OVERVIEW

What is a simple renal cyst?

A simple renal cyst is the most common cystic kidney disease. It primarily manifests as fluid-filled sacs (cysts) on or within the kidney that do not connect to the outside. These cysts are mostly round or oval, with clear boundaries, and typically contain pale yellow fluid without kidney tissue. They may appear as single or multiple cysts and can occur in one or both kidneys. Simple renal cysts vary in size, ranging from less than 1 cm to over 10 cm, with most being smaller than 2 cm in diameter.

Simple renal cysts are acquired and non-genetic. The widely accepted hypothesis is that they result from abnormal proliferative responses due to kidney ischemia or injury. This condition is more common in adults, particularly men, and its incidence increases with age. However, there is no need for excessive concern, as simple renal cysts are usually asymptomatic, do not affect normal life, and generally do not require treatment ^[1-3].

Another congenital kidney abnormality related to renal cysts is polycystic kidney disease, which is hereditary and differs in symptoms and treatment. For more details, click: Polycystic Kidney Disease.

Are simple renal cysts and renal cysts the same thing?

Simple cysts are the most common type of renal cysts.

Renal cysts can be classified as simple renal cysts and complex renal cysts. Generally, when people refer to renal cysts, they mostly mean simple renal cysts, which account for about 65%–70% of all renal cyst cases ^[1].

How common are simple renal cysts?

It’s difficult to generalize how common they are, but relatively speaking, older individuals are more prone to the condition.

The incidence of simple renal cysts increases with age. They are more frequently seen in adults over 50, with men being more commonly affected.

According to Campbell-Walsh Urology, the incidence remains relatively stable from newborns to individuals under 18, averaging 0.22%, meaning about 2 in 1,000 people under 18 may have the condition. In 40-year-olds, the incidence rises to 20%, or about 20 in 100 people. For those aged 50 and above, the incidence can exceed 50%, meaning over 50 in 100 people may have simple renal cysts ^[2].

SYMPTOMS

What are the manifestations of simple renal cysts?

Most simple renal cysts do not cause noticeable symptoms. However, when the cyst enlarges (6–8 cm) and compresses surrounding tissues, affecting kidney function (occurring in about 2%–4% of cases), the following manifestations may appear ^[1,3]:

1. Discomfort or pain in the lower back or abdomen: Enlargement of the cyst or rupture due to spontaneous causes or blunt abdominal trauma may lead to dull or aching pain in the lower back or abdomen, localized to one or both sides, possibly radiating downward or to the back.
2. Hematuria: In rare cases, blood may be visible in the urine, or red blood cells may be detected under a high-power microscope.
3. Abdominal mass: Occasionally, if the cyst is very large, a palpable lump may be felt on the skin surface.
4. Proteinuria: Generally, the amount of protein in the urine is small.
5. Hypertension: Compression of the kidney by the cyst, leading to renal ischemia, may cause high blood pressure.
6. Secondary infection: In addition to worsening pain, fever (normal body temperature ranges between 36–37.2°C) and general discomfort may occur.

Can simple renal cysts affect kidney function?

In rare cases, they can, typically when a simple renal cyst obstructs the renal calyx (a structure near the center of the kidney). Younger patients with simple renal cysts may also experience mild reductions in kidney function (though still within the normal range).

Simple renal cysts account for 65%–70% of renal cystic diseases, and they are increasingly detected during physical examinations. Many people worry about whether these cysts will affect kidney function. Although enlargement of the cyst can compress the renal parenchyma, most simple renal cysts do not impair kidney function ^[1].

Can simple renal cysts become cancerous?

Simple renal cysts are benign and almost never turn cancerous, with a malignancy risk of less than 1%.

There is no need to panic if a cyst is accidentally discovered, but it is important to follow up with an ultrasound every six months to a year. Surgical intervention is necessary if the cyst meets certain conditions, such as a large diameter (4 cm or more), rapid growth, or symptoms like back pain, hematuria, or hypertension. Prompt treatment is crucial to avoid complications such as infection, bleeding, or cyst rupture ^[3].

CAUSES

What Causes Simple Renal Cysts?

