A supernumerary kidney (SK) is an additional kidney with its own capsule and blood supply that is not fused with the ipsilateral kidney (IK). Because individual case reports indicate a high morbidity rate, the aim of this retrospective study was a detailed analysis of this rare anatomical variant. Our systematic imaging-based search for SKs, conducted in the period from 2000 and to 2017, yielded 9 cases in total (5 men, 4 women; mean age: 51.8 ± 22.8 years). The SKs were observed on the right in six and on the left side in three cases. In six subjects (66%) they were located caudal and in three cases (33%) cranial to the ipsilateral kidney. Calculi were found in three (33%) of the renal collecting systems. Five (56%) of the SKs had hydronephrosis grade IV and one SK had recurrent pyelonephritis (11%). Two of the ureters opened into the ipsilateral seminal vesicle (22%). Two (22%) SKs were functional but atrophic. Clinically relevant findings were made in 33% of the IKs: atrophy (n = 2), calculi (n = 1), and reflux with recurrent pyelonephritis (n = 1); another 33% had anatomical anomalies without functional impairment. The correct diagnosis of a SK is possible using CT imaging in all subjects. The prevalence of SK based on CT imaging can be estimated to be 1:26750. CT is the method of choice for visualizing SKs. The correct diagnosis is crucial in preventing dispensable surgical procedures and for providing optimal patient treatment and outcome.

中文翻译：

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肾外肾：9例临床和影像学分析

产后肾脏（SK）是另外一个肾脏，其自身的胶囊和血液供应未与同侧肾脏（IK）融合。由于个别病例报告显示较高的发病率，所以本回顾性研究的目的是对该罕见的解剖变异进行详细分析。我们在2000年至2017年期间进行了系统的基于影像学的SK搜寻，共发现9例病例（5例男性，4例女性;平均年龄：51.8±22.8岁）。在右侧观察到的SK中有6例，在左侧观察到的3例中有SK。在六名受试者（66％）中，他们位于同侧肾脏的尾部，三例（33％）在颅骨中位于同侧肾脏。在三个（33％）的肾脏收集系统中发现了结石。五个（56％）的SK患者患有IV级肾盂积水，而一个SK患有复发性肾盂肾炎（11％）。输尿管中有两个开向同侧的精囊（22％）。2个（22％）SK起作用但萎缩。在33％的IK中有临床相关的发现：萎缩（n = 2），结石（n = 1）和复发性肾盂肾炎反流（n = 1）；另有33％的人解剖结构异常，无功能障碍。使用CT成像可以在所有受试者中正确诊断SK。基于CT成像的SK患病率估计为1：26750。CT是可视化SK的首选方法。正确的诊断对于防止不必要的手术程序以及提供最佳的患者治疗和结果至关重要。以及复发性肾盂肾炎返流（n = 1）；另有33％的人解剖结构异常，无功能障碍。使用CT成像可以在所有受试者中正确诊断SK。基于CT成像的SK患病率估计为1：26750。CT是可视化SK的首选方法。正确的诊断对于防止不必要的手术程序以及提供最佳的患者治疗和结果至关重要。以及复发性肾盂肾炎返流（n = 1）；另有33％的人解剖结构异常，无功能障碍。使用CT成像可以在所有受试者中正确诊断SK。基于CT成像的SK患病率估计为1：26750。CT是可视化SK的首选方法。正确的诊断对于防止不必要的手术程序以及提供最佳的患者治疗和结果至关重要。