Diagnostic Ultrasound - Abdomen and Pelvis

Horseshoe Kidney
TERMINOLOGY
Definitions
• Congenital anomaly in which kidneys are fused at their
lower poles in midline
○ In contradistinction to crossed fused ectopia (CFE),
where both kidneys are located on 1 side
IMAGING
General Features
• Best diagnostic clue
○ Kidneys on opposite sides with lower poles fused in
midline
• Location
○ Ectopic, lies lower than normal kidneys
– Isthmus caudally positioned at L3-L5
– Isthmus most often anterior to great vessels
□ Rarely, can be posterior, or less commonly in
between aorta and inferior vena cava (IVC)
• Morphology
○ 2 types of fusion
– Midline or symmetric fusion (90% of cases)
– Uncommonly can have lateral or asymmetric fusion
Ultrasonographic Findings
• Grayscale ultrasound
○ Kidneys often low-lying
– Renal pelvis frequently anteriorly oriented
○ Boomerang shape: Bent on sagittal imaging, with lower
poles pointed away from transducer
○ Presence of isthmus is defining feature
– Crosses midline anterior to spine and aorta, but
posterior to inferior mesenteric artery
– Can be difficult to visualize isthmus in subjects with
large body habitus, or if isthmus is composed of
fibrous tissue
– Transverse midline view of aorta for best visualization
○ Look for associated collecting system dilatation
(ureteropelvic junction obstruction), calculi, or signs of
infection
CT Findings
• CECT
○ Often incidentally detected
○ Define structural abnormalities
– Degree and site of fusion: Midline or lateral fusion
– Renal malrotation
– Collecting system abnormalities (e.g., hydronephrosis
secondary to ureteropelvic junction [UPJ] obstruction)
– Complication
□ Calculi
□ infection
□ Malignancy
○ Presence of enhancing tissue distinguishes functional
renal tissue in isthmus vs. fibrous tissue
• CTA
○ Useful in demonstrating variant arterial supply
○ Multiple renal arteries, including arteries arising from
vessels other than abdominal aorta
– e.g., common, external or internal iliac arteries,
inferior mesenteric artery
○ Isthmus located posterior to inferior mesenteric artery
MR Findings
• Similar utility as CT in defining structure as well as
associated abnormalities and complications, without
ionizing radiation
• MRA can be utilized to define arterial supply
Nuclear Medicine Findings
• Tc-labeled radiotracers are taken up by functional renal
tissue and thus can demonstrate horseshoe kidney
Imaging Recommendations
• Best imaging tool
○ US often sufficient but can be difficult to appreciate
isthmus
○ CT or MR best demonstrates horseshoe kidney in its
entirety
DIFFERENTIAL DIAGNOSIS
Renal Ectopia
• Kidney congenitally abnormal in position
• Crossed renal ectopia: 2 kidneys on same side of the body
(left kidney more often ectopic than right)
○ Fused CFE: 90%
– Kidneys on same side of spine; ureter of crossed
kidney crosses midline to insert into bladder at
contralateral trigone
○ Nonfused
– Kidneys on same side of spine, although without
fusion
– As in CFE, ureter of crossed kidney inserts at
contralateral trigone
○ Bilateral: Left and right kidneys arise on wrong side, both
ureters cross midline to insert into bladder
• Ipsilateral or simple ectopia
○ Kidney on same side of body as its ureter; most often
located in pelvis
Paraaortic Lymphadenopathy/Retroperitoneal Mass
• Soft tissue mass at midline anterior to spine
• Note normal appearance and location of bilateral kidneys,
as well as absence of typical reniform shape of
retroperitoneal tissue
PATHOLOGY
General Features
• Etiology
○ Proposed theory of fusion of nephrogenic blastomas at
4 weeks gestational age when still located in pelvis,
preventing appropriate ascent
– Abnormal flexion or growth of developing spine
brings nephrogenic elements together
– Isthmus cannot ascend due to inferior mesenteric
artery
○ Alternate theory that horseshoe kidney develops due to
abnormal migration of posterior nephrogenic
precursors, creating an isthmus
• Genetics
Diagnoses: Urinary Tract
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