It is possible for an individual to have four kidneys, though this is an exceptionally rare occurrence. This condition is termed a supernumerary kidney. It is a congenital anomaly, meaning the extra organ develops from birth due to deviations during embryonic development. Many individuals with this condition remain unaware of its presence throughout their lives.
What Are Supernumerary Kidneys?
A supernumerary kidney is an additional kidney beyond the typical two. This rare congenital anomaly has its own distinct capsule, separate blood supply, and collecting system, distinguishing it from a duplex kidney with a single continuous capsule. Fewer than 100 cases have been documented, underscoring its rarity.
This anomaly arises during embryonic development, often from an extra ureteric bud or an abnormal division of the nephrogenic cord. This results in two kidneys on the same side, potentially with a duplicated ureteral bud. The accessory kidney is frequently smaller than the native kidney and may be found in an abnormal position, often located lower than the existing kidney. It can exist completely separate from the native kidney or be partially fused to it.
How Functional Are Extra Kidneys?
The functionality of supernumerary kidneys varies considerably among individuals. Some are fully functional, contributing to filtering processes, while others are partially functional or non-functional. Many individuals remain asymptomatic throughout their lives, never experiencing symptoms or realizing they possess an extra kidney.
While often silent, supernumerary kidneys can lead to complications. These include recurrent urinary tract infections, kidney stones, or hydronephrosis (swelling of a kidney due to urine backup). Rarely, there is an increased risk for tumors, such as Wilms tumor or clear cell carcinoma, especially if the extra kidney is poorly developed or has abnormal urine drainage. The presence of complications often depends on the additional kidney’s anatomy and urine flow patterns. Supernumerary kidneys have also been observed in conjunction with other congenital anomalies affecting the genitourinary system or other body systems, such as fusion anomalies, ectopic ureteric openings, or even cardiac defects.
Diagnosis and Management
Supernumerary kidneys are frequently discovered incidentally during imaging studies for unrelated medical reasons. Techniques like ultrasound, computed tomography (CT) scans, or magnetic resonance imaging (MRI) identify these extra kidneys. If found, further evaluation typically involves detailed imaging to assess its anatomy, functionality, and any associated complications or anomalies.
For asymptomatic individuals whose extra kidney causes no problems, specific treatment is generally not required. Management often involves watchful waiting and regular monitoring through follow-up imaging to detect potential changes or complications. However, if complications arise, such as persistent infections, significant obstruction, or pain, medical or surgical intervention may be necessary. Surgical options range from addressing the specific complication to, in rare instances, removing the supernumerary kidney if it is non-functional or problematic.