CYSTIC KIDNEY DISEASE
The most common form of this disease is autosomal dominant polycystic kidney disease (ADPKD), which is inherited as an autosomal dominant with almost complete penetrance in patients who live to be 80 years old. ADPKD is the most common hereditary disorder affecting approximately 1 in every 1250 live births and accounts for 10 to 12% of all cases of end-stage renal disease. Other cystic kidney diseases include medullary sponge kidney and medullary cystic kidney. Infantile and childhood forms of PKD do occur, but they are uncommon.
Adult polycystic kidney disease generally becomes symptomatic in the fourth or fifth decade of life. Initial symptoms include abdominal or flank pain, hematuria which may be either gross or microscopic, mild proteinuria, or hypertension. Sudden and severe pain may be secondary to hemorrhage of a cyst, infection, or calculus (usually calcium oxalate or uric acid stones). Hypertension is a common symptom and if uncontrolled it will accelerate the progressive decline in renal function associated with ADPKD. Urinary tract infections are a common complication of this disorder, and are often secondary to gram-negative organisms. Infections of renal parenchyma cysts are often difficult to diagnose and require treatment with lipid-soluble antibiotics (trimethoprim-sulfamethoxazole, chloramphenicol, clindamycin, erythromycin, or ciprofloxacin) as these agents will penetrate the cyst wall. In patients with infected cysts who remain febrile despite treatment with the above lipid-soluble antibiotics, identification of the infected cyst or cysts with imaging techniques followed by aspiration should be considered. Other complications include intracranial aneurysms, polycystic liver disease, cysts in the pancreas, ovary and spleen, and colonic diverticula.
Intracranial aneurysms occur with a frequency of 5 to 10% with the majority located in the middle cerebral artery. Although there does not appear to be an increase in the frequency of aneurysm rupture in patients with ADPKD, there does appear to be a tendency for the aneurysm to rupture at an earlier age when they do rupture. There is usually not an associated family history of aneurysms in these patients; however, if the patient does relate a positive family history, then an aggressive screening for intracranial aneurysms should be undertaken. Associated neurologic symptoms in a patient with ADPKD should also prompt physicians to screen patients for the possibility of intracranial aneurysms. High-resolution CT and magnetic resonance imaging (MRI) are adequate screening tests; however, magnetic resonance angiography is the procedure of choice as it more effectively detects smaller aneurysms than the above-mentioned studies. If no aneurysms are detected, patients should be subjected to regular screening at 5-year intervals.
Associated laboratory abnormalities are nonspecific and are not always present. If renal insufficiency is present, the serum BUN and creatinine will be elevated, the urinalysis may show hematuria or proteinuria, and the hematocrit is often higher than would be expected in patients with the same degree of chronic renal failure from other causes. If there is an associated urinary tract infection, the urinalysis may also reveal pyuria.
Once the diagnosis is suspected, ultrasonography and CT scanning are effective radiographic procedures for establishing the diagnosis. Ultrasonography will detect cysts as small as 1 cm whereas CT scanning is more sensitive and can detect cysts as small as 0.5 cm. The criteria for diagnosing ADPKD are a positive family history plus the presence of at least one cyst in each kidney, with one of the kidneys displaying multiple (two or more) cysts. In persons without a family history, the diagnosis is established by demonstrating three or more renal cysts, with bilateral parenchymal involvement.
ADPKD is a progressive disorder that often (45% of cases) continues until end-stage renal disease. Blacks develop end-stage renal disease approximately 10 years early than their Caucasian counterparts, and those patients with sickle cell disease have the earliest onset. Therapy is supportive and aimed at slowing the progression to end-stage renal disease. Aggressive control of hypertension is the most effective method of slowing disease progression. Angiotensin converting enzyme inhibitors or angiotensin receptor blockers are the most commonly used first-line agents.
Medullary sponge kidney (MSK) is characterized by malformation of the collecting ducts with cyst formation. In MSK the terminal collecting ducts in the pericalyceal region of the renal pyramids are malformed by the presence of microscopic and macroscopic cysts. Frequently, calculi are contained within the cysts. Unlike ADPKD, the renal cortex in MSK is unaffected and progressive loss of renal function does not occur. Both MSK and ADPKD may occur within a single family or within the same patient. This disorder is usually asymptomatic and discovered when an intravenous pyelography (IVP) is performed for some other reason. The diagnosis is confirmed by the characteristic abnormalities noted on IVP, which include pericalyceal opacifications with the appearance of a brush radiating out from the calyx. Bunches of small calculi noted around the affected calyces may also be noted. Because this is a benign disorder, therapy is not required except for symptomatic treatment for infections or stone passage.
Medullary cystic kidney disease (MCD) is a rare inherited disorder noted in children that progresses to end-stage renal disease. This disorder is characterized by small cysts (1 mm to 1 cm in size) located at the corticomedullary junction and medulla and which arise from intact distal tubules and collecting tubules. The disorder is asymptomatic until patients develop nocturia or symptoms of uremia (anorexia, nausea, or fatigue) develop as a result of a progressive decline in renal function. The diagnosis may be established by ultrasonography or CT scanning. This disorder eventually progresses to end-stage renal disease requiring dialysis.