Regarding the causes of simple renal cysts, the hypothesis widely accepted by experts is the abnormal proliferative response of cysts induced by renal ischemia or injury.

Additionally, advancing age, male gender, impaired renal function (creatinine >1.5 mg/dL), hypertension, and smoking are also risk factors for the development of simple renal cysts ^[3].

1. Aging: The incidence gradually increases with age.
2. Smoking: Harmful components such as nicotine in tobacco can cause renal vascular diseases, leading to renal ischemia.
3. Male gender: Studies indicate that the incidence of simple renal cysts is higher in males than in females.
4. Impaired renal function: When creatinine exceeds 1.5 mg/dL, impaired renal function may trigger simple renal cysts.
5. Hypertension: The number and size of cysts are closely associated with elevated blood pressure ^[3].

DIAGNOSIS

What are the examination methods for simple renal cysts?

Almost all simple renal cysts can be detected through kidney B-ultrasound, kidney MRI, kidney CT, cyst puncture, and cyst fluid examination, which are commonly used and effective diagnostic methods.

It should be noted that if the examination results meet the ultrasound criteria for benign simple renal cysts, no further testing is required. CT is only used for further confirmation when the results are unclear (indicating potential malignancy) ^[1].

What diseases should simple renal cysts be differentiated from?

Simple renal cysts need to be distinguished from renal cancerous masses, hydronephrosis, extrarenal tumors, renal abscesses, polycystic kidney disease, cystic renal cell carcinoma, and parapelvic cysts. Differentiation can be achieved through kidney B-ultrasound, CT, renal puncture biopsy, or renal puncture fluid analysis.

TREATMENT

How to treat simple renal cysts?

Simple renal cysts are mostly asymptomatic and have little impact on kidney function or surrounding tissues, requiring no special treatment. Follow-up examinations with ultrasound should be performed every 6 months to 1 year, along with blood tests if necessary, to monitor cyst size and kidney function.

However, if the cyst becomes infected, prompt medical attention is required for anti-infection treatment, such as antibiotics like ciprofloxacin and levofloxacin, which can penetrate the cyst for better efficacy. Cyst puncture and drainage may also be performed, leading to cyst disappearance or shrinkage ^[1,2].

If the cyst diameter is large (exceeding 4 cm) or causes compression symptoms, ultrasound-guided aspiration and sclerotherapy can be performed, though recurrence rates are high.

Currently, for cysts larger than 4 cm that significantly compress the renal parenchyma, renal pelvis, or calyces, or cause ureteral obstruction with obvious symptoms, cyst decortication surgery is recommended ^[2].

With the advancement of laparoscopic techniques, laparoscopic decortication for simple renal cysts is widely recognized as a safe, minimally invasive, and effective treatment with low recurrence rates.

Can simple renal cysts heal on their own?

Simple renal cysts do not resolve spontaneously and will not disappear without treatment.

Small cysts (5–10 mm) typically grow slowly and have minimal impact on kidney function. If asymptomatic, no intervention is needed, and ultrasound monitoring every 6 months or 1 year is sufficient.

For cysts larger than 4 cm, rapidly growing cysts, or those causing compression or calyceal obstruction, surgical treatment is necessary ^[1,4].

When should patients with simple renal cysts seek medical attention?

Patients with small cysts who do not require immediate treatment should undergo regular check-ups. Seek prompt medical attention if any of the following symptoms occur:

1. Cyst enlargement leading to significant compression of renal parenchyma, pelvis, or calyces, or suspected malignancy on imaging;
2. Urinary frequency, urgency, pain, abnormal urine tests, or recurrent urinary tract infections;
3. Severe flank or abdominal pain after intense activity or trauma;
4. Unexplained recent increase in blood pressure ^[1].

Is surgery always necessary for simple renal cysts?

Generally, cysts smaller than 4 cm do not require surgery but need regular monitoring.

Surgery is recommended for cysts larger than 4 cm, rapidly growing cysts, or those causing symptoms like back pain, hematuria, hypertension, infection, rupture, or suspected malignancy ^[4].

