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What are kidney cysts?
Acquired kidney cysts are fluid-filled sacs that can form in one or both kidneys. A person may develop a single cyst or multiple cysts at once, and these are typically round or oval in shape.
The size of a kidney cyst can vary considerably, ranging from as small as a pea to as large as a golf ball, or occasionally even larger.
Simple kidney cysts are generally harmless. Unlike more serious kidney conditions, they do not typically cause the kidney to become enlarged, they do not replace or damage the normal structure of the kidney tissue, and therefore they do not usually affect overall kidney function. This is an important distinction from patients with polycystic kidney disease, a genetic condition in which numerous cysts progressively replace healthy kidney tissue and significantly impair kidney function over time (this distinction is explored in more detail later in this article).
Because simple kidney cysts are so common and typically cause no symptoms, understanding when they require attention, and when they can simply be monitored, is important for anyone who receives this diagnosis, often unexpectedly, during imaging performed for an unrelated reason.
Causes
It is estimated that simple kidney cysts affect up to 1 in 10 people in the general population, with the prevalence rising to as many as 1 in 3 people among those over the age of 70. Despite how common this condition is, the exact underlying causes that explain why these cysts form in the first place remain unclear.
Current medical understanding suggests that simple kidney cysts likely develop due to a combination of factors related to the natural aging process of kidney tissue, including gradual changes in the tiny tubules within the kidney that filter blood and produce urine. Over time, small pouches or outpouchings can form along these tubules, gradually filling with fluid and developing into the fluid-filled sacs known as simple cysts.
Risk Factors
Several factors are considered to increase the likelihood of developing kidney cysts, including:
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Age: the prevalence of kidney cysts increases steadily as people get older, with cysts being relatively uncommon in younger adults but very common in older adults.
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Male sex: men appear to develop simple kidney cysts somewhat more frequently than women, for reasons that are not yet fully understood.
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Impaired kidney function: individuals with reduced kidney function or underlying kidney disease appear to be at increased risk of developing cysts.
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High blood pressure: hypertension has been associated with an increased likelihood of kidney cyst formation, although the exact nature of this relationship, meaning whether high blood pressure directly causes cysts or whether the two conditions simply tend to occur together as part of general kidney aging, is still being studied.
It is worth emphasizing that having one or more of these risk factors does not mean a person will necessarily develop kidney cysts, and many people with kidney cysts have no identifiable risk factors at all.
Symptoms
Isolated simple kidney cysts generally do not cause any symptoms or noticeable problems. In fact, they are very frequently discovered entirely by accident, identified incidentally during imaging tests performed for a completely unrelated medical reason, such as an abdominal ultrasound or CT scan ordered to investigate a different concern.
However, when a kidney cyst grows large enough to become clinically significant, it can occasionally cause problems, including:
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Pressure on surrounding bones or organs, which can cause pain or discomfort in the back or abdomen.
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Obstruction of blood flow through the kidney, or of urine flow through the ureters (the tubes that carry urine produced by the kidneys down to the bladder), which can occasionally lead to more serious kidney problems if left unaddressed.
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Infection, which can cause fever, pain in the kidney area, and a feeling of tension or pressure in the flank or lower back.
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Rupture, meaning the cyst bursts, which can cause pain and the presence of blood in the urine, a condition known as hematuria.
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An increase in blood pressure, for reasons that are not yet fully understood, though this is thought to potentially relate to the cyst’s effect on local blood flow within the kidney.
Multiple kidney cysts are also commonly found in patients with chronic kidney disease, particularly in those who have been undergoing long-term dialysis for several years. This particular pattern, sometimes referred to as acquired cystic kidney disease, is distinct from simple kidney cysts and is thought to result from the long-term changes that occur within kidney tissue in patients with advanced kidney disease.
Complications
The vast majority of simple kidney cysts are entirely harmless and clinically insignificant. For most patients, a simple kidney cyst represents nothing more than an incidental finding that requires no treatment and has no meaningful impact on overall health or kidney function.
That said, in the relatively uncommon situations described above, where a cyst becomes large, infected, or ruptures, prompt medical attention is important to prevent further complications and to properly manage any associated pain, infection, or bleeding.
