| disease | Simple (Isolated) Renal Cyst |
Simple renal cysts are usually unilateral and solitary, but can also be multiple or multilocular, rarely occurring bilaterally. Compared to polycystic kidney disease, they differ in both clinical and pathological manifestations.
bubble_chart Pathogenesis
It remains unclear whether simple renal cysts are congenital or acquired. Their origin may be similar to polycystic kidneys, differing only in degree. On the other hand, simple renal cysts can be induced in animals by causing renal tubule obstruction and local ischemia, suggesting that such lesions may also be acquired. As the cyst enlarges, the pressure it exerts can damage the kidney parenchyma, though not enough to impair renal function. An isolated cyst may compress the ureter, leading to progressive hydronephrosis, which can subsequently be complicated by infection. Feiner, Katz, and Gallo noted in 1980 that acquired renal cystic disease is common in long-term dialysis patients. Kessel and Tynes observed in 1981 two cases of spontaneous regression of renal cysts.
bubble_chart Pathological Changes
Simple renal cysts often involve the renal root of the nose. Symptomatic cysts have an average diameter of about 10 cm, but a few may grow large enough to fill the flank. The cysts usually contain clear amber-colored fluid. The cyst wall is very thin, and the cyst often appears as a "blue dome." Occasionally, calcification of the cyst wall may be observed. Approximately 5% of cysts contain bloody fluid, with half of these potentially having papillary carcinoma on the cyst wall. Simple renal cysts commonly occur on the kidney's surface but can also be located deep within. When a cyst is deep-seated, its wall is tightly adhered to the epithelial lining of the renal pelvis and calyces, making separation difficult, though the cyst does not communicate with the renal pelvis. Microscopic examination may reveal grade III fibrosis and hyaline degeneration in the cyst wall, along with calcified areas, and adjacent renal tissue may also show compression-induced fibrosis. Many cases of simple renal cysts in children have been reported, but giant cysts remain rare in this age group. In such cases, the possibility of malignancy must first be ruled out. In urography, multilocular renal cysts can be confused with tumors. Ultrasound imaging can confirm the diagnosis, though CT or MRI may occasionally be required.
bubble_chart Clinical Manifestations(1) Symptoms: Common pain is located in the flank and back, usually presenting as intermittent dull pain. When hemorrhage causes the cyst wall to expand, sudden severe pain may occur. Gastrointestinal symptoms may occasionally appear, raising suspicion of peptic ulcer or gallbladder disease. Patients may discover an abdominal mass themselves, although such large cysts are rare. When the cyst becomes infected, patients often complain of flank pain, general discomfort, and fever.
(2) Signs: Physical examination is mostly normal, though occasionally a mass may be palpated or percussed in the renal area. If the cyst becomes infected, there may be tenderness in the flank.
(3) Laboratory tests: Urinalysis is usually normal. Microscopic hematuria is rare. Renal function tests are also normal unless the cysts are multiple or bilateral (rare). Even with extensive surface destruction of one kidney, the contralateral kidney can maintain normal overall renal function through compensatory hypertrophy.
(4) X-ray examination: Plain abdominal films often show partial enlargement of the renal shadow or a mass compressing it. The weight or position of the cyst may cause renal torsion, leading to abnormal renal axis. Sometimes, streaky calcifications can be seen at the edge of the mass. Excretory urography can confirm the diagnosis. In films taken 1-2 minutes after intravenous contrast injection, the vascular density of the kidney parenchyma increases, while the space occupied by the cyst does not, as it contains no blood vessels. Serial urography films demonstrate the presence of a mass. One or several calyces or the renal pelvis around the cyst often become widened, flattened, or even lose their lumen due to indentation or bending. Oblique or lateral views are also helpful for diagnosis. When the mass occupies the renal hilum, the upper ureter may shift toward the spine. The kidney itself may also rotate. The psoas muscle may be visible through the radiolucent cyst fluid. When conventional urography cannot effectively distinguish the radiopaque kidney parenchyma, renal tomography can enhance the contrast between vascularized kidney parenchyma and the cyst. Occasionally, kidney parenchymal tumors may also be relatively avascular, making them easily confused with cysts. In rare cases, the cyst wall may develop tumors, necessitating further differential diagnostic tests.
(6) Renal ultrasound: Ultrasound plays a significant role in non-invasive diagnostic techniques for differentiating renal cysts from solid masses. When ultrasound findings are consistent with a cyst, the cyst can be punctured and aspirated under ultrasound guidance.
(7) Isotope scanning: Linear scanning can show the outline of the mass but is difficult to distinguish between cysts and tumors. Technetium scanning can reveal avascular masses.
