Ross M. Decter, MD, FRCS
Paul H. Smith III, MD
BASICS
DESCRIPTION
• Reanl fusion is a congenital condition in which the renal units are joined
• Horseshoe kidney and crossed-fused ectopia are the most common variants
– Horseshoe kidney:
Most common renal fusion anomaly
Poles of the kidney are fused by the isthmus
Fusion occurs at the lower poles in 95%
• Crossed-fused ectopia:
– 2nd most common renal fusion anomaly
Kidney is on opposite side of where ureter inserts (ureter crosses midline)
The ectopic renal unit is fused to its companion in 90% of cases
– Crossing from left to right is the most common morphology
EPIDEMIOLOGY
Incidence
• Male > Female
• Horseshoe kidney occurs in 1 in every 400–500 live births
• Crossed-fused ectopia occurs in 1 in every 1,000–2,000 live births (1)
Prevalence
• Horseshoe Kidney: ∼1:400–500
• Crossed-fused ectopia: ∼1:3,000
RISK FACTORS
• Crossed-fused ectopia is frequently seen with vertebral anomalies such as myelomeningocele and sacral agenesis (1)
• Horseshoe kidney is present in 60% of female patients with Turner syndrome and 20% of patients with trisomy 18
Genetics
Specific genetic causes unknown, but renal fusion anomalies commonly seen in association with a variety of chromosomal and congenital abnormalities (Turner syndrome, trisomy 18)
PATHOPHYSIOLOGY
• Metanephric blastema is the embryologic precursor to the adult kidney
• Development of the kidney begins in the 4th–5th wk with ingrowth of the ureteric bud, an outpouching of the mesonephric duct, into the surrounding metanephric blastema
• Proper renal development is coordinated through interactions between the metanephric blastema and ureteric bud
• The developing kidney ascends and rotates medially to reach its usual anatomic position by the 9th wk of gestation
• Several theories have been offered to explain crossed ectopia
– One theory proposes that an aberrantly oriented ureteric bud induces renal development in the contralateral mesonephric blastema
– An alternate theory suggests that the developing kidney is channeled/displaced to the contralateral side during its ascent by the presence of an aberrant umbilical or common iliac artery or other pelvic structures
• The developing left and right metanephric blastemas are in close proximity to each other within the pelvis and, if abutting, may merge to form a horseshoe kidney or other fusion anomaly (2)
ASSOCIATED CONDITIONS
• Other congenital anomalies are present in up to a one-third of patients with horseshoe kidney
– Skelet al, cardiovascular, neural tube, and anorectal anomalies are the most common
• Other GU anomalies associated with horseshoe kidney
– Cryptorchidism or hypospadias in 4% of males
– Vesicoureteral reflux (VUR) in >50% of patients
Voiding cystourethrogram (VCUG) is routine part of evaluation
• 2–8 times increased risk of Wilms tumor
• 2–4 times increased risk of TCC
• Imperforate anus in 4% with crossed ectopia
• Horseshoe kidney associated with imperforate anus and Meckel’s diverticulum
• Increased risk of urolithiasis due to both anatomic and metabolic factors
GENERAL PREVENTION
• Preventative measures aim to minimize risk factors for future renal deterioration:
– Prophylactic antibiotics or surgical correction if VUR present
– Decompression of obstructed moieties (pyeloplasty)
DIAGNOSIS
HISTORY
• Most are asymptomatic and are incidentally discovered
• May be diagnosed on prenatal ultrasound (US)
• Symptoms are usually the result of infection, stones, or obstruction of the abnormally positioned collecting system (UPJO)
– Nonspecific abdominal pain, nausea, vomiting, hematuria
PHYSICAL EXAM
• Palpable abdominal mass (hydronephrosis)
• CVA tenderness (stone or pyelonephritis)
DIAGNOSTIC TESTS & INTERPRETATION
Lab
• Urinalysis (hematuria)
• Serum creatinine (elevated with obstruction)
• Metabolic evaluation for urolithiasis
– Metabolic etiologies for stone disease common in patients with horseshoe kidney
Serum chemistries
24-hr urinalysis
Imaging
• Renal US: Hydronephrosis
• VCUG:
– High incidence of VUR
• Diuretic renography (MAG3):
– If clinical or radiographic concern for obstruction
• Contrast-enhanced CT or gadolinium-enhanced MRI with delayed images can accurately characterize the renal, collecting system, and vascular anatomy for surgical planning
Diagnostic Procedures/Surgery
Karyotype in females with horseshoe kidney if dysmorphic features suggestive of Turner syndrome
Pathologic Findings
• Wilms tumor in children and TCC in adults are more common in horseshoe kidneys:
– Unclear if carcinoma related to embryologic mechanisms, urinary sepsis, or infection
DIFFERENTIAL DIAGNOSIS
• Renal mass
• Supernumerary kidney:
– An accessory organ with its own blood supply and collecting system
– It may not be reniform, but possesses a distinct capsule surrounding a parenchymal mass
• Malrotated kidneys: Can look like a horseshoe kidney on radiographic imaging
TREATMENT
GENERAL MEASURES
No treatment if asymptomatic. Specific management dictated by complicating features.
