Article Type : Clinical Image
Authors : Choayb S, El Harras Y, Allali N, Chat L and El Haddad S
Keywords : Horseshoe kidney; Parenchymal or fibrous isthmus
Horseshoe kidney is the most frequent congenital
fusion abnormality of the urinary system. It associates malrotation, ectopia,
and vascular changes. It’s usually the lower pole of both kidneys that are
merged by a parenchymal or fibrous isthmus. An inverted horseshoe kidney is
called when the isthmus connects the superior pole.
Horseshoe kidney is the most frequent congenital
fusion abnormality of the urinary system. It associates malrotation, ectopia,
and vascular changes. It’s usually the lower pole of both kidneys that are
merged by a parenchymal or fibrous isthmus. An inverted horseshoe kidney is
called when the isthmus connects the superior pole [1].
The classic theory implies that during the 4th week of gestation, while the kidneys are still in the pelvis, their lower poles are connected and fuse in the midline leading to a horseshoe kidney with fibrous isthmus [1]. Normally around the 7th and 8th week of gestation both kidneys ascent at the level of the L2 vertebral body and rotate 90° permitting hilar turning from anterior to medial. Nevertheless, ascension and rotation are prevented by the inferior mesenteric artery. And thus, the lower poles point toward each other, and the hilar remains anteriorly directed [2].
Figure: Transverse sonogram showing renal fusion by a connecting isthmus.
A second theory implies an abnormal migration of the
posterior nephrogenic cells that forms a parenchymal isthmus [1]. Median fusion
of the kidneys in a symmetrical position on either side of the spine forms a
U-shaped horseshoe kidney. On the other hand, a lateral fusion between vertical
and horizontal kidneys with an isthmus laterally to the midline forms an
L-shaped horseshoe kidney.
This condition is usually asymptomatic, but in case of
hydro nephrosis, lithiasis, or infection the most common symptom is intense
abdominal pain radiating to a low lumbar position. Gastrointestinal
manifestations such as nausea and vomiting may also be associated [1].
HSK may be complicated by ureteropelvic junction
obstruction, lithiasis especially large staghorn stones, infection secondary to
urine stasis, and renal tumours secondary to chronic infection.
On ultrasound, detecting the isthmus is the finding
making the diagnosis. It can be complicated in the case of a fibrous isthmus or
a large body habitus, and it should not be mistaken for a retroperitoneal mass.
On CT, the imaging of enhancing tissue distinguishes functional from the
fibrous isthmus.
Both CT and MRI provide useful information on the
kidney anatomy and vascular supply that is commonly abnormal with multiple
accessory vessels. It also allows detailed information about the relationship
with other structures, and the associated pathological conditions [1,2].
The authors declare that there is no conflict of
interests regarding the publication of this paper.
All authors contributed equally to this work