Horseshoe kidney
Horseshoe kidney is the most common of all renal fusion anomalies,
accounting for 90% of all such anomalies,
and occur in approximately 0.25% of the population,
with a predilection for males in the order of approximately 2:1 [2].
Embryology
During ascent towards the renal fossa,
the foetal kidneys cross the umbilical arteries.
Any aberration in position of the umbilical arteries may result in fusion of the bilateral nephrogenic blastemas.
When this fusion occurs at the lower poles of the kidneys,
a U-shaped single kidney is formed – resembling the shape of a horseshoe.
Imaging Findings
Horseshoe kidneys may be found anywhere along the path of normal renal migration that occurs during foetal development.
They usually lie anterior to the great vessels (the aorta and inferior vena cava).
However,
cases have been described where they are positioned posterior to,
or in between the great vessels [2].
The isthmus of a horseshoe kidney consists of either functional renal parenchyma or fibrous tissue,
and is most often located inferior to the origin of the inferior mesenteric artery [2]. Fig. 2
Blood supply is variable,
with up to 30% of cases receiving blood from a single renal artery to each kidney [2].
For the remainder of cases,
there may be duplicate or triplicate renal arteries to one or both kidneys,
whilst the isthmus may receive blood from branches of the renal arteries or from branches arising from other nearby major vessels,
such as the aorta,
inferior mesenteric artery and iliac arteries [2].
The inferior pole of each kidney points medially,
which results in the ureters passing anterior to the isthmus.
This abnormality of renal axis orientation leaves the pelvicalyceal system of horseshoe kidneys prone to obstruction,
which results in a higher risk of infection,
hydronephrosis and calculus formation.
Key Points
- A single horseshoe-shaped kidney
- Results from fusion of the lower poles
- Located anywhere along the path of normal renal migration,
but usually lower than normal,
ascent impeded by the inferior mesenteric artery
- Variable blood supply
Pancake kidney
Pancake kidney is a rare fusion anomaly that occurs in approximately 0.04% of the population [3].
Also known as ‘cake kidney’,
‘disc kidney’,
‘lump kidney’ or ‘fused pelvic kidney’,
the defining characteristics are that of a single fused renal mass that lies within the pelvis. Fig. 3
Embryology
The anomaly results from fusion of both nephrogenic blastemas with early arrested renal migration during foetal development.
Migration is arrested whilst the fused renal mass is still within the pelvis during embryogenesis,
preventing the normal cephalad ascent of the kidneys.
During this ascent,
the blood supply is derived from nearby major arteries and is re-established progressively as the renal masses reach their final position.
Hence,
in pancake kidney,
the blood supply reflects the fact that renal migration has been stopped.
The single renal mass receives blood from branches of nearby major arteries,
such as the lower abdominal aorta or common iliac arteries,
which increases the risk of haemorrhage in cases of pelvic trauma.
Imaging Findings
The shape of a pancake kidney is non-reniform,
lobulated and often flattened.
Commonly two short ureters are present,
which arise from an anteriorly positioned collecting system and enter the bladder in a normal relationship; however,
cases have been described where only a single ureter is present [3].
A pancake kidney may be located in the presacral or prevertebral space,
or at the level of the aortic bifurcation.
Concomitant anomalies in other extrarenal organs such as: tetralogy of Fallot,
vaginal absence,
sacral agenesis and spina bifida have been reported in many diagnosed cases of pancake kidney [3].
This highlights the importance of screening for such associated abnormalities when the diagnosis is made.
Key Points
- A single fused renal mass in the pelvis
- Results from abnormal fusion of the kidneys and failure to ascend
- Blood supply from foetal vessels originating from local major arteries
- Concomitant congenital abnormalities in other organs
Cross fused ectopia
Cross fused ectopia is the second most common fusion anomaly after horseshoe kidney,
with an estimated incidence of up to 0.08% [4].
It is both a fusion and ectopic anomaly characterised by one kidney that has crossed the midline and fused with the kidney on the opposite side.
Embryology
The aetiology of cross fused ectopia is not exactly known.
Proposed mechanisms include: effect of teratogens,
a wandering ureteral bud causing nephrogenesis in an atypical location,
abnormally located umbilical arteries hindering the normal cephalad migration of the kidneys causing one to cross the midline along the path of least resistance,
and aberrant development of the caudal end of the foetus with malrotation [4].
Imaging Findings
The most common configuration is that the left kidney is ectopic and located on the right,
with fusion of it’s upper pole to the lower pole of the normally positioned right kidney [4].
As a result,
the ureter arising from the ectopic kidney crosses the midline and enters the bladder in a normal relationship. Fig. 4
The arterial supply and venous drainage of this anomaly is usually grossly abnormal with a variety of possibilities.
The ectopic kidney usually receives blood from a vessel on the ipsilateral side,
however cases have been described where the arterial supply originates on the contralateral side.
There can be up to a total of six major arteries supplying the fused renal mass,
with 25% of them receiving blood from branches of the upper abdominal aorta and the remainder of cases receiving blood from branches of either the lower abdominal aorta or the iliac arteries [4].
As with other fusion and ectopic abnormalities,
there tends to be a higher risk of developing hydronephrosis,
renal calculi and infection.
