Every year when we dissect the kidneys in our cadavers, we find what appear to be cysts. Some are small, about the size of your fingernail, but othersare quite large, about 1/4 of the total kidney. What are these and what causes them? Is it a normal process of aging?
Kidney cysts are a relatively common occurrence. We see usually several examples in our laboratory each year, where we dissect 40 cadavers a year.
What you describe sounds like simple renal cysts. Simple renal cysts are very common acquired lesions. They are found in up to half of persons over 50 years of age. Unless they become very large, they are asymptomatic, and are only found upon autopsy or dissection. These cysts are fluid-filled, and may be isolated or multiple. They are usually found in the cortex where they may bulge beneath the capsule. Less frequently, they are found in the medulla. Tereda et al. (2002) report that the prevalence of these cysts increase dramatically with age. They report the prevalence varies from 5.1 percent in the fourth decade to 36.1 percent in the 8th decade of life. (These cysts were located by ultrasound, and this may account for the lower prevalence than that found through autopsy and dissection.) They report an annual increase in size of 2.82 mm. (6.3 percent) annually. Cysts grow at a greater annual rate in those under 50 than in those over 50 years of age. They further found that multiloculated (many-spaced) cysts grew more rapidly than simple cysts, and suggest that they multiloculated cysts may have a different natural history than simple cysts.
Cysts within the kidney can result from several other sources as well. One common cause of these cysts is polycystic kidney disease. Polycystic kidney diseases are a group of heterogeneous congenital and acquired disorders that result in distortion of the renal perynchema and by the presence of numerous cysts. These include:
Autosomal dominant (adult) polycystic kidney disease (ADPKD):
ADPKD affects about 1:400 to 1:1000 persons in the U.S. About half of all these patients end up with end-stage renal failure. This disorder is responsible for about 10 percent of all renal disorders that require dialysis or transplant. This form is usually genetically transmitted, but can result from spontaneous mutations. Cysts arise in portions of the renal tubules from a few cells that proliferate abnormally. The walls of the tubules become covered with undifferentiated epithelium. A defective basement membrane allows for dilation of the tubule. Initially the cyst is filled by fluid derived from the glomerular filtrate. However, eventually the cysts become separated from the tubules, and the fluid is replaced by fluid derived from transepithelial secretion. It appears that only about 2 percent of tubules are affected, but the enlarging cysts crowd out the healthy tissue, and renal failure results from the loss of healthy tissue. In ADPKD, the kidneys are generally enlarged, and the surface of the kidneys are distorted by the presence of multiple cysts. About a third of patients with ADPKD also have hepatic cysts with a lining that resembles bile duct epithelium.
Autosomal recessive (infantile) polycystic kidney disease (ARPKD):
ARPKD is a disease of infants, and is much rarer than ADPKD. It occurs in about 1:10,000 to 1:50,000 live births. About 75% of these infants die in the perinatal period, largely because the enlarged kidneys compromise the expansion of the lungs. In some cases the kidneys are so large that delivery of the infant is impeded. In exceptional cases, this form may also occur in older children and even adults. In ARPKD the external surface of the kidney remains smooth, and the involvement is always bilateral. The cysts are fusiform enlargement of the collecting ducts, and have a radiate arrangement parallel to the renal capsule. Interstitial fibrosus and tubular atrophy are commonly associated, especially when the disease is present in older children. There is also usually associated congenital hepatic fibrosis.
Nephronophthisis-medullary cystic disease complex:
This disorder is a group of autosomal recessive diseases characterized by renal cysts, sclerotic kidneys and progressive renal failure. It is probably related to developmental defects in tubular basement membranes. In this disorder, the kidneys are usually small and display multiple, variably sized cysts up to 1 cm. in diameter. The cysts arise in the distal part of the nephrons. The kidney displays atrophic tubules with markedly thickened basement membranes. This stage proceeds the formation of cysts. Cases in which cysts do not develop have been described. The cysts eventually develop, however, and the remainder of the parenchyma becomes atrophic. Glomerular sclerosis and interstitial fibrosis usually follow.
Medullary sponge kidney:
Medullary sponge kidney is characterized by small cysts less than 5 mm. in diameter in one or more renal papillae. The cysts are lines by columnar or cuboidal epithelium, and communicate with the collecting ducts in the renal papillae. In about 75 percent of cases, the disorder is present bilaterally. The disease is usually appears in patients between 30 and 60 years of age, and is characterized by flank pain, dysuria, hematuria, or “gravel” in the urine resulting from stones formed in the cysts. The disease itself is usually not dangerous, but the cysts may predispose the patient to secondary pyelonephritis.
Acquired cystic renal disease:
This disorder is usually restricted to patients undergoing long term dialysis. Multiple cortical and medullary cysts develop. The cysts are initially lined by flat to cuboidal epithelium, but hyperplastic and neoplastic epithelium may develop.
Rubin, E, and Farber, JL (1999) Pathology. Third Edition. Lippincott, Williams and Wilkins. Baltimore. pp. 866-868.
Sciotti, VM, Ph.D. (2004) Personal communication. Department of Basic Sciences, New York Chiropractic College, Seneca Falls, NY.
Terada, N, et al (2002) Natural History of Simple Renal Cysts. Journal of Urology, 167 (1): 21-23.
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