A simple kidney cyst is a pocket of fluid which originates from the surface of the kidney and is contained by a thin wall. One or more simple cysts may form within the kidneys. The simple cyst is the most common type of renal cyst.
Simple renal cysts are acquired, not inherited. The exact cause is unknown. Risk factors are advancing age and male sex. The presence of simple cysts increases after age 40. Renal cysts are most often identified at autopsy or incidentally in radiological studies.
Simple renal cysts are extremely common, and incidence increases with age. Approximately 25% of people who are 40 years or older and approximately 50% of people who are 50 years and older have simple kidney cysts. Simple renal cysts can double in size over 10 years. Solitary renal cysts are usually incidentally discovered and are typically asymptomatic. Multiple renal cysts are often seen in patients with chronic renal disease on dialysis.
The exact pathophysiology for renal cysts is unclear. Diverticula on the distal tubule of the nephron may be the starting point for cyst formation. The basal membrane involution with age, combined with a degree of urinary obstruction, may result in simple cyst formation.
Simple renal cysts are typically filled with serous fluid and have a simple epithelial lining. They can have a single layer of cuboidal or flattened epithelial. A simple cyst can have an atrophic epithelial layer.
Simple cysts typically do not cause any symptoms. They are most often detected incidentally during a radiological study performed for other reason such as abdominal pain. Simple cysts usually do not interfere with kidney function and do not typically affect renal function tests. On rare occasion, simple cysts can lead to impaired renal function. Most people are not aware of having them. Rarely, a simple cyst can rupture and bleed, become infected, or grow so large that it causes mass effect on other organs and abdominal pain and discomfort. There is an association between simple cysts and high blood pressure, but a causal relationship has not been proven.
Renal cysts are often detected incidentally during a CT scan or ultrasound. The sonographic features of a simple cyst are as follows: anechoic, thin imperceptible wall, and no internal septations or debris.
On non-contrast, unenhanced CT scan, a simple cyst can be confidently diagnosed if the cyst measures simple fluid of 20 Hounsfield units or less, has no septations, and has no central or peripheral calcification, and has a thin wall. On a contrast-enhanced CT Scan, a simple cyst is diagnosed if there is no enhancement, it measures the simple fluid, it has a thin wall, and contains no calcifications.
MRI features of a simple cyst are as follows: homogenously low signal intensity on T1 weighted imaged, homogenously high signal intensity on T2 weighted images, and no demonstrable enhancement after IV contrast. Subtraction images can be helpful in difficult cases of questionable enhancement.
The majority of cysts do not need any intervention or follow up. If a cyst becomes infected, antibiotics are necessary. If a cyst is so large that it is exerting mass effect or pressure on another organ, the two most common procedures for treating simple kidney cysts are (1) aspiration and sclerotherapy and (2) laparoscopic surgery to remove the cyst. Ethanol is most commonly used in sclerotherapy.
Cyst aspiration without sclerotherapy has a high recurrence rate ranging from 20% to 80%. Simple cysts are not treated with surgical exploration.
Complex renal cysts need to be differentiated based on imaging criteria to assess the risk of malignancy. In 1986, Morton Bosniak developed a classification of renal cysts based on CT findings that have become the standard to ascertain which complex cysts have a high incidence of malignancy and require surgery or close follow up imaging. The Bosniak classification is currently used by both radiologists and urologists to manage complex renal cysts. It was originally based on CT findings but has been expanded to MR imaging findings as well. Simple cysts fall into the Bosniak category 1 and do not require further workup or imaging.
To evaluate complex renal cysts by CT scan or MRI, it is imperative to obtain the scan through the kidneys before IV contrast and after IV contrast during the same exam with the same parameters. The Bosniak classification system is used to guide treatment.
Bosniak I lesions
Treatment: Bosniak lesions in category I are benign cysts and do not require further imaging or intervention.
Bosniak II lesions
Treatment: Bosniak lesions in category II are benign cysts and do not require further imaging or intervention.
Bosniak IIF lesions
Treatment: For Bosniak lesions category IIF, the F stands for follow up. Follow up cross-sectional imaging with IV contrast is recommended at 6 months, 12 months, and then yearly for 5 years. If the lesions grow in size and develop internal enhancing soft tissue, they become category III lesions and require surgical treatment or ablation therapy.
Bosniak III lesions
Treatment: Bosniak lesions category III require excision or can be treated with ablation therapy. They have a 50% to 80% chance for malignancy.
Bosniak IV lesions
Treatment: Bosniak lesions category IV require excision or can be treated with ablation therapy. Bosniak category IV lesions have a greater than 90% chance of malignancy.
Surgical exploration and treatment of simple renal cysts are no longer employed.
Radiation therapy is not a treatment for simple cysts.
Simple cysts do not affect mortality or morbidity.
Rarely, simple cysts can become infected and require antibiotic treatment.
Also, simple cysts rarely become so large as to cause mass effect on other organs and block the flow of urine. This would require drainage of the cyst with sclerotherapy or laparoscopic removal with surgery.