Congenital malformations - Edocr

Chapter 40
Horseshoe Kidney
PRAVEEN KUMAR
INTRODUCTION
Horseshoe kidney is a common congenital anomaly
of the kidney which is characterized by an isthmus
connecting right and left kidney. The isthmus can
be a band of fibrous tissue or a rim of functional
renal parenchyma and crosses the mid-plane of
the body. While most horseshoe kidneys are fused
at the inferior pole, fusion of the superior pole
and of both poles (sigmoid kidney) have been
described in 5–10% of patients with horseshoe
kidney. 1 A classification of horseshoe kidney
proposed the following types: A (a)—fused at
the superior pole, A (b)—fused at the inferior
pole, B (a)—fused by fibrous tissue, B (b)—fused
directly and, B (c)—fused by mediators. 1 However,
this classification is not frequently used or
described by other authors.
EPIDEMIOLOGY
The reported prevalence of horseshoe kidneys
varies from 1 in 300 to 1 in 1800 but most reports
cite a prevalence of 1 in 400–500. 1–3 These estimates
are based primarily on data from patients
requiring renal evaluations and epidemiologic
postmortem studies. Based on data from three
large population based congenital malformation
registries from Europe and the United States,
Harris et al reported a much lower prevalence
range of 0.25–0.61 per 10,000 births. 4 It is likely
that asymptomatic cases of horseshoe kidneys
were not identified and contributed to the lower
incidence in this report. Tsuchiya et al screened
5700 healthy 1-month-old infants in Japan and
identified only one case of horseshoe kidney in
their population. 5 It is likely that the low incidence
was because only healthy infants with no
known malformations were included in this study.
Horseshoe kidney may be seen in as many as
20% of patients with trisomy 18 and 7% of cases
with Turner syndrome. However, these two studies
raise the possibility that the true prevalence
of horseshoe kidney may be lower than the previously
cited rate of 1 in 400. Overall a slight male
predominance has been reported.
EMBRYOLOGY
The horseshoe kidney results from fusion of the
two kidneys probably around the sixth week of
gestation. Initially the human kidneys lie close
to each other in the pelvis and ventral to the
sacrum. With the subsequent growth of the embryo,
the kidneys migrate cranially and rotate
medially almost ninety degrees to lie in their
adult position by about the ninth week. Abnormal
contact between the developing kidneys leads to
fusion. It has been proposed that a slight alteration
in the position of the umbilical or common
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