Paroxysmal Nocturnal Hemoglobinuria (PNH)

Paroxysmal nocturnal hemoglobinuria, usually called PNH, is a rare blood disorder in which the body’s own immune defenses destroy otherwise healthy red blood cells. The destruction happens because a genetic mutation in bone marrow stem cells produces blood cells that are missing protective surface proteins, leaving them vulnerable to a part of the immune system called the complement system.¹ Despite its name suggesting symptoms that come and go at night, PNH is a chronic, 24-hour condition whose effects reach well beyond dark-colored urine, touching blood clotting, kidney health, energy levels, and daily quality of life.

What Causes PNH

PNH traces back to a mutation in a gene called PIG-A, which sits on the X chromosome.² This gene is responsible for the first step in building tiny molecular anchors that hold certain proteins onto the surface of blood cells. When PIG-A is mutated in a bone marrow stem cell, every blood cell descended from that stem cell lacks these anchors and, by extension, lacks the proteins that would normally sit on their surface.³

PNH is not inherited. The mutation happens spontaneously in one or a few stem cells during a person’s lifetime, so the affected blood cells exist alongside perfectly normal ones. The proportion of abnormal cells in the bloodstream, called the clone size, varies enormously from person to person. Some people carry a tiny PNH clone that causes no noticeable problems, while others have a large clone that dominates their blood production and drives severe symptoms.⁴ Roughly two-thirds of PNH clones turn up in people whose main problem is bone marrow failure rather than the classic PNH symptoms, and those patients tend to have smaller clones.⁵

How the Complement System Destroys Red Blood Cells

Two surface proteins matter most here: CD55 and CD59. On normal red blood cells, these proteins act as brakes on the complement system, a set of immune proteins that patrol the bloodstream looking for foreign or damaged cells. CD55 blocks an early amplification step in the complement cascade, while CD59 prevents the final punch, the formation of a pore that punches holes in cell membranes. PNH red blood cells, missing both proteins, cannot stop the cascade once it gets going.⁶

The result is chronic intravascular hemolysis, meaning red blood cells burst open inside blood vessels around the clock. This releases free hemoglobin into the plasma, which triggers a cascade of downstream problems.⁷ The hemolysis involves both the so-called alternative pathway, which acts upstream in the complement cascade and is always ticking in the background, and the terminal pathway further downstream. That dual vulnerability is one reason PNH red blood cells are so fragile.

Symptoms Beyond Dark Urine

The name “paroxysmal nocturnal hemoglobinuria” comes from the observation that some patients pass dark or reddish urine in the morning, a sign that red blood cells were destroyed overnight. But most of the burden of PNH has nothing to do with urine color. When red blood cells break apart, the free hemoglobin released into the bloodstream soaks up nitric oxide, a molecule that normally keeps blood vessels relaxed and smooth muscle functioning. The depletion of nitric oxide accounts for a long list of seemingly unrelated complaints: recurring abdominal pain, difficulty swallowing, erectile dysfunction, and crushing fatigue.⁸

Fatigue is the symptom that most consistently wrecks daily life. Studies measuring quality of life in PNH patients find that even those receiving treatment report significantly worse functioning across nearly every measured domain compared with the general population, including physical health, cognitive function, and shortness of breath during activities.⁹ The fatigue tends to be severe. In one cohort that included PNH patients alongside people with aplastic anemia and myelodysplastic syndromes, the average fatigue scores across all three groups fell in the severe range.¹⁰ Patients with worse fatigue also reported worse outcomes in virtually every other quality-of-life measure, with the one exception being sleep disturbance, which did not track as tightly with fatigue severity.¹¹

The Thrombosis Problem

Blood clots are the most dangerous complication of PNH and a leading cause of death in untreated patients. The clot risk in PNH is strikingly high compared with other blood disorders, and researchers have found that no single mechanism fully explains it. Instead, several processes pile on top of each other.¹²

Free hemoglobin from burst red blood cells can directly injure the lining of blood vessels. At the same time, tiny cell fragments called microparticles shed from damaged white blood cells carry tissue factor, a potent trigger of clotting. Platelets lacking their normal surface proteins behave abnormally, becoming more prone to clumping. And the nitric oxide depletion mentioned earlier narrows blood vessels and makes the whole system more pro-clotting. On top of all that, PNH cells may be missing a receptor involved in breaking down clots, meaning even small clots that form are slower to dissolve.¹³ Clots can form in unusual locations, including the veins draining the liver or the brain, and may occur even in patients with relatively small PNH clones.

