Almost 27% of the population over the age of 50 years has one simple renal cyst. Simple cysts could enlarge with age, and even double over 10 years. However, simple cysts rarely lead to complications.

Most of the time, parenchymal lesions or cystic renal masses are incidentally found during medical checkups. It is the number of cysts, as well as their nature that holds a lot of clinical significance. Doctors use the Bosniak renal cyst classification system to differentiate simple renal cysts from the complex cysts or malignant masses. Simple renal cysts are usually not a cause of serious concern, however, hemorrhage could occur in a simple cyst, or a cyst may form as a result of the liquefaction of a traumatic hemorrhage within the kidney parenchyma. In about 6% of cases of simple cysts, complications arise due to hemorrhage, which in turn could occur due to trauma, enlargement, or predisposition to bleeding. It must be noted that hemorrhage, infection, or inflammation can cause non-malignant renal cysts to have a complex appearance on a CT scan. Such cysts might develop features that are similar to those observed in atypical complex cysts. When hemorrhagic cysts resolve, features such as calcification, thickening of the wall, and septa (divisions within the wall) could develop, giving it the appearance of a complex cyst in imaging studies. Therefore, doctors generally seek the help of imaging procedures in order to take a closer look at the characteristics of renal cysts. When the imaging studies reveal irregular characteristics, it becomes important to assess whether the renal cyst is malignant or not. Besides CT scans, tests such as a renal ultrasound, MRI, and urinalysis might be conducted for formulating a diagnosis. Dr. Morton Bosniak, a faculty member at the NYU Langone Medical Center in New York City, was the first person to come up with the classification of renal cysts on the basis of the contrast-enhanced CT findings in 1986. Morphological and enhancement characteristics on CT scans form the basis of this classification. As per this classification, kidney cysts are classified into five categories: ➠ Category I ➠ Category II ➠ Category II F ➠ Category III ➠ Category IV Category I cysts are simple cysts, which means that they are thin-walled and don't contain septa, calcifications, or solid material. The density of the cyst resembles that of water. Even after the injection of the contrast dye, the cyst doesn't become enhanced. Category II cysts are usually benign cysts that are considered to be minimally complex, but these contain a few thin divisions within the walls (septa), with fine calcifications in the wall or the septa. High-density cysts that are filled with blood or proteinaceous fluid are placed in this category, if they are sharply-demarcated, uniformly attenuated lesions which are less than 3 cm and don't get enhanced with contrast dye. Cysts that come under category II F (F to suggest the need for radiologic follow-up and further evaluation) are the ones that are well-defined and have sharp edges. These might have several divisions within the walls, which may or may not appear enhanced on the injection of the contrast dye. There also include uniformly attenuated lesions with sharp edges that are greater than 3 cm, which do not enhance with contrast dye. The risk of malignancy in this category is estimated to be 5%. Category III cysts are indeterminate cystic masses that have irregular, thick walls and septa which appear very dense or get enhanced on the injection of the contrast dye. The risk of malignancy is around 40 to 50%. Partial nephrectomy is usually recommended, but in case of elderly or those who cannot undergo the surgery, RF ablation is suggested. Category IV cysts are clearly malignant cystic masses in which the characteristics of category III are present along with soft tissue components that enhance when contrast is given. Partial or total nephrectomy is suggested in such cases. The Bosniak classification is very helpful in determining both the risk of kidney cancer and the need for follow-up and/or treatment. While category I and II don't carry the risk of malignancy, follow-up is required for category II F, III, and IV. While there's a considerable chance that category III cysts could either be benign or malignant, the risk for malignancy is very high in category IV. Simple cysts are fluid-filled sacs that have clear fluid or blood enclosed within their walls. Simple cysts that are filled with blood are referred to as hyperdense renal cysts. At times, a renal cyst might hemorrhage or even rupture. What follows after such a hemorrhage will depend on the size, location, and the nature of the cyst. The reason behind the hemorrhaging of the cyst could be the pressure exerted on an enlarged and growing cyst present on the surface of the kidney by the surrounding organs. This can lead to the rupturing of the cyst. Trauma during surgery might also cause the cyst to hemorrhage or rupture. In severe cases, one might experience symptoms such as back pain, flank pain, pain in the abdomen, and blood in urine. Hematuria, which is characterized by the presence of blood in the urine, is one of the common symptoms of a hemorrhagic renal cyst. While hemorrhage of a simple cyst might resolve on its own, this might not be the case with complex cysts or malignant renal masses. If the fluid inside the cyst was infected, then the hemorrhage could lead to sepsis. The pain one might experience might be a result of a blood infection. If not treated soon, a life-threatening situation might arise. Simple cysts are generally asymptomatic. It is usually rare for one to experience severe symptoms due to the presence of a simple renal cyst, but if the cyst has become enlarged and has ruptured, one cannot rule out the possibility of complications. Taking analgesics will not suffice in such cases. A blood infection is one of the biggest complication that can arise. Once the doctors have analyzed the condition of the patient, they can suggest the most suitable treatment option. Percutaneous aspiration is one of the common procedures that is used for the treatment of symptomatic cysts in the kidney. This is a minimally invasive procedure which involves draining out of the fluid by using a needle. At times, the fluid might get filled up again. Therefore, this procedure is often followed by injection of a sclerosing agent. Generally, alcohol is used as sclerosing agent. Since it scars the space within the cyst, there is less likelihood of the cyst getting filled again. The contents of the cyst can also be studied so as to confirm the diagnosis about the nature of the cyst. If percutaneous aspiration and sclerotherapy don't seem to help, doctors might consider laparoscopic cyst decortication. Under this procedure, the doctor makes small incisions, inflates the space with gas, and removes the walls of the renal cyst. If laparoscopic deroofing of the renal cyst doesn't seem to work, surgical drainage and removal of the cyst through laparoscopic cyst ablation or open surgery might be recommended. If the cyst is malignant, it must be surgically removed. The patient must have his/her kidney function monitored at regular intervals with the help of imaging studies.