Diagnosis: Crossed Fused Renal Ectopia
The diagnosis is crossed fused renal ectopia (CFRE), a rare congenital anomaly where both kidneys are fused together and located on the same side of the midline (in this case, the right side), leaving the left renal fossa empty 1, 2.
Key Diagnostic Features
The clinical presentation described—left kidney fused to the right kidney with an empty left renal fossa—is pathognomonic for CFRE. This represents one of the most uncommon congenital renal anomalies, with an estimated incidence of approximately 1 in 7,500 autopsies 1.
Anatomical Characteristics to Confirm
- Both kidneys located on the right side with parenchymal fusion (most commonly inferior fusion pattern)
- Empty left renal fossa on imaging
- Crossed ureter: The ectopic (left) kidney's ureter crosses the midline to insert normally into the left side of the bladder 1, 3
- Aberrant vascular anatomy: Multiple renal arteries often arise from the aorta, common iliac, or other anomalous sources 1, 4
- Preserved corticomedullary differentiation in both moieties if functional 1
Recommended Imaging Workup
Contrast-enhanced CT (CECT) is the gold standard for definitive diagnosis and anatomical mapping 1, 3. This provides:
- Complete anatomical delineation of both fused kidneys
- Vascular mapping showing anomalous arterial supply and venous drainage
- Ureteral course visualization (confirming midline crossing)
- Assessment of collecting systems (typically two separate systems)
- Detection of associated complications (stones, obstruction, masses)
Alternative Imaging Modalities
- Ultrasound: Useful for initial detection, showing bilaterally empty renal fossae and a lobulated reniform mass on one side 1
- MR urography: Radiation-free alternative providing excellent anatomical and functional data, though requires sedation in children and has limited availability 5
Clinical Significance and Management Implications
Most Cases Are Asymptomatic
CFRE typically remains clinically silent throughout life and is discovered incidentally 1, 6. The patient in the reported case from Ghana remained asymptomatic until age 55 with preserved renal function (eGFR 76 mL/min/1.73 m²) 1.
Potential Complications Requiring Surveillance
- Urolithiasis: Stone formation is more common due to urinary stasis 2, 3
- Urinary tract infections: Related to anatomical abnormalities
- Obstruction: Can occur from stones, ureteral anomalies, or compression between fused moieties 4
- Malignancy: Rare but reported, including renal cell carcinoma in CFRE 7
Critical Surgical Considerations
Awareness of aberrant vascular anatomy is essential before any surgical or interventional procedure 1, 4. The vascular supply is highly variable with:
- Multiple small arterial pedicles
- Anomalous origins from iliac vessels or lower aorta
- Possible malrotation with anterior-facing hilum
- Single or multiple renal veins 3
Failure to recognize CFRE preoperatively can lead to catastrophic vascular injury during abdominal surgery 1.
Common Pitfalls to Avoid
- Misdiagnosing as renal agenesis: An empty renal fossa does NOT equal absent kidney—always search the contralateral side and pelvis 8
- Assuming "cake kidney": CFRE shows distinct fusion with identifiable separate moieties, unlike horseshoe or pancake kidneys 6
- Inadequate vascular mapping: Never proceed to surgery without detailed CT angiography showing all arterial and venous anatomy 4
- Overlooking the crossed ureter: This anatomical detail confirms the diagnosis and is crucial for surgical planning 1, 3
When to Intervene vs. Observe
Asymptomatic CFRE requires no intervention—only surveillance 6. Active treatment is indicated only for:
- Symptomatic obstruction requiring drainage or pyeloplasty
- Infected or obstructing stones necessitating percutaneous nephrolithotomy 2
- Non-functional moiety causing damage to the functional kidney 4
- Confirmed malignancy requiring nephrectomy 7
The 2017 case report demonstrates that even with a small calculus, patients can remain stable with conservative management and regular follow-up without active intervention 6.