What Is Paroxysmal Nocturnal Hemoglobinuria?

Medically Reviewed by Jennifer Robinson, MD on May 02, 2023
3 min read

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare blood disorder that causes red blood cells in the bloodstream to burst.  A person with PNH also usually has bone marrow that can't make enough blood cells for their body.

PNH may lead to a form of anemia where red blood cells are destroyed faster than your body can make them. It can also cause dangerous blood clots.

There are mild and severe cases of PNH. Some people who have it can lead normal lives, while others need medicine or blood transfusions. And some people with severe PNH don’t live as long as their healthy peers.

People aren’t born with PNH, and you can’t pass it along to your child.

It happens when a certain gene (called PIGA) in your bone marrow’s stem cells mutates. This affects the way these stem cells create red blood cells.

Once the gene mutates, the stem cells make red blood cells that are missing a protein. Without that protein, your immune system can’t tell that the red blood cells are supposed to be there, so your body targets and destroys them.

Experts aren’t sure why the PIGA gene mutates or why some people make abnormal red blood cells.

When red blood cells break apart, they release a substance called hemoglobin, which gives red blood cells their color. The body rids itself of this hemoglobin through urine, which may make yours look red, brown, or darker than usual.

Only about half of people with PNH notice their urine change color. It may be darkest at night or first thing in the morning.

Aside from reddish or dark-colored urine, people with PNH may:

  • Bruise or bleed easily, or have trouble stopping minor cuts from bleeding
  • Notice small red dots beneath the skin’s surface, a sign of bleeding under the skin
  • Get headaches
  • Feel tired
  • Have shortness of breath
  • Get blood clots in the veins (which doctors call thrombosis)

In severe cases, men might have problems keeping an erection.

Experts say PNH is more likely if:

  • You have weakened bone marrow.
  • You’ve had aplastic anemia, a bone marrow disorder that can weaken the bone marrow.
  • You're of Asian descent.

Usually, PNH happens to adults in their 30s. But it can happen to children and teens.

Because PNH is rare and the symptoms may overlap with other health problems, it may take a while for a doctor to diagnose it. The process may take longer in people whose urine doesn’t change color, if doctors don’t suspect PNH.

To diagnose it, your doctor will ask about your symptoms and medical history. They may give you different blood tests. One, called a flow cytometry test, is often the best tool to figure out if someone has PNH. It allows doctors to see if -- and how many -- red blood cells are missing the crucial protein.

There are a few ways to treat PNH, including:

  • Eculizumab (Soliris) or ravulizumab (Ultomiris), medications that can keep the immune system from destroying red blood cells and that may prevent blood clots
  • Steroids, which may keep red blood cells from being destroyed
  • Blood thinners, which may help prevent blood clots
  • Blood transfusions to make red blood cell counts higher

Doctors might also recommend treatments to help your body make more red blood cells, like:

  • Folic acid
  • Human growth factor
  • Androgen, a male hormone

A bone marrow transplant can cure PNH. But it has risks, so doctors usually offer other treatments first.