Kidneys are organs responsible for the elimination of most of the unnecessary and harmful substances from the human body, and for excess water elimination. They are very complex organs composed of microscopic units called nephrons which create primary urine. Primary urine then passes through the system of tubules and converts to secondary (final) urine.
For the purposes of proper functioning, the optimal number of healthy kidneys is two. In some cases, when one kidney is damaged by any pathological process, the remaining kidney has the ability to enhance its action and compensate the function of the diseased kidney.
On the contrary, there are cases in literature where more than two kidneys are found in human body, which is called supernumerary kidney. This can happen due to anomalies in the embryonic development or it can be hereditary. Clinical presentation, diagnosis, and eventual need for treatment are different from case to case.
Although not many cases are documented and investigated, clinical presentation is assessed for some of them. A person with three kidneys can be completely asymptomatic, but they also can have severe symptoms of renal dysfunction. The supernumerary kidney is most commonly below the left normal kidney and smaller than the normal kidney. Functional studies have shown that the supernumerary Kidney often has decreased function in comparison with normal kidneys. The third kidney can obstruct the ureters or vascular and nerve elements in the abdomen, thus producing pain, problems in vascularization of pelvic organs, and signs of urinary obstruction, similar to those seen in kidney stones.
In most cases, a supernumerary kidney is discovered by accident when performing tests such as ultrasound imaging, intravenous urography, computerized tomography, or magnetic resonance imaging for other reasons. Ultrasound imaging and MRI are the most valuable techniques for structural evaluation of the supernumerary kidney, but functional tests are required too.
The need for treatment is determined based on the results of the functional tests and patient's complaints. If the function of normal kidneys is not jeopardized by the presence of a supernumerary kidney, no treatment is needed, but periodical reassessment of renal function is necessary. It is also not recommended to remove the supernumerary kidney even if it is not functional, as every surgery carries its risks.
On the other hand, if the function of normal kidneys is affected by the third kidney, it has to be removed as soon as possible, preferably in the childhood, to avoid further damage and preserve renal function.
It is a controversial topic whether it is a good idea for a person with three kidneys to donate the supernumerary kidney, as problems with renal function can appear later in life.
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