Introduction
A radiologist may encounter a variety of congenital renal anomalies in his or her practice. While horseshoe kidneys and crossed-fused ectopia are relatively common, other renal fusion anomalies are rare. These uncommon fusions can prove diagnostically challenging, as their imaging features may mimic other pathologies. However, a systematic approach utilizing multiple imaging modalities, including radionuclide studies, can help elucidate these complex anomalies. This article reviews three of the rarest types of congenital renal fusions - lump kidney, cake kidney, and crossed unfused ectopia - and demonstrates how modern imaging techniques can be leveraged to make a confident diagnosis in these unusual cases. With an awareness of these anomalies and their imaging characteristics, the radiologist can avoid potential diagnostic pitfalls and determine the most appropriate management for these patients.
What Are the Radionuclide Imaging Techniques for Diagnosing Renal Fusion Anomalies?
Specialized nuclear medicine imaging techniques are often required to diagnose rare congenital renal fusion anomalies properly. Radionuclide imaging using 99mTc-DMSA, 99mTc-MAG3, and 99mTc-DTPA can accurately demonstrate renal fusion anomalies that may remain occult on ultrasound and CT.
99mTc-DMSA renal scintigraphy utilizes a radiotracer that is avidly taken up by functioning renal cortical tissue. It can precisely delineate renal moieties' number, size, shape, and location in patients with horseshoe kidneys or crossed fused ectopia. The degree of cortical fusion or separation between renal moieties can also be assessed.
99mTc-MAG3 renal scintigraphy employs a radiotracer that is cleared by the renal tubules and can be used to evaluate differential renal function in complex renal fusions. It can also unmask subtle collecting system anomalies that may impact drainage and increase infection risk.
99mTc-DTPA renal scintigraphy uses a radiotracer filtered by the glomerulus and can demonstrate early uptake, excretion, and drainage of the radiotracer through the renal collecting systems and ureters. It is beneficial for delineating complex drainage anomalies associated with specific renal fusions like horseshoe kidneys.
What Is a Horseshoe Kidney?
A horseshoe kidney is the most common renal fusion anomaly, occurring in about one in 400 people. In this condition, the kidneys fuse during development in the womb. The kidneys ascend from the pelvis into the abdomen but remain joined at their inferior poles, forming a horseshoe shape.
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Diagnosis is often incidental and detected during imaging for other reasons. Symptoms, when present, include abdominal or flank pain, recurrent urinary tract infections, and kidney stones.
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The diagnosis and evaluation of the anatomy require imaging studies. Intravenous urography provides an anatomical outline, but CT urography and magnetic resonance urography offer fine anatomical detail without radiation exposure. Radionuclide imaging using Tc-99m-mercaptoacetyltriglycine (MAG3) assesses differential renal function and drainage.
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Most horseshoe kidneys require no treatment. However, some patients may benefit from surgery to correct complications like kidney stones, hydronephrosis, or recurrent infections. In rare cases, kidney transplantation may be needed for end-stage renal disease.
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Patients with a horseshoe kidney should be monitored periodically with blood and urine tests to check for signs of reduced kidney function. Imaging may be repeated if there are concerning symptoms or significant changes in laboratory values.
With an understanding of the anatomy and physiology of horseshoe kidneys, radiologists and urologists can work together to diagnose and manage this congenital condition properly, helping patients maintain health and quality of life.
What Is Crossed-Fused Renal Ectopia?
Crossed fused renal ectopia (CFRE) occurs when the developing kidneys fail to ascend to their normal positions, resulting in fusion and malrotation. During normal development, the kidneys ascend from the pelvis into the abdomen. The left kidney moves to the left side, and the right kidney moves to the right. In CFRE, the kidneys fail to separate and ascend properly, leading to fusion and ectopic positioning of one kidney.
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Ectopic Fusion: In the most common type of CFRE, the left kidney crosses the midline and fuses with the right kidney. The fused kidneys are located on the right side of the abdomen. Radionuclide imaging demonstrates the ectopic left kidney fused with the right kidney on the right side. The ureters may insert into the bladder in a normal orthotopic or ectopic manner.