MEDICATION
First Line
• Antibiotics:
– VUR treated the same as in those without fusion anomalies
– Antibiotic prophylaxis may be used until resolution for low-grade reflux
Second Line
N/A
SURGERY/OTHER PROCEDURES
• General operative considerations:
– Horseshoe and ectopic kidneys often have abnormal and complex renal vasculature
Renal vessels may arise from the aorta, common iliac artery, or both, and typically enter the kidney anteriorly
Angiography (including CT and MR angiogram) may be useful to delineate renal vascular anatomy for operative planning
– The renal pelvis and ureteropelvic junction of the horseshoe kidney often have an abnormal configuration, which can result in urinary stasis or obstruction. This anatomic abnormality contributes to the majority of the symptoms associated with horseshoe kidney, including stone formation, infection, hydronephrosis, and frank UPJO.
– In the horseshoe kidney, the lower poles are joined by an isthmus, which is typically situated just caudal to the inferior mesenteric artery. The isthmus may be divided if necessary during nephrectomy.
• UPJO, best managed with dismembered pyeloplasty:
– Endoscopic management of UPJO is feasible (3)[B]
• Stone procedures:
– PCNL:
Provides the highest stone-free rates for larger renal stones and is typically the primary procedure for renal calculi in the horseshoe kidney. Access to the collecting is usually best achieved through a posterior upper pole calyx. (4)[B]
– ESWL:
It may be difficult to target stones for ESWL due to the abnormal location of the kidney. ESWL is most appropriate for stones <1.5 cm associated with unobstructed collecting systems. Repeat treatments may be required in order to achieve a stone-free status (4)[B].
– Ureteroscopy:
Safe and effective approach for calculi associated with horseshoe or ectopic renal moieties; however, the smaller ureteroscopic instruments generally limit this approach to smaller stone burdens
ADDITIONAL TREATMENT
Radiation Therapy
N/A
Additional Therapies
N/A
Complementary & Alternative Therapies
N/A
ONGOING CARE
PROGNOSIS
Related to complicating features. Outcomes comparable to those for anatomically normally positioned kidneys.
COMPLICATIONS
• Possible increased risk of malignancy
• UTI
• Urolithiasis
• UPJO
• VUR
FOLLOW-UP
Patient Monitoring
• Due to slight increase in incidence of Wilms tumor in children, some advocate imaging every 6 mo once the diagnosis is made
• The abnormalities and conditions for crossed-fused ectopia are similar to horseshoe kidneys, and treatment often follows similar lines
Patient Resources
National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC) http://kidney.niddk.nih.gov/kudiseases/pubs/ectopicKidney/index.aspx
REFERENCES
1. Glodny B, Petersen J, Hofmann KJ, et al. Kidney fusion anomalies revisited: Clinical and radiological analysis of 209 cases of crossed fused ectopia and horseshoe kidney. BJU Int. 2009;103:224–235.
2. Glassberg KI. Normal and abnormal development of the kidney: A clinician’s interpretation of current knowledge. J Urol. 2002;167:2339–2351.
3. Yohannes P, Smith AD. The endourological management of complications associated with horseshoe kidney. J Urol. 2002;168:5–8.
4. Gupta NP, Mishra S, Seth A, et al. Percutaneous nephrolithotomy in abnormal kidneys: Single-center experience. Urology. 2009;73(4):710–714.
ADDITIONAL READING
• Boyan N, Kubat H, Uzum A. Crossed renal ectopia with fusion: Report of two patients. Clin Anat. 2007;20:699–702.
• Decter RM. Renal duplication and fusion abnormalities. Pediat Clin N Am. 1997;44(5):1323–1341.
See Also (Topic, Algorithm, Media)
• Horseshoe Kidney
• Malrotated Kidney/Renal Malrotation
• Renal Ectopia
• Renal Fusion Anomalies Image 
• Vesicoureteral Reflux, Pediatric
CODES
ICD9
753.3 Other specified anomalies of kidney
ICD10
• Q63.1 Lobulated, fused and horseshoe kidney
• Q63.2 Ectopic kidney
CLINICAL/SURGICAL PEARLS
• Renal fusion anomalies often associated with other congenital anomalies.
• Ureteropelvic junction obstruction (UPJO) in up to 1/3 of patients with horseshoe kidney.
• Renal vasculature usually aberrant.
• Increased risk of urolithiasis due to medical and anatomic abnormalities.