am. Oct 1997;44(5):1323-41. [Medline].
Evans WP, Resnick MI. Horseshoe kidney and urolithiasis. J Urol. May 1981;125(5):620-1. [Medline].
Fernbach SK, Davis TM. The abnormal renal axis in children with spina bifida and gibbus deformity--the pseudohorseshoe kidney. J Urol. Dec 1986;136(6):1258-60. [Medline].
Grainger R, Murphy DM, Lane V. Horseshoe kidney--a review of the presentation, associated congenital anomalies and complications in 73 patients. Ir Med J. Jul 1983;76(7):315-7. [Medline].
Göksel O, Cinar B, Kömürcü G, et al. Surgical treatment of abdominal aortic aneurysms associated with horseshoe kidney. Vascular. Jan-Feb 2006;14(1):27-31. [Medline].
Koff SA, Wise HA. Anomalies of the kidneys. Adult Ped Urol. 1996;3:2178-93.
Kolln CP, Boatman DL, Schmidt JD. Horseshoe kidney: a review of 105 patients. J Urol. Feb 1972;107(2):203-4. [Medline].
Krishnan B, Truong LD, Saleh G. Horseshoe kidney is associated with an increased relative risk of primary renal carcinoid tumor. J Urol. Jun 1997;157(6):2059-66. [Medline].
Nino-Murcia M, deVries PA, Friedland GW. Congenital anomalies of the kidneys. Clin Uroradiol. 2000;1:690-763.
Pitts WR Jr, Muecke EC. Horseshoe kidneys: a 40-year experience. J Urol. Jun 1975;113(6):743-6. [Medline].
Schacht MJ, Sakowicz B, Rao MS. Intermittent abdominal pain in a patient with horseshoe kidney. J Urol. Oct 1983;130(4):749-51. [Medline].
Segura JW, Kelalis PP, Burke EC. Horseshoe kidney in children. J Urol. Aug 1972;108(2):333-6. [Medline].
Stein RJ, Desai MM. Management of urolithiasis in the congenitally abnormal kidney (horseshoe and ectopic). Curr Opin Urol. Mar 2007;17(2):125-31. [Medline].
Viola D, Anagnostou T, Thompson TJ, et al. Sixteen years of experience with stone management in horseshoe kidneys. Urol Int. 2007;78(3):214-8. [Medline].
Whitehouse GH. Some urographic aspects of the horseshoe kidney anomaly-a review of 59 cases. Clin Radiol. Jan 1975;26(1):107-14. [Medline].
Further Reading
Keywords
midline renal fusion, L-shaped kidney, fusion anomaly, fusion anomalies, fused kidneys, symmetric horseshoe kidneys, asymmetric horseshoe kidneys, isthmus, lateral renal fusion, crossed fused ectopia