Such complications may be the reason a patient is imaged and the diagnosis made,
however a significant proportion of cases are asymptomatic and therefore diagnosed incidentally during imaging for other indications.
Key Points
- A single fused renal mass located on one side of the body
- Two ureters,
with one crossing the midline
- Blood supply grossly abnormal
Renal Agenesis
Renal agenesis is the absence of renal tissue due to failure in embryogenesis,
and can occur unilaterally or bilaterally.
Bilateral renal agenesis is a fatal condition that may result in miscarriage or death in the early neonatal period.
It is one of the causes of oligohydramnios,
which results in Potter sequence.
Unilateral renal agenesis is also known as solitary kidney and has an incidence of approximately 1 in 1000 to 1 in 2000 newborns [5].
Embryology
The aetiology in many cases of renal agenesis is currently unknown,
but is thought to be multi-factorial.
An early vascular insult to the developing ureteric bud has been proposed.
Embryologically,
renal agenesis results from a failure of the proper development of the metanephros resulting in complete absence of a renal structure.
Imaging Findings
In unilateral renal agenesis,
the contralateral kidney is often hypertrophic as a compensatory mechanism.
There is usually no renal vein,
artery or ureter on the affected side,
although in some cases a fibrotic cord may be seen in lieu of the ureter.
This cord will usually end ectopically rather than enter the bladder [5].
The adrenal gland of the affected side is present in 90% of cases [5]. Fig. 5
Unilateral renal agenesis is associated with other congenital anomalies of the kidneys and urinary tract in approximately 32% of cases,
most commonly vesicoureteral reflux[6].
Extra-renal anomalies are found in up to 31% of cases [6].
Hence,
the early diagnosis of a solitary kidney is important as the condition requires clinical follow-up.
Key Points
- Absence of one kidney
- Fibrotic cord corresponding to the ureter may be seen
- Absence of the corresponding renal artery and vein
- Often associated with other renal,
extra renal and urinary tract abnormalities
Pelvic kidney
This is an ectopic anomaly,
where one kidney remains fixed retroperitoneally within the bony pelvis.
Embryology
The ectopia arises from a failure of the kidney to ascend cranially during embryogenesis,
hence ectopic kidneys may be found anywhere along the pathway of normal renal ascent - such as the pelvis and lower abdomen.
In rare cases,
ectopic kidneys may be found in the thorax due to excessive cranial migration prior to diaphragmatic closure.
Imaging Findings
Ectopic kidneys are often small,
lobulated,
malrotated and can have extrarenal calyces.
Their blood supply is usually derived from surrounding vessels,
such as the iliac arteries.
Typically,
patients with pelvic kidney(s) are asymptomatic and the diagnosis is usually made incidentally during imaging for other indications.
Due to the ectopic location of the kidney,
referred pain caused by pyelonephritis or calculi may be atypical,
and patients may be misdiagnosed,
for example with acute appendicitis. Fig. 6
Complications such as vesio-ureteric reflux or pelvi-uereteric junction (PUJ) obstruction are common. These complications may be a consequence of a malrotated kidney or abnormal blood supply which obstructs the collecting system.
The ectopic location and blood supply present a treatment challenge in regards to surgery or trauma in the pelvis as the risk of injury to aberrant vessels,
overlying nerves and viscera is increased.
Key Points
- One kidney remains in the pelvis,
whilst the other is normally located
- Results from a failure to ascend during embryogenesis
- Abnormal blood supply from surrounding vessels
- High incidence of vesico-ureteric reflux and PUJ obstruction
Duplex kidney
Duplication of the renal collecting system is the most common congenital abnormality of the urinary tract,
with an estimated incidence of up to 5% [7].
The abnormality may be complete,
in which the affected kidney has two pelvicalyceal systems,
with two ureters that enter the bladder separately.
Partial duplication results in two pelvicalyceal systems with either a single ureter draining both,
or a bifid ureter that joins distally to enter the bladder via a single orifice.
Of the two,
partial duplication is more common and accounts for over 95% of cases [7].
Embryology
Duplication anomalies occur very early during embryogenesis.
Complete duplication is thought to arise as a result of two ureteric buds which join with the metanephric blastema,
whilst partial duplication is thought to result from a single ureteric bud that bifurcates prior to it joining the metanephric blastema.
Imaging Findings
In a complete duplication,
the upper pole renal moiety is drained by its own single ureter,
whilst the lower pole moiety has its own single ureter.
The position of the insertion sites of the distal ends of the ureters into the bladder have been observed to be nearly constant and are described by the Weigert-Meyer rule,
which states that the ureter draining the lower pole moiety enters the bladder in an orthoptic location that lies more lateral and cephalad,
whilst the ureter draining the upper pole moiety enters the bladder in an ectopic location that is more medial and caudal and most often ends in a ureterocoele. There have only been a few rare reports of cases where there are exceptions to this rule [7].
The consequence of the Weigert-Meyer rule is that the ureter draining the lower pole moiety is prone to vesico-ureteric reflux,
whilst the ureter draining the upper pole moiety is prone to obstruction. Fig. 7
Key Points
- Two pelvicalyceal systems draining a single kidney
- Single bifid ureter,
or two separate ureters
- Normal renal vascular supply and drainage
- Complete duplication follows the Weigert-Meyer rule