Definition, Etiology, PathogenesisTop
Simple renal cysts are isolated acquired cysts that usually occur in patients without severe chronic kidney disease who do not fulfill the diagnostic criteria for polycystic kidney disease. The prevalence rate in adults is ~30% and is very strongly associated with increasing age. Simple renal cysts may increase in size with age and are usually asymptomatic; however, large cysts (>50 mm) may manifest as flank or lower back pain, a feeling of abdominal distention or pressure, or nonspecific gastrointestinal symptoms. Possible complications of simple renal cysts include hematuria and infection.
The key diagnostic test is ultrasonography. Sonographically, a simple cyst must have no definable wall thickness, an anechoic lumen without internal vascularity, a clearly defined back wall, and posterior acoustic enhancement. Cysts with these characteristics or small cysts with <1 mm thin septations do not require further imaging.Evidence 1Strong recommendation (downsides clearly outweigh benefits; right action for all or almost all patients). Moderate Quality of Evidence (moderate confidence that we know true effects of intervention). Quality of Evidence lowered due to the relatively small number of observations (imprecision). Clayman RV, Surya V, Miller RP, Reinke DB, Fraley EE. Pursuit of the renal mass. Is ultrasound enough? Am J Med. 1984 Aug;77(2):218-23. PubMed PMID: 6465172. Cysts that do not meet these criteria should be investigated with computed tomography (CT) scanning and classified as per the Bosniak renal cyst classification system. (See Can Urol Assoc J. 2010 Apr;4:98-9.)
Patients with asymptomatic simple renal cysts require no further monitoring or imaging. Bosniak 2F cysts should be investigated by comparison with previous films (if available), perhaps further characterized with contrast-induced magnetic resonance imaging (MRI), and if truly 2F, they should be followed with yearly ultrasonography at a minimum. Changes should be investigated with CT.Evidence 2Strong recommendation (benefits clearly outweigh downsides; right action for all or almost all patients). Moderate Quality of Evidence (moderate confidence that we know true effects of intervention). Quality of Evidence lowered due to the relatively small number of cases (imprecision). Israel GM, Bosniak MA. Follow-up CT of moderately complex cystic lesions of the kidney (Bosniak category IIF). AJR Am J Roentgenol. 2003 Sep;181(3):627-33. PubMed PMID: 12933451. Bosniak 3 and 4 cysts should be referred to a surgeon, who may elect to biopsy a category 3 cyst or follow it closely. In rare cases where large simple cysts cause symptoms (in particular flank or abdominal pain) or compression of adjacent organs, treatment may be required. The available options are cyst drainage and sclerotherapy (injection of 95% ethanol into the cyst) or surgical resection of the cyst.