A person can be born with four kidneys, a condition referred to as a supernumerary kidney. This congenital anomaly results in the development of more than the typical pair of kidneys. The defining feature is the presence of an extra kidney that functions as a distinct, separate organ. Each supernumerary kidney possesses its own fibrous capsule, a dedicated blood supply, and an individual collecting system for urine drainage. The biological pathways that lead to this formation are extremely rare, making it one of the least frequently reported congenital anomalies of the urinary tract.
Defining Supernumerary Kidneys
A supernumerary kidney is characterized by a complete separation from the native kidney, each having its own parenchymal tissue, blood vessels, and capsule. This differentiates the condition from a duplex collecting system, where a single kidney has a split ureter. The true supernumerary kidney is an accessory organ that may be attached to the native kidney or be completely free-standing.
Medical literature has documented fewer than 100 cases globally, establishing the condition as an extreme rarity. Some studies suggest it may occur in approximately 1 in 26,750 individuals. The extra organ is typically smaller than the two native kidneys and is often positioned lower, or caudal, to the main kidney on the same side of the body.
The extra kidney may possess a separate ureter that drains independently into the bladder, or it may share a common ureter with the adjacent kidney, forming a bifid system. While these accessory organs are structurally complete, they often exhibit reduced function compared to the primary pair.
The Embryological Mechanism of Formation
Normal kidney development, called nephrogenesis, involves the reciprocal interaction of two structures: the metanephric blastema and the ureteric bud. The metanephric blastema forms the functional filtering units of the kidney. The ureteric bud, an outgrowth from the mesonephric duct, forms the collecting system, including the ureter and renal pelvis. The ureteric bud penetrates the blastema, inducing it to differentiate and form the mature kidney structure.
The formation of a supernumerary kidney results from a biological error that duplicates the initial inducing signal. The most widely accepted mechanism involves an abnormal and complete splitting of the ureteric bud. If this splitting occurs early, two separate buds form, each migrating and inducing its own distinct portion of the metanephric blastema.
The timing of this error is significant; a partial or late splitting would lead only to a duplex kidney, where the collecting system is split but the parenchyma remains fused. Alternatively, an entirely separate, ectopic ureteric bud may arise from the mesonephric duct, distinct from the primary bud. This secondary bud then induces a separate metanephric blastema, leading to the formation of an accessory organ.
Clinical Implications and Detection
Many individuals with a supernumerary kidney remain asymptomatic, with the condition being discovered incidentally during imaging for unrelated issues. When symptoms occur, they are generally related to the accessory organ’s anatomical anomalies, particularly concerning its drainage system. The atypical connection or position of the supernumerary ureter can lead to complications.
The most common problems include an increased risk of urinary tract infections due to poor drainage or reflux. Obstruction of the urine flow can also occur, leading to hydronephrosis (swelling of the kidney due to fluid backup). Furthermore, altered flow dynamics can predispose the individual to the formation of kidney stones.
Confirming the diagnosis requires sophisticated diagnostic imaging techniques. Ultrasound is often the initial screening tool, but confirmation relies on cross-sectional imaging such as Computed Tomography (CT) scans or Magnetic Resonance Imaging (MRI).
In cases where the extra kidney is non-functional or is the source of recurrent, symptomatic issues like persistent infection or severe hydronephrosis, surgery may be indicated. If the supernumerary kidney is functional and remains free of complications, the prognosis is favorable, and the patient is typically managed with observation and regular follow-up.