(8) Percutaneous cyst aspiration under cystography: When the above tests still leave doubts about differentiating cysts from tumors, puncture and aspiration are necessary.
bubble_chart Treatment Measures
(1) Special Treatment:
① When excretory urography, renal tomography, ultrasonography, and CT still fail to provide a definitive diagnosis, angiography may be chosen, with percutaneous cyst aspiration as the next diagnostic step. This can be performed under fluoroscopic or ultrasonic guidance. The discovery of clear cyst fluid is an encouraging sign, but cytological examination of the aspirate should still be conducted. The fat content should also be measured, as elevated levels are consistent with a tumor diagnosis. Subsequently, the cyst fluid should be fully drained and replaced with a contrast agent. Images are then taken in different positions to assess the smoothness of the cyst wall and the presence of any neoplastic growth. Before withdrawing the contrast agent, 3 ml of Lipiodol is injected into the cyst cavity to reduce the likelihood of recurrence caused by fluid reaccumulation. In 1981, Bean introduced the method of injecting 95% alcohol into the emptied cyst, reporting only 1 recurrence among 29 patients treated this way. Simple cyst aspiration, in contrast, results in recurrence in most cases. If the aspirate is bloody, surgical exploration may be considered, as this could indicate severe seasonal disease or even malignant transformation.
③ When diagnostic uncertainty persists, surgical exploration may be warranted. Ambrose et al. (1971) preferred surgical exploration even in most confirmed cyst cases. Among their 55 surgical cases, 5 (9%) were confirmed as malignant. Typically, only the extrarenal portion of the cyst is excised. Nephrectomy is indicated only in cases of severe renal damage, though this is uncommon.
(2) Treatment of Complications: When a cyst becomes infected, intensive antibiotic therapy is required, although Muther and Bennett (1980) found that antibiotic concentrations in cyst fluid are often low. Percutaneous drainage is frequently necessary. If percutaneous drainage fails, surgical excision of the extrarenal cyst wall combined with drainage has proven effective. In cases of hydronephrosis, removing the obstructive cyst wall can relieve ureteral obstruction. Pyelonephritis involving the kidney suggests urinary tract obstruction with impaired ureteral drainage. Cyst removal naturally relieves urinary pressure, enhancing the effectiveness of antibiotic therapy.
The use of ultrasound imaging and CT scans makes the diagnosis of simple renal cysts extremely accurate. Ultrasound imaging is an excellent method for annual follow-up to observe changes in size, shape, and internal texture. If there are signs of malignancy, a CT scan can be performed, and if confirmation is needed, cyst fluid aspiration can follow. Most cysts have a favorable prognosis.
Spontaneous infection is rare in simple renal cysts, but once it occurs, it is difficult to distinguish from a kidney abscess. Sometimes, bleeding may occur within the cyst, and when it happens suddenly, it can cause severe pain. The bleeding may originate from an associated cancerous tumor on the cyst wall. If the cyst is located at the renal root of the nose and presses against the ureter, it can worsen hydronephrosis, and the pressure of urine on the renal pelvis may lead to back pain. This obstruction can also lead to kidney infection.
(1) Renal carcinoma: Presents as a space-occupying lesion, but tends to occur in the deeper parts, leading to more pronounced calyceal distortion. Hematuria is common, whereas it is absent in cysts. When a solid renal tumor overlies the psoas muscle, the muscle margin may be obscured on a plain abdominal film, whereas it remains visible with cysts. Evidence of metastasis (e.g., weight loss, lack of strength, palpable supraclavicular lymphadenopathy, metastatic nodules on chest X-ray), polycythemia, hypercalcemia, or an elevated ESR suggests carcinoma. It should be remembered that the wall of a cyst may also undergo malignant transformation. If the renal vein is occluded by tumor, the excretory urogram may be faint or absent. Ultrasonography and CT are ultimately used for differential diagnosis. Angiography and nephrotomography may show a contrast "pool" in highly vascular tumors, whereas cysts show no such density. It is prudent to assume that all renal space-occupying lesions are carcinomas until proven otherwise.
(2) Polycystic kidney disease: As demonstrated by urography, this condition is almost always bilateral, with diffuse distortion of the calyces and pelvis being the rule. Simple renal cysts are usually solitary and unilateral. Polycystic kidneys are often associated with renal impairment and hypertension, whereas renal cysts are not.
(3) Renal abscess: This condition is rare. A history of skin infection weeks before the sudden onset of fever and localized pain may be elicited. Urography shows lesions similar to cysts and tumors, but the renal outline and psoas shadow are obscured due to perinephritis. The kidney is often fixed, as confirmed by comparing its position in supine and upright views. Angiography reveals an avascular lesion. Gallium-67 scanning may demonstrate the inflammatory nature of the lesion, though an infected simple renal cyst can present similarly.
(4) Hydronephrosis: Symptoms and signs may be identical to those of a simple renal cyst, but the urogram is distinctly different. Cysts deform the kidney, whereas hydronephrosis shows dilation of the calyces and pelvis due to obstruction. Acute or subacute hydronephrosis often causes more localized pain due to increased intrapelvic pressure and may be complicated by infection.
(5) Extrinsic renal tumors (e.g., adrenal, mixed retroperitoneal tumors): May displace the kidney but rarely invade it or distort the calyces.
(6) Echinococcosis: When the cyst does not communicate with the renal pelvis, it is difficult to distinguish from a simple renal cyst, as hydatid elements will not yet appear in the urine. Calcification of the cyst wall is often seen on X-ray. Skin sensitivity tests may aid in diagnosis. {|105|}
