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Background

One third of people older than 50 years develop renal cysts. Although most are simple cysts, renal cystic disease has multiple etiologies. Broad categories of cystic disease include the following:

• Developmental -Multicystic dysplastic kidney (MCDK)
• Genetic -Autosomal recessive polycystic kidney disease (ARPKD), autosomal dominant polycystic kidney disease (ADPKD),juvenile nephronophthisis (JNPHP), medullary cystic kidney disease (MCKD), glomerulocystic kidney disease (GCKD)
• Cysts associated with systemic disease -Von Hippel-Lindau syndrome (VHLS), tuberous sclerosis (TS)
• Acquired - Simple cysts, acquired cystic renal disease, medullary sponge kidney (MSK)
• Malignancy -Cystic renal cell carcinoma (RCC)
The most common larger cysts includeacquired cysts, simple cysts, and cysts associated with ADPKD. Smaller cysts characterize ARPKD, JNPHP, MCKD, and MSK. In adults, renal angiomyolipomas and RCC may also have cystic components.
Pathophysiology

Cysts develop from renal tubule segments and most detach from the parent tubule after they grow to a few millimeters in size. Cyst development is generally attributed to increasedproliferation of tubular epithelium, abnormalities in tubular cilia, and excessive fluid secretion.

Developmental cystic renal disease

MCDK represents abnormal development or formation of the kidney and may involve part, or all of, one or both kidneys. This condition is thought to be secondary to dysfunctional genetics, abnormal differentiation of the metanephros or in utero ureteral obstruction.Patients are observed unless complications arise directly from the kidney or its associated conditions.
Inherited cystic renal disease

ADPKD is due to mutations in the genes PKD1 and PKD2, which encode polycystin proteins. The genetic mechanism of cyst development requires a "second hit," a somatic mutation of the normal PKD allele, which accounts for the onset of ADPKD, usually in those aged30-50 years. Symptoms primarily include pain, hypertension and renal failure. The goal of treatment is to control blood pressure and to slow the onset of renal failure. ADPKD is associated with involvement of other organs, particularly intracranial aneurysms.

ARPKD is due to mutations in PKHD1, a large gene that encodes fibrocystin/polyductin, which plays critical roles in collecting-tubule andbiliary development. This disease carries a high neonatal mortality rate, and many individuals who survive eventually require renal transplantation. Symptoms include hypertension and liver disease. Diagnosis is often made in utero. Treatment is supportive in severe cases but otherwise is similar to that for ADPKD.

GCKD is often confused with ADPKD, as it is common in individuals with a familyhistory of ADPKD. This disease is distinguished histologically and symptoms and treatment are similar to those in ADPKD.

JNPHP and medullary cystic disease are two diseases that some consider a disease complex.1 They share similar pathologic features but are due to different genetic mutations and have different inheritance patterns. JNPHP is inherited in an autosomal recessive manner andpresents in childhood, while MCKD is inherited autosomal dominantly and affects adults. Both diseases present with symptoms of salt wasting and polyuria.
Systemic disease with associated renal cysts

TS is caused by mutations in the suppressor genes TSC1 and TSC2, which encode hamartin and tuberin, respectively. Renal cysts and angiomyolipomas are part of a syndrome that includes seizures anddermatologic findings.

VHLS is due to mutations in the VHL gene, which increases the risk for malignancy, including RCC. Affected individuals develop cysts in multiple organs, including the kidney, pancreas, liver, and epididymis.
Acquired cystic renal disease

The exact cause of this disease is not known. It occurs exclusively in patients on dialysis. The severity of disease is directly related...