Ectopic kidneys

The kidneys may develop anywhere along the urogenital ridge of the fetus and so, if they are not in a normal position, they should be searched for along the psoas muscles and down into the pelvis (see Fig. 21.10). Ectopic kidneys are more common on the left side, some 80% of them being found at, or below, the level of the iliac crest on the ipsilateral side.46 An ectopic kidney can be recognised by its overall similarity in shape, size and structure to normal kidneys, although like horseshoe kidneys (see below) ectopic kidneys are associated with malrotation and a predisposition to dilatation of the collecting system.47 However, pelvic kidneys may develop with unusual shapes and degrees of rotation and also may show some dilatation of the collecting system.48 These unusual features may make an ectopic kidney difficult to recognise as a kidney, especially if it is an unexpected finding and presents as a mass during an examination (Fig. 21.22A). The overall appearance may lead to confusion with a bowel tumour or lymph node mass, by the

Figure 21.21 Duplex collecting system. Image of the left kidney showing a column or bar of cortex separating the renal sinus echoes into two parts in a patient with duplication of the collecting system and upper ureters.

Unwary; colour Doppler is of value as it usually shows the vascular architecture compatible with renal vessels (Fig. 21.22B). Conversely the colon may lie in the vacant renal fossa and this may give a false impression of a normally situated kidney.49

The ectopic kidney usually lies on the correct side of the abdomen or pelvis; much more rarely, approximately 1 in 1000 births, it lies on the opposite side under a normally positioned kidney (crossed renal ectopia). The crossed, ectopic kidney is almost always fused


Figure 21.22 Ectopic kidney. A: Ectopic kidney in the right iliac fossa. B: Colour Doppler shows regular renal arrangement of vessels.


Figure 21.23 Crossed fused renal ectopia. A: A large (13.2 cm) reniform mass lies on the right psoas muscle. No kidney could be seen in the left abdomen. B: Another crossed fused ectopia in which the two components are easily distinguished.


With the other kidney (crossed fused renal ectopia) to produce a long, kidney-shaped flank mass, usually with two sinus echo complexes (Fig. 21.23).48 In crossed renal ectopia the ureter from the ectopic kidney crosses the midline to enter the bladder on the normal side.12

Pancake kidneys are fused ectopic kidneys, usually lying centrally in a low abdominal or pelvic position. The normal renal outlines are not clearly seen and distortions of the renal sinus may add to the difficulties in recognition, although the absence of normal kidneys in the renal fossae should alert the examiner to the possibility (Fig. 21.24).48 Again, colour Doppler is of value in demonstrating a renal pattern of vessels. If there is doubt over the nature of a possible ectopic kidney a DMSA isotope scan will confirm the presence of renal tissue but CT or arteriography may be required in a few cases for full assessment.

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