It is also worth noting that, in rare cases, what initially appears to be a simple cyst on imaging can actually represent a more complex cystic lesion, which may carry a small risk of being cancerous. This is one of the key reasons why proper imaging characterization, discussed in the diagnosis section below, is so important whenever a kidney cyst is identified.
Diagnosis
Because acquired kidney cysts typically cause no symptoms, the diagnosis is very often made incidentally, discovered while reviewing images obtained from imaging tests ordered for a completely different reason. Common imaging methods used to identify kidney cysts include:
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Ultrasound, often the first imaging test to reveal the presence of a kidney cyst, and a useful tool for basic characterization of the cyst’s appearance.
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CT scan, which can provide more detailed information about the size, shape, and internal characteristics of the cyst.
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MRI (magnetic resonance imaging), which may be used in certain cases, particularly when additional detail is needed or when contrast dye used in CT scanning needs to be avoided.
The images collected through these tests are generally sufficient to reliably distinguish a harmless simple cyst from a kidney tumor, based on well-established imaging criteria that assess the wall of the cyst, its internal contents, and whether it enhances (lights up) with contrast dye. Radiologists commonly use a standardized classification system, known as the Bosniak classification, to categorize kidney cysts based on their appearance on CT or MRI imaging. This system ranges from category I, representing a straightforward simple cyst with essentially no risk of cancer, to category IV, representing a clearly complex cystic mass with a high likelihood of malignancy.
In addition to imaging, doctors will typically order urine tests and blood tests to confirm that overall kidney function remains normal and to rule out any signs of infection or other kidney-related problems.
Treatment
One or more kidney cysts that are not causing any symptoms generally do not require any specific treatment. In these cases, the recommended approach is typically simple observation, which may include periodic follow-up ultrasound imaging to monitor the cyst over time and confirm that it remains stable in size and appearance, without any concerning changes developing.
The situation is different when a kidney cyst is causing pain or other symptoms, such as an obstruction to the normal flow of urine. In these cases, active treatment may be recommended. The two most commonly used treatment approaches are:
Sclerotherapy
Sclerotherapy is a minimally invasive technique used to drain a symptomatic kidney cyst. Using ultrasound guidance, a doctor inserts a long, thin needle through the skin and directs it precisely into the cyst. The fluid inside the cyst is then drained out, after which the empty cyst cavity is filled with an alcohol-based solution. This solution helps harden the inner lining of the cyst wall, reducing the likelihood that fluid will reaccumulate in the future. This procedure is typically performed on an outpatient basis under local anesthesia, meaning the patient can usually go home the same day.
Surgery
If a kidney cyst is particularly large, or if sclerotherapy is not appropriate or has not been successful, surgical treatment may be necessary. This is typically performed using a laparoscopic approach, meaning the surgeon makes several small incisions in the abdomen through which specialized surgical instruments, including a small camera, are inserted. During this procedure, the cyst is first drained of its fluid contents, and the outer wall of the cyst is then either surgically removed or treated with heat energy to destroy the remaining cyst lining and prevent it from reforming. This procedure requires a hospital setting and typically involves the use of general anesthesia.
Compared to sclerotherapy, laparoscopic surgery is generally considered a more invasive and involved procedure, but it also tends to provide a more definitive, long-term solution with a lower risk of the cyst returning in the future.
Kidney Cyst Size: When Should They Be Removed?
There are currently no universally established, formal guidelines dictating an exact size threshold at which a kidney cyst should automatically be treated or removed. However, based on available clinical research, it is generally considered that a kidney cyst becomes more likely to cause symptoms once it exceeds approximately 5 to 6 centimeters in diameter. Cysts of this size may make a patient a reasonable candidate for treatment, although intervention is generally only recommended when a patient is actually experiencing specific, bothersome symptoms directly attributable to the cyst, rather than based on size alone.
In other words, the decision to treat a kidney cyst should always be based primarily on whether it is causing a genuine clinical problem, such as pain, infection, or obstruction, rather than being pursued purely because of its size on an imaging report.
Research comparing treatment approaches has suggested that the laparoscopic surgical approach may offer somewhat greater long-term effectiveness compared to sclerotherapy, particularly in terms of lower recurrence rates. However, this increased effectiveness comes at the cost of a more invasive, more complex surgical procedure, which carries its own set of risks and requires a longer recovery period. For this reason, the choice between sclerotherapy and surgery should always be made on an individual basis, taking into account the specific characteristics of the cyst, the severity of the patient’s symptoms, and the patient’s overall health and personal preferences, in consultation with a qualified urologist or nephrologist.