The decision for surgery and treatment method depends on the patient’s condition and cyst severity.

What is sclerotherapy for simple renal cysts?

Sclerotherapy is a simple, safe, and effective treatment. Under ultrasound guidance, a fine needle aspirates the cyst fluid, followed by injection of a sclerosing agent (e.g., 95% ethanol) to prevent recurrence. Potential side effects include ethanol absorption affecting renal tissue or leakage causing dizziness or nausea.

Tetracycline may also be used as a sclerosing agent, offering both sclerosing and anti-infection effects with fewer side effects.

This method is suitable for elderly patients. Ethanol-allergic patients should be closely monitored, and alternative agents may be used if adverse reactions occur ^[4].

What surgical methods are commonly used for simple renal cysts?

For large cysts (≥4 cm), cyst decortication is performed. If malignancy is confirmed, prompt surgery is required.

Laparoscopic surgery is the most common approach, offering minimal invasiveness, fast recovery, and reliable outcomes ^[4].

Is recurrence likely after surgery for simple renal cysts?

Recurrence is possible.

Post-laparoscopic recurrence rates are around 1%, often due to insufficient cyst wall removal. Some polycystic kidney patients may experience recurrence or worsening after decortication ^[4].

Puncture and aspiration have higher recurrence rates, as reduced intracystic pressure may accelerate regrowth.

Which department should patients with simple renal cysts visit?

Nephrology or urology.

Asymptomatic patients should consult nephrology for initial evaluation and monitoring.

Patients with large cysts (≥4 cm) should visit urology for surgical assessment.

DIET & LIFESTYLE

What are the lifestyle precautions for patients with simple renal cysts?

1. Low-salt, low-protein, and low-fat diet: Maintain a diet low in salt (2–3 g per day is recommended), low in protein (such as milk, eggs, fish, etc.), and low in fat. Avoid foods high in potassium (such as corn, crucian carp, etc.) to reduce kidney burden. Eat more foods rich in vitamins and plant fiber (such as vegetables and fruits) to ensure smooth bowel movements.
2. Drink plenty of water and avoid holding urine: Drink enough water (at least 2000 mL per day) and avoid holding urine to prevent bacterial growth in the urinary tract and kidney infections. This also helps reduce the occurrence of urinary stones ^[5].
3. Control blood pressure and blood sugar: Hypertension and diabetes can lead to renal artery hardening, causing kidney ischemia and worsening kidney damage. Therefore, it is important to manage these risk factors.
4. Moderate exercise: Develop a habit of regular physical activity. Choose preferred exercises such as jogging, swimming, or badminton, aiming for mild sweating.
5. Avoid trauma: Reduce intense or risky activities to prevent rupture or bleeding of simple renal cysts, which could lead to kidney infections.

PREVENTION

How to Prevent Simple Renal Cysts?

1. Maintain a positive and optimistic attitude: Keeping an optimistic mindset and emotions can boost immunity.
2. Exercise moderately: Exercise should be done in moderation. Intense sports (such as soccer or rugby) or a lack of exercise can negatively affect the kidneys. Additionally, avoid abdominal trauma ^[1].
3. Avoid hypertension: Blood pressure is closely related to simple renal cysts, so it is important to prevent high blood pressure ^[3].
4. Follow a balanced diet: Ensure adequate intake of high-fiber and high-vitamin foods. Avoid picky eating—grains, fresh vegetables, fruits, lean meats (beef, lamb, pork), poultry, eggs, milk, fish, and seafood are all suitable. Avoid alcohol, spicy foods, and fried foods like spicy hot pot or fried chicken. Refrain from blindly taking supplements or trusting advertised health products to prevent kidney damage or increased burden on the kidneys.
5. Prevent colds and infections: Colds can lower immunity and impair kidney function. Additionally, infections in other parts of the body can alter the body's internal environment through nerves and body fluids, affecting kidney function and potentially leading to simple renal cysts or other kidney diseases. During seasonal changes, stay warm and adjust clothing promptly. During flu outbreaks, avoid crowded places, wear masks when going out, and wash hands thoroughly upon returning home.