Why PNH Is Often Diagnosed Late

The gold standard for diagnosing PNH is flow cytometry, a lab technique that tags blood cells with fluorescent markers to see whether they are missing the surface proteins that PNH cells lack.¹⁴ This test is sensitive and reliable, but it has to be ordered in the first place, and that is where the trouble starts.

PNH’s symptoms are notoriously nonspecific. Fatigue, anemia, and abdominal pain can point toward dozens of more common diagnoses. There is also substantial clinical overlap with bone marrow failure conditions like aplastic anemia and myelodysplastic syndromes, which leads to patients being diagnosed with one of those first and the PNH clone being caught later, sometimes years later.¹⁵ Hematology guidelines now recommend screening for PNH in any patient with unexplained hemolytic anemia, unexplained blood clots in unusual locations, or bone marrow failure syndromes, but awareness outside specialty centers remains uneven.¹⁶

Kidney Damage From Chronic Hemolysis

One of the less publicized consequences of long-standing PNH is kidney injury. When red blood cells break apart continuously, the kidneys filter out excess hemoglobin and iron. Over time, iron deposits accumulate in the kidney tubules, causing direct toxic damage. Repeated episodes of tiny blood clots in the kidney’s small vessels add to the problem.¹⁷ Free heme also triggers inflammation and oxidative stress in kidney tissue, depletes nitric oxide locally, and can cause ischemia.¹⁸

The numbers are sobering. In a large cohort of PNH patients, about two-thirds already had some degree of kidney dysfunction or damage at the time they were evaluated, and roughly one in five had progressed to moderate-to-severe kidney disease.¹⁹ This underscores why getting the hemolysis under control matters for more than just anemia: every day that red blood cells are bursting unchecked is a day of cumulative organ damage.

Complement Inhibitor Therapy

The treatment landscape for PNH changed dramatically with the introduction of eculizumab, a monoclonal antibody that blocks a key step late in the complement cascade (the C5 protein). By preventing the complement system from assembling the pore that punches holes in red blood cells, eculizumab sharply reduces intravascular hemolysis. Ravulizumab, a longer-acting version of the same concept, allows for less frequent dosing and has shown durable control of hemolysis for up to six years in follow-up data. Compared with untreated patients tracked in an international registry, ravulizumab reduced the risk of death roughly five-fold and kept the rate of blood clots and other serious vascular events low.²⁰

These drugs are genuinely life-changing for many patients, but they come with a serious trade-off. By disabling part of the complement system, they leave patients more vulnerable to certain bacterial infections, especially from Neisseria meningitidis, the bacterium that causes meningococcal meningitis.²¹ All patients starting complement inhibitor therapy are vaccinated against meningococcal disease, and many centers add prophylactic antibiotics. The UK National PNH Service, which has more than 18 years of experience managing this risk, uses a combination of vaccination on the first day of treatment, a short course of ciprofloxacin in the early days, and ongoing daily penicillin (or erythromycin for those allergic to penicillin).²²

The Limits of Blocking C5

For all their benefits, drugs that block C5 do not fix everything. When the terminal pathway is shut down, the earlier steps of complement activation still proceed. The complement protein C3 still tags PNH red blood cells, coating them with fragments. These C3-coated cells are not destroyed inside blood vessels, but they become targets for immune cells in the spleen and liver, which engulf them in a process called extravascular hemolysis.²³ This phenomenon emerges in virtually all PNH patients on eculizumab, and it limits the blood-count improvements that some patients can achieve. About one in four patients on eculizumab remain dependent on transfusions because of this ongoing extravascular destruction.²⁴

This limitation drove the development of drugs targeting the complement system higher upstream. Pegcetacoplan, which inhibits C3, and iptacopan, which blocks a protein called factor B, both aim to prevent the C3 tagging that causes extravascular hemolysis in the first place. Head-to-head comparisons between the two newer agents and C5 inhibitors suggest they offer improvements in hemoglobin levels, markers of red blood cell destruction, and fatigue scores.²⁵ However, targeting complement earlier in the cascade raises its own concerns about infection risk, and long-term safety data are still accumulating.