Kidney Cysts Versus Polycystic Kidney Disease
It is important for patients to understand the difference between simple kidney cysts, the focus of this article, and a separate, more serious condition known as polycystic kidney disease (PKD). While both conditions involve fluid-filled cysts within the kidneys, they are fundamentally different in their cause, severity, and long-term implications.
Simple kidney cysts, as described throughout this article, are typically acquired over time, most likely related to the natural aging process of kidney tissue. They usually occur as one or a few isolated cysts, do not significantly enlarge the kidney, do not replace healthy kidney tissue, and do not impair overall kidney function.
Polycystic kidney disease, by contrast, is an inherited genetic condition in which numerous cysts develop throughout both kidneys over time. As these cysts grow and multiply, they progressively replace healthy kidney tissue, causing the kidneys to become significantly enlarged and gradually impairing kidney function. Over time, PKD can lead to chronic kidney disease and, in many cases, eventual kidney failure requiring dialysis or kidney transplantation. PKD is also frequently associated with other complications, including high blood pressure, cysts in other organs such as the liver, and an increased risk of certain vascular abnormalities.
Given these important differences, it is essential that any diagnosis of kidney cysts be properly evaluated by a healthcare professional, who can determine whether the finding represents simple, harmless cysts or whether further investigation is warranted to rule out an underlying genetic condition such as polycystic kidney disease, particularly in patients with a family history of kidney disease or in those found to have numerous cysts affecting both kidneys.
Frequently Asked Questions
Are kidney cysts dangerous?
In the vast majority of cases, no. Simple kidney cysts are generally harmless and do not affect kidney function or overall health. However, in rare cases, a cyst may become large enough to cause symptoms, become infected, or, very rarely, be associated with an underlying cancerous growth, which is why proper imaging evaluation is important.
Do kidney cysts need to be removed?
Most kidney cysts do not need to be removed and can simply be monitored over time with periodic imaging. Treatment is generally only recommended when a cyst is causing significant symptoms, such as pain, infection, or obstruction of normal urine flow.
Can kidney cysts turn into cancer?
Simple kidney cysts themselves do not turn into cancer. However, some cystic kidney lesions can have more complex features that carry a higher risk of being, or becoming, cancerous. This is why doctors use standardized imaging classification systems to carefully evaluate the specific characteristics of any kidney cyst identified.
Can diet or lifestyle changes help prevent or shrink kidney cysts?
There is currently no strong scientific evidence that specific dietary or lifestyle changes can prevent simple kidney cysts from forming or cause existing cysts to shrink. However, maintaining good overall kidney health, including staying well hydrated, managing blood pressure, and avoiding excessive use of medications that can harm the kidneys, is always a reasonable and beneficial general health practice.
How often should a kidney cyst be monitored?
For simple, asymptomatic kidney cysts, many doctors recommend periodic follow-up ultrasound imaging, often on a yearly basis or as otherwise advised by your physician, primarily to confirm that the cyst remains stable over time. The exact frequency of monitoring should be determined individually based on the specific characteristics of the cyst and any relevant risk factors.
Sources and Bibliography
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). “Simple Kidney Cysts”. Available at: https://www.niddk.nih.gov/health-information/kidney-disease/simple-kidney-cysts
- National Kidney Foundation. “Simple Kidney Cysts”. Available at: https://www.kidney.org/atoz/content/simple-kidney-cysts
- Terada N, et al. “The natural history of simple renal cysts”. BJU International. Available at: https://bjui-journals.onlinelibrary.wiley.com/doi/abs/10.1111/j.1464-410X.2004.04844.x
- National Center for Biotechnology Information (NCBI). “Prevalence and Risk Factors of Renal Cysts”. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8925956/
- National Center for Biotechnology Information (NCBI). “Management of Symptomatic Simple Renal Cysts”. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4719512/
- National Center for Biotechnology Information (NCBI). “Comparison of Sclerotherapy and Laparoscopic Surgery for Renal Cysts”. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4376217/
- Mayo Clinic. “Simple Kidney Cysts”. Available at: https://www.mayoclinic.org
