Polycystic kidney disease (PKD) is a genetic condition where numerous fluid-filled sacs, called cysts, develop within the kidneys. These cysts disrupt the kidneys’ structure and impair their ability to filter waste from the blood. The condition manifests in two primary forms: Autosomal Dominant Polycystic Kidney Disease (ADPKD) and Autosomal Recessive Polycystic Kidney Disease (ARPKD). Although they share a name because of the characteristic kidney cysts, they are fundamentally distinct disorders. Their differences stem from unique genetic origins, which dictate separate clinical paths, symptoms, and timelines for disease progression.
Defining Genetic Inheritance Patterns
The primary difference between the two forms of PKD is their pattern of genetic inheritance. Autosomal dominant polycystic kidney disease follows a dominant inheritance model, meaning only one parent needs to carry and pass on a single copy of the mutated gene for their child to be affected. The primary genes associated with ADPKD are PKD1, located on chromosome 16, and PKD2, on chromosome 4. A mutation in the PKD1 gene is more common, accounting for approximately 85% of all ADPKD cases.
Each child of a parent with ADPKD has a 50% chance of inheriting the mutated gene and developing the disorder. This inheritance pattern explains why ADPKD is often seen in multiple generations of the same family. The proteins produced by the PKD1 and PKD2 genes, known as polycystin-1 and polycystin-2, are involved in the development and function of kidney tubules, though their exact function is still being researched.
In contrast, autosomal recessive polycystic kidney disease is governed by a recessive inheritance pattern. For a child to develop ARPKD, they must inherit two copies of a specific mutated gene—one from each parent. The gene responsible for ARPKD is PKHD1, which provides instructions for a protein called fibrocystin. While its function is not fully understood, the protein is found in the kidneys, liver, and pancreas.
Parents who each have one copy of the mutated PKHD1 gene are known as carriers. They do not show any signs or symptoms of the disease themselves. When two carriers have a child, there is a 25% chance with each pregnancy that the child will inherit two mutated copies and be born with ARPKD. There is a 50% chance the child will be a carrier and a 25% chance the child will inherit two normal copies of the gene. This explains why ARPKD is much rarer than ADPKD and does not appear in every generation of a family.
Contrasting Symptoms and Age of Onset
The clinical presentation and timeline for ARPKD and ADPKD are markedly different, largely defined by the age at which symptoms first appear. ARPKD is a condition of early life, with signs often becoming apparent at birth or even during prenatal development. One of the most prominent features is the presence of massively enlarged kidneys, which can cause a visibly swollen abdomen in a newborn.
This kidney enlargement can be significant enough to restrict the development of other organs. It may lead to underdeveloped lungs, causing severe respiratory distress immediately after birth. Another characteristic of ARPKD is consistent liver involvement. Nearly all individuals with ARPKD develop a condition known as congenital hepatic fibrosis, which is scarring of the liver that impairs its function and can lead to serious complications.
ADPKD, on the other hand, is an adult-onset disease. While cysts may begin to form in the kidneys during childhood or adolescence, symptoms do not emerge until an individual is between 30 and 50 years old. The initial signs can include high blood pressure (hypertension), pain in the back or sides, blood in the urine, recurrent urinary tract infections, or the development of kidney stones.
Over time, the progressive growth of these cysts leads to a gradual decline in kidney function. Unlike in ARPKD, liver cysts in ADPKD are common but not universal, and tend to cause problems later in life. Individuals with ADPKD also have an increased risk of developing cysts in other organs, such as the pancreas, and face a heightened risk of brain aneurysms, which are bulges in the walls of blood vessels in the brain.
Diagnostic Processes for Each Condition
The diagnostic approach for each form of PKD is tailored to its age of onset and clinical signs. For ARPKD, a diagnosis is frequently made before a child is even born. During routine prenatal ultrasounds, technicians may observe that the fetus has unusually large and bright-appearing kidneys (echogenicity) or a lower-than-normal volume of amniotic fluid.
If not detected prenatally, the diagnosis is often made shortly after birth when a newborn presents with a swollen abdomen or respiratory difficulties. An abdominal ultrasound confirms the diagnosis, revealing enlarged kidneys filled with minuscule cysts. Because of the strong association, imaging of the liver is also performed to assess for signs of fibrosis.
For ADPKD, diagnosis begins in adulthood. It is often prompted by symptoms like high blood pressure or back pain, or by a known family history of the disease. Ultrasound imaging is again the standard diagnostic tool. Specific criteria, which correlate the number of cysts present with the patient’s age, are used to make a definitive diagnosis.
In both forms, genetic testing can provide a conclusive diagnosis by identifying the specific gene mutation. This testing is particularly useful in cases where imaging results are ambiguous or for individuals with a family history who are planning to have children. However, genetic testing is not always necessary, as imaging results are often clear.
Disease Management and Prognosis
Management and prognosis for PKD diverge significantly between the two types. For newborns with ARPKD, medical care is immediate and intensive. The initial focus is on providing respiratory support for underdeveloped lungs and managing severe high blood pressure. Nutritional support is also a priority to ensure proper growth.
Since ARPKD invariably affects the liver, ongoing management of hepatic fibrosis and its complications, like portal hypertension, is required. The prognosis for infants with ARPKD can be severe, especially for those with significant respiratory problems at birth. Many children with ARPKD will require dialysis or a kidney transplant within the first few years of life.
Managing ADPKD aims to slow the progression of kidney damage and manage symptoms. A key part of this strategy is rigorous blood pressure control, as hypertension can accelerate cyst growth and kidney decline. Lifestyle modifications, such as a low-sodium diet and maintaining a healthy weight, are also recommended.
Specific medications are available to treat ADPKD. Tolvaptan is a drug that can slow the rate of cyst growth and kidney function decline in some adults at risk of rapid disease progression. While many individuals with ADPKD will eventually require dialysis or a kidney transplant, this occurs much later in life, often in their 50s, 60s, or even later.