Overview of Polycystic Kidney Disease (PKD)
Polycystic kidney disease (PKD) is a genetic disease of the kidneys in which there are dilations of all parts of the nephron, the main filtering mechanism. The nephron, composed of glomeruli (singular, glomerulus)–a tuft of capillaries surrounding a collecting tubule, is the interface where waste products are filtered and collected as urine.
PKD is due to a dominant gene, with a 50% chance of passing it down to one’s offspring. There is a much rarer recessive gene, seen mostly in children, fifty times less frequent than the dominant version.
PKD causes increased kidney size, as much as 5 times normal, due to microcysts, with progressively larger cysts causing a decrease in renal function. Nephrectomy–removal of the kidney–is indicated when there are recurrent infections, incapacitating pain, renal cell carcinoma, or hematuria requiring repetitive transfusions. When the severity of kidney dysfunction reaches end-stage renal disease (ESRD), either dialysis or kidney transplant is necessary.
Signs and Symptoms
- Hematuria: visibly blood-tinged urine
- Decrease in concentration of urine
- Proteinuria: Release of protein from the blood into the urine, which increases with PKD severity
- Kidney stones: primarily of uric acid. (Most stones in non-PKD patients are calcium oxalate)
- Pain: flank or abdominal, from infections, kidney stones, or cyst hemorrhage
- Higher risk of kidney cancer (renal cell carcinoma)
Diagnosis of Polycystic Kidney Disease
Patients with polycystic kidney disease (PKD) can present with hypertension, hematuria, proteinuria, or renal insufficiency. These can be discovered by routine laboratory urinalysis and blood work:
- CMP (complete metabolic profile)
Prior to these later signs, due to flank pain, stones, or urinary tract infection (UTI) being the most common symptoms reported by patients, anyone presenting with one or more of these should be considered for diagnostic tests.
Since many symptoms are delayed until a certain degree of kidney dysfunction, screening tests should be done on persons over 18 with a family history of PKD, since it has a dominant expression in its inheritance risk. (Diagnosis in those <18 in otherwise asymptomatic young people can have adverse consequences in careers or commitment to relationships and should be delayed.)
- Ultrasound: Safe, reliable, and inexpensive, and it can tell whether the criteria for diagnosis are satisfied: at least three kidney cysts–unilateral or bilateral
Fewer cysts (1-2), insufficient for diagnosis, still indicate high risk for PKD, prompting increased surveillance and genetic testing.
In symptomatic individuals due to cysts large enough to cause the symptoms, large cysts and enlarged kidneys are easily seen on ultrasound.
- CT and MRI: Which are more sensitive and can discover many very small cysts, do not have the same criteria for diagnosis as ultrasound. Instead, >10 cysts are needed for a diagnosis of PKD
- Genetic testing: Indicated in those with uncertain ultrasound results or in screening of a related potential kidney donor
- Kidney function testing: When a person is diagnosed with PKD, the progression of renal impairment can be followed by tests such as glomerular filtration rate, urinalysis, CMP, and CBC
Simple kidney cysts are a normal age-related occurrence in the general non-PKD population, so the more accurate CT or MRI may be necessary to separate these benign age-related cysts from the pathology of PKD.
Management of Polycystic Kidney Disease
Renal function usually remains normal until a polycystic kidney disease (PKD) patient reaches his or her forties. After that, the glomerular filtration rate (GFR–measurement of kidney function) declines steadily and irreversibly.
Since there are comorbidities associated with (or caused by) PKD, treatment of them is part of a combined approach to global care.
Elevations in blood pressure are common in PKD patients, often before any decrease in kidney function. Due to the kidney’s role in blood pressure maintenance (the renin-angiotensin system), an angio-converting enzyme (ACE) inhibitor is used as an initial antihypertensive medication. When kidney failure progresses enough to cause a rise in serum creatinine, a beta blocker can be used to keep the blood pressure in normal range (for PKD patients, <130/80).
Low Sodium Diet
As PKD progresses, more sodium is excreted. This can be counteracted by decreasing sodium intake (dietary alteration–low sodium diet). Sodium restriction can slow the progress of PKD.
Statins for Dyslipidemia
Abnormal cholesterol and triglyceride levels should be treated aggressively with statin drugs, since chronic kidney disease is a major risk factor for coronary heart disease.
Vasopressin, also called “Anti-diuretic Hormone” (ADH), is a hypothalamic-made, pituitary-released hormone that controls water conservation in the kidney. Tolvaptan is a drug that suppresses vasopressin, and this action has been shown to slow down the growth of cysts and decrease the rate of GFR decline. Because it can cause liver toxicity and other side effects, however, it is primarily used in those who are high-risk for rapid PKD progression as determined by serial ultrasonography and GFR testing.
Increased Fluid Intake
Increasing fluids will suppress the natural vasopressin levels, which is a safer way than using tolvaptan, and therefore no risk to all patients with PKD. Increased fluids also dilutes minerals, decreasing the risk of stone formation.
Natural somatostatin inhibits the release of insulin, glucagon, and other pancreatic products. Octreotide, a somatostatin analog (mimics somatostatin), has been shown to decrease the accumulation of fluid in PKD cysts and slow the enlargement of the kidneys and liver that is part of the disease. It also slows GFR decline.
Treatment for ESRD
Eventually, end-stage renal failure requires another method to clear toxic wastes from the body other than the failed kidneys. This consists of dialysis, kidney transplant, or dialysis until a kidney is available for transplant.
Prevention of Polycystic Kidney Disease
The organs and processes that are responsible for homeostasis (equilibrium of physiologic processes) are crucial for life. Any of them that become dysfunctional can become life-threatening, and the kidneys are major participants in this balance. The severity of kidney dysfunction that comes with polycystic kidney disease (PKD) will eventually eliminate the kidneys’ contribution to homeostasis and require replacement via dialysis or kidney transplant.
Prevention of PKD is impossible due to its genetic inheritance. Instead, the concept of prevention is based on:
- Delay of end-stage renal failure (ESRF)
- Mitigating the morbidities that accompany PKD, such as anemia, fluid and electrolyte imbalances, hypertension, cardiac disease, hepatic and pancreatic cysts, and malignancy.
Much of the treatment of PKD, such as vasopressin antagonists, somatostatin agonists, increased fluid intake, and low sodium diet are also as preventative as they are therapeutic, thought to slow the pace of the deterioration in kidney function.
The “extra-renal” (outside of the kidney) problems associated with PKD are mitigated by preventative strategies:
- Antihypertensive and statin medications should be used to prevent or decrease the cardiovascular disease seen in PKD also weight reduction and smoking cessation.
- Cerebral aneurysm: The most serious complication of PKD, should be addressed before any rupture, which causes hemorrhagic stroke. Poorly controlled hypertension, smoking, heavy alcohol consumption, stimulants, illicit drug abuse, and excessive exertions should all be avoided until which time surgical intervention is warranted to prevent rupture and stroke.
- The pain of hepatic cysts can be prevented by drainage via ultrasonographic guidance.
- Echocardiography: Can diagnose coronary aneurysms and other cardiac disease early to prevent serious cardiac events that contribute to the mortality of PKD.
- Vigilance for malignancy can prevent late-stage unexpected consequences of cancer, the risk of which is increased with PKD.