This lesson will discuss an inherited disorder known as polycystic kidney disease as well as the latest understanding of why it occurs in people with this condition and how it can be diagnosed and treated.

Bubble Wrap Kidneys

The kidneys in your body serve to filter your blood. They remove waste products, form urine, and therefore keep you healthy and happy. But the most common inherited disorder of the kidneys, one we’ll cover in this lesson, disfigures the kidneys over time, resulting in their failure and a person’s death. The kidneys end up looking like bubble-wrap kidneys in this disease, and you’ll find out how and why in just a second.

Polycystic Kidney Disease

Polycystic kidney disease is an inherited disorder characterized by the formation of cysts within the kidneys. A cyst is a fluid, semi-solid, or air-filled membranous sac. Don’t get a cyst confused with an abscess, though, as this refers to a collection of pus. Polycystic kidney disease (PKD) is actually inherited in an autosomal dominant fashion (although autosomal recessive inheritance is possible). What this means is that during dominant inheritance, only one copy of the defective gene is necessary to cause the disorder.

The Pathophysiology of PKD

When a person inherits this genetic defect, their kidneys’ function and structure becomes compromised. Normally, the kidneys filter the blood using the glomerulus. The glomerular filtrate flows into the renal tubule, from there into the collecting duct, and thereafter out of the kidneys.

The renal tubule that is involved in this process has little hair-like projections, called cilia, that sense the flow rate of the glomerular filtrate. They look and function like those windsocks you see at regional airports that sense direction and speed of the wind – except the cilia sense the flow of urine instead of air. It is suspected that the mutations in PKD lead to the dysfunction of these cilia. As a result of this dysfunction, the cells of the renal tubule begin to divide in a haphazard fashion.

This improper hyperplasia (which refers to proliferation) of renal tubular cells results in an out-pouching of the renal tubule that is still connected to the tubule itself. You can imagine taking a balloon filled with water and using your fingers to gently pinch off a bit of the main balloon out to the side. The balloon is like the renal tubule and the part you have separated out just a little bit is the out-pouching. The out-pouching initially fills with glomerular filtrate passing by, which causes the out-pouching to dilate more and more with time.

This, and further hyperplasia, causes the out-pouching to eventually separate from the tubule it originated from completely, resulting in a separated cyst. The cyst continues to fill with fluid by way of transepithelial secretion, which is the secretion of fluid through the membrane of the cyst itself. All of this, combined with further cell proliferation, dilates the cyst even more with time. Unfortunately, the dilating cyst is not only dysfunctional itself, but its size, location, and growth compresses and interferes with the normal structure and function of the kidney, leading to eventual kidney failure.

The other problem with PKD is that it is ‘poly’cystic, where ‘poly‘ is a term that refers to ‘many’ or ‘more than one.’ Therefore, the kidneys develop many cysts all over the place, which only further exacerbates kidney dysfunction and damage. Some of these cysts are small and others are really big. It’s like bubble wrap. It comes in all sorts of sizes, small and large.

Imagine placing individual bubbles from the bubble wrap of all sorts of sizes on and within your kidneys. That’s what polycystic kidney disease does, except the bubble wrap is filled with fluid. To put the damage this does to your kidneys into perspective for you, these cysts can number in the thousands and cause a kidney, which normally weighs about one fourth of a pound, to weigh upwards of 30 pounds as a result of all of that fluid accumulation!

Signs, Diagnosis, and Treatment

In the beginning, this usually slow-progressing disease may not result in any noticeable problems. With time, however, people may experience hypertension, or high blood pressure, partially as a result of the activation of the water and sodium conserving system known as the renin-angiotensin-aldosterone system. This only further exacerbates the expansion of the cysts and subsequently their damage upon the kidneys. PKD will also lead to urinary tract infections, kidney stones, blood in the urine, back or side pain, and of course, as I said before, kidney failure.

Diagnosis of this disease is made through the use of an ultrasound examination, which uses sound waves to help visualize the kidneys and their cysts. There is no direct treatment for this inherited disorder and thus treatment must be aimed at treating the complications of the disease process. For instance, medication to lower blood pressure, antibiotics for infections, and pain medication may be necessary.

Lesson Summary

For this lesson, remember that polycystic kidney disease is an inherited disorder characterized by the formation of cysts within the kidneys. A cyst is a fluid, semi-solid, or air-filled membranous sac, and ‘poly‘ is a term that refers to ‘many’ or ‘more than one.’ Therefore, this condition causes many cysts to form in the kidneys. As they grow bigger and bigger, they compromise the function of the kidneys, resulting in kidney failure. The major suspected cause of PKD is a mutation in renal cilia that results in haphazard renal tubular epithelial hyperplasia and consequently cyst formation and renal failure.

Learning Outcomes

Following this lesson, you should have the ability to:

  • Describe what occurs in polycystic kidney disease (PKD)
  • Identify the major suspected cause of PKD
  • List signs and symptoms of PKD
  • Explain how PKD is diagnosed and the treatment options available