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Diagnosis and Management: CFRE is often detected incidentally during evaluation for other conditions. Radionuclide renal scanning is useful for confirming the diagnosis and assessing renal function. Management is typically conservative, monitoring renal function and screening for complications like infection, stone formation, or malignancy. Surgical intervention may sometimes be required for pain, infection, or stone disease.
CFRE is a rare congenital anomaly but an important one to recognize. Radionuclide renal scanning is essential in establishing the diagnosis and guiding management. Careful monitoring and follow-up are needed to screen for potential complications in these complex renal fusions.
What Is a Lump Kidney?
A lump kidney, a solitary fused kidney, is an extremely rare congenital renal fusion anomaly in which the left and right kidneys join together during fetal development to form a single, enlarged kidney. This occurs when the metanephric blastemas, the embryonic anlagen that gives rise to the kidneys, fuse at an early stage of organogenesis. The result is a solitary kidney in the midline or to one side of the abdomen.
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Imaging Findings: On radionuclide imaging, a lump kidney will demonstrate a single enlarged kidney in the midline or paramedian position, often with a characteristic beak- or boomerang-shaped appearance. The ureters, collecting system, and renal pelvis are fused, though some duplication may exist. The function is usually normal or near-normal, though complications like infection, stone formation, and reduced glomerular filtration rate can occur.
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Diagnosis and Management: The diagnosis of a lump kidney is often made incidentally during evaluation for other conditions. Prenatal ultrasound may detect the anomaly, prompting further postnatal workup. In addition to renal scintigraphy, CT and MR urography can fully characterize the anatomy and rule out complications.
Most patients with a lump kidney require no treatment and can live normal lives with regular monitoring. However, some may need surgery to correct complications like obstruction, stone disease, or recurrent infection. Close follow-up is required to monitor renal function, especially in children, as the solitary kidney is responsible for 100 percent of renal function.
Is a Renal Scan or Renal Cortical Scan More Appropriate?
A renal scan using 99mTc-DMSA is ideal for evaluating renal fusion anomalies. It provides high-resolution static images allowing detailed anatomic assessment of the kidneys and ureters. A renal cortical scan using 99mTc-glucoheptonate can also be used but may provide a slightly lower resolution. Either scan can detect the presence of fused or ectopic kidneys.
What Patient Preparation Is Required?
No special patient preparation is required for either scan. Patients can eat and drink as usual prior to the exam. They will be asked to void immediately before imaging to ensure an empty bladder, which can interfere with image acquisition and interpretation.
How Are the Scans Performed and Interpreted?
The radiotracer is injected intravenously; then, the patient lies under a gamma camera as it rotates around the abdomen to capture images. The number, size, shape, and location of the kidneys, as well as the presence of any structural abnormalities, are determined by a nuclear medicine physician after reviewing the images. The relative uptake and excretion of the radiotracer are also evaluated.
What Are the Risks and Limitations?
Radionuclide imaging using 99mTc poses very little risk, as the amount of radiation exposure is minimal. However, the scans may be limited in patients with impaired renal function or urinary tract obstruction. They provide functional information but limited anatomical detail. For a more comprehensive evaluation, other imaging modalities such as ultrasound, CT, or MRI complement the nuclear medicine study.
How Are Results Reported?
The interpreting physician generates a report describing the findings, impressions, and recommendations for follow-up testing or procedures. They may also review the results with the referring physician to discuss the implications of the diagnosis and the next steps in patient management or treatment.
Conclusion
Radionuclide imaging is important in diagnosing and managing rare congenital renal fusion anomalies. While these conditions are uncommon, nuclear medicine techniques allow for accurate noninvasive evaluation when ultrasound is limited. Familiarity with the scintigraphic appearances of the horseshoe kidney crossed fused ectopia, and lump kidney enables the clinician to suggest the appropriate diagnosis and determine the presence of complications. Though complex, understanding the embryology and anatomy of these anomalies is key to interpreting the imaging findings. Radionuclide studies provide functional data that complements anatomic imaging, allowing for a comprehensive assessment of these unusual entities. With a multimodality approach, nuclear medicine helps unlock the mysteries of the rare.
