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THE PAEDIATRIC ABDOMEN 223
When there is a strong clinical suspicion of an
ectopic ureter an intravenous urogram or MR
urogram will be required to identify a duplex
kidney and site of ureteric insertion.
Renal fusion and ectopia
The horseshoe kidney is the most common form of
renal fusion, in which the lower poles of the kidneys
are fused with a central isthmus or ‘bridge’ across
the front of the spine (Fig. 9.6). The isthmus frequently
lies behind gas-filled bowel and can be difficult
to detect. The sonographer should be suspicious
of a horseshoe kidney when the lower poles of the
kidneys cannot be clearly outlined, particularly when
both kidneys look a little smaller than expected for
age. Always ensure you see the outline of the lower
poles clearly by turning the child prone or by scanning
coronally through the side if necessary.
A dimercaptosuccinic acid (DMSA) scan may
demonstrate the isthmus or bridge of renal tissue
(when the ultrasound scan is equivocal) but only if
it is functioning. In some cases the bridge is composed
of non-functioning, fibrous tissue.
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.
Ectopic kidneys occur most frequently in the
pelvis (Fig. 9.7). In rare cases the kidney may be
situated in the thorax.
Ectopic and horseshoe kidneys are often associated
with a degree of malrotation of the kidney.
This can be associated with a degree of obstruction
at the pelviureteric junction, and predispose to the
development of renal calculi.
RK
A
B
Figure 9.6 (A) The lower pole of this right kidney (RK)
could not be successfully demonstrated. Horseshoe kidney
was suspected, and confirmed on coronal scanning planes.
(B) Longitudinal section (LS) in the midline shows the
renal isthmus of a horseshoe kidney anterior to the aorta.
(C) Transverse section (TS) through the lower abdomen
demonstrates the isthmus anterior to the spine.
C