Breakthrough hemolysis remains a practical clinical challenge even with newer treatments. In a recent international multicenter study, more than half of breakthrough episodes were triggered by infections, and about one in ten episodes in patients on oral complement inhibitors were linked to missed doses.²⁶ That finding matters because several of the newer agents are oral pills rather than infusions, which is more convenient but also makes adherence the patient’s own responsibility in a way that clinic-administered infusions do not.

Bone Marrow Transplant as a Potential Cure

The only treatment that can actually eliminate the PNH clone and cure the disease is an allogeneic stem cell transplant, in which a patient’s bone marrow is replaced with marrow from a healthy donor. This approach effectively wipes out the mutated stem cells and replaces them with normal ones.²⁷ However, transplant carries significant risks, including graft-versus-host disease, infections during the period of immune suppression, and transplant-related mortality. Because complement inhibitors now control the disease effectively in most patients, transplant is generally reserved for people who do not respond to medical therapy, who develop life-threatening clotting despite treatment, or who have severe concurrent bone marrow failure.

PNH and Pregnancy

Pregnancy in PNH has historically been considered high-risk for both mother and baby. The normal physiological changes of pregnancy, including increased blood volume and a more pro-clotting state, compound the risks PNH already poses. A systematic review with meta-analysis found that overall live birth rates were about 78%, but outcomes were significantly better when the mother was treated with eculizumab: fetal survival was 82% in eculizumab-treated pregnancies versus 68% in those without it, a difference largely driven by higher rates of spontaneous miscarriage in untreated women.²⁸

Managing PNH during pregnancy involves a balancing act between controlling hemolysis, preventing blood clots with anticoagulation, and anticipating an increased need for transfusions. Eculizumab is increasingly used in this setting, and short-term outcomes for both mothers and infants appear reassuring, though long-term data on children exposed in utero remain limited.²⁹ Interestingly, the review found no statistically significant link between the use of blood thinners during pregnancy and fetal survival, suggesting that complement control may matter more for pregnancy outcomes than anticoagulation alone.³⁰

The Cost of Living With PNH

Complement inhibitor therapies rank among the most expensive medications in the world. A real-world study of U.S. insurance claims found that total health care costs averaged about $712,000 per year for patients on eculizumab and about $625,000 per year for those on ravulizumab. The complement inhibitor itself accounted for roughly 80 to 86 percent of those totals.³¹ These figures make PNH one of the costliest diseases to treat on a per-patient basis, and access to therapy varies dramatically by country and insurance system. In many lower-resource settings, complement inhibitors are simply unavailable, and patients depend on transfusions, anticoagulation, and supportive care.

The financial pressure also shapes clinical decisions in subtle ways. Because the newer oral agents, while still expensive, may eventually reduce the logistical burden of clinic-based infusions, the field is watching closely to see whether a shift toward oral therapies could affect both adherence patterns and cost structures. For now, though, the economics of PNH treatment remain a significant barrier for patients and health systems alike.

The Overlap With Bone Marrow Failure

PNH does not always present as a standalone disease. It frequently coexists with aplastic anemia, a condition in which the bone marrow fails to produce enough blood cells. The relationship runs deep: the immune attack that drives aplastic anemia may paradoxically give PNH stem cells a survival advantage, allowing the mutant clone to expand to fill the gap left by destroyed normal stem cells. This is why flow cytometry screening for PNH clones is standard practice in anyone newly diagnosed with aplastic anemia.

Clone size matters for prognosis and treatment decisions. Patients with small PNH clones detected in the context of bone marrow failure typically do not need complement inhibitor therapy. Their primary problem is underproduction of blood cells, not complement-driven destruction, and their treatment focuses on immunosuppression or transplant for the aplastic anemia. Only when the PNH clone grows large enough to drive clinically significant hemolysis or thrombosis does complement inhibition enter the picture.³² The distinction between “PNH clone present” and “PNH disease” is one of the more important nuances in managing these patients, and it is one that can confuse both patients and generalist physicians who encounter the diagnosis for the first time.