Horseshoe kidney
Anna-Maria Nahm and
Eberhard Ritz
Sektion Nephrologie, Klinikum der Universität Heidelberg, Heidelberg, Germany
Correspondence and offprint requests to:
Prof. E. Ritz, Sektion Nephrologie, Med. Universitatsklinik, Bergheimer Str. 56a, D-69115 Heidelberg Germany.
The horseshoe kidney is one of the most frequent malformations of the urogenital tract. During normal development, the kidneys rotate in such a way that the ureters leave the kidney from its medial aspect and the axes of the kidneys diverge (Figure 1a
); the horseshoe kidney is malrotated, i.e. the normal rotation of the kidney is incomplete, so that the ureters leave the kidney from its ventral aspect and the longitudinal axes of the kidneys converge (Figure 1b
). Because the lower poles of horseshoe kidneys extend ventro-medially they can usually not be properly delineated. As the kidney cannot be visualized in its entirety, one common error is the diagnosis of a small kidney. The fused lower poles are connected by an isthmus (bridge), consisting either of renal parenchyma or fibrous tissue, which lies ventrally to the aorta. This constellation often leads to the erroneous diagnosis of a retroperitoneal tumour, particularly lymphoma.

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Fig. 1. (a) Normal position of the kidneys with divergent axes and medial exit of the ureters. (b) Horseshoe kidney in deep lumbar position with convergent axes and ventral exit of the ureters.
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The position of the fused kidneys can be anywhere from the normal lumbar location to the pelvis.Special points are: (i) often minimal dilatation of the pelvicalyceal system, but seldom hydroureter or hydronephrosis secondary to pelviureteral stenosis or narrowing of the ureter where it crosses the isthmus; (ii) renal stone formation; (iii) accessory renal arteries or abnormal arterial supply, e.g. from the mesenteric artery. When should a horseshoe kidney be suspected?(i) When the lower pole cannot be properly delineated.
(ii) When the renal axes converge and/or,
(iii) when the kidney(s) is (are) abnormally small for unknown reasons.
(iv) When there is a `preaortic mass'.
The diagnosis is missed if ultrasonography is only performed with the patient in prone position. This is a strong argument to start the examination of the kidneys with the patient in supine position and then in the left/right side position. This position permits to scan the kidney from a ventral, especially ventro-lateral, lateral and dorsal direction. Examples are given in Figure 2
.

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Fig. 2. (a) Small right kidney, abnormal direction of the longitudinal axis, lower pole indistinct and not clearly delineated. (b) Same features, left kidney. (c) Ultrasonographic scan in a lower and more ventral plane. Note that the lower pole of the kidney extends towards the aorta. (d) This is more clearly visible in this plane which shows an anterior mass to the aorta connecting the right and left kidneys.
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Teaching point
Think of the possibility of horseshoe kidneys whenever you diagnose:
(i) unexpectedly small kidneys and/or
(ii) a preaortic mass.
References
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Cochlin D, Dubbins P, Goldberg B, Archie A. Urogenital UltrasoundA Text Atlas, Chapman & Hall: 1994
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Koeppen-Hagemann I, Ritz E. Nierensonographie. Thieme, Stuttgart: 1992
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Lange S. Lehratlanten der radiologischen DiagnostikNiere und ableitende Harnwege. Thieme, Stuttgart; 1993
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Rettenmaier G, Seitz K. Sonographische Differentialdiagnostik, Bd. 1, edition medizinVHC, Weinheim: 1990