Updated on August 17, 2026.
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare disorder that occurs due to genetic mutations that affect blood-producing stem cells in the bone marrow. These mutations are not inherited from a person’s biological parents. PNH mutations are acquired, meaning they occur at some point after a person is conceived.
Normally, blood cells have proteins that protect them from the immune system. In PNH, blood cells lack this protein. As blood cells are released into the bloodstream, they are vulnerable to attack by a part of the immune system called the complement system.
The complement system destroys the unprotected blood cells, and the components are then filtered out of the bloodstream and expelled from the body in urine. This causes the main symptom of PNH, dark or blood-colored urine, often occurring in the morning after sleep. This is also the symptom that gives PNH its name—paroxysmal (sudden episodes), nocturnal (occurring at night), hemoglobinuria (a red blood cell component called hemoglobin in the urine).
All parts of the body depend on a steady supply of oxygen to function and remain healthy, and red blood cells are the body’s main delivery system for oxygen. As red blood cells flow through blood vessels in the lungs, they collect oxygen molecules. These oxygen molecules are then carried to cells, tissues, and organs.
Without enough red blood cells, numerous problems can occur, including fatigue, shortness of breath, headaches, abdominal and/or chest pain, rapid heartbeat, easy bruising and bleeding, difficulty swallowing, and anemia. Severe cases can cause life-threatening complications.
Blood clots (thrombosis) are the most common serious complication and leading cause of death among people with PNH.
What is a blood clot?
A blood clot is a clump of cells, proteins, and platelets. Clotting is an important mechanism that helps the body stop bleeding when there is an injury. Blood clots can form inside blood vessels, obstructing or completely blocking the flow of blood. Blood clots can cause permanent damage to limbs and organs, including the heart, lungs, kidneys, and brain.
What are the symptoms of blood clots?
Blood clots can cause different symptoms depending on where in the body the clot occurs. Clots can also break away from their original location and move to other parts of the body.
Seek emergency medical care if you experience any of the following:
- Sudden shortness of breath
- Chest pain or pressure
- Coughing up blood
- Sudden weakness or numbness, especially on one side of the body
- Difficulty speaking, understanding speech, or sudden confusion
- Sudden vision changes
- Severe, sudden headache
- Fainting or loss of consciousness
- Swelling, pain, warmth, and/or tenderness in an arm or leg, especially if the swelling is in one limb only or is accompanied by chest pain or shortness of breath
How can blood clots be prevented and treated?
PNH is treated with medications called complement inhibitors, which reduce activity in the complement system.
Mentioned above, this is the part of the immune system that goes to work breaking down the defective blood cells seen in PNH. This process releases inflammatory substances into the bloodstream, activates clotting agents, and damages blood vessel walls, which creates an ideal environment for blood clots to occur (keep in mind that this is a very simplified explanation of a complex biological process).
Complement inhibitors can significantly reduce the occurrence of blood clots, though they do not remove the risk entirely. Complement inhibitors also carry a risk of serious side effects, including severe infections of the brain and spinal cord. Suddenly stopping a complement inhibitor can worsen symptoms and increase the risk of complications.
Anticoagulant medications, commonly called “blood thinners,” may be prescribed to treat or prevent blood clots. Excessive bleeding is the most common side effect of anticoagulant medications.
It’s important that your healthcare providers have an up-to-date list of all medications you are taking. Some medications can increase the risk of blood clots (for example, medications that contain estrogen). There is also a risk of unintended drug interactions. Your list should include all prescription medications, over-the-counter medications, and supplements (including herbal products).
Also talk to your healthcare provider about other risk factors for blood clots, including pregnancy, smoking, air travel, high altitudes, injuries, obesity, coexisting health conditions, and sedentary lifestyle.
