Paroxysmal Nocturnal Hemoglobinuria

Introduction

If you or someone close to you has recently been diagnosed with paroxysmal nocturnal hemoglobinuria — usually shortened to PNH — you are probably trying to make sense of a condition that very few people have heard of. PNH is rare, the name is long and unfamiliar, and the symptoms can have been confusing for months or years before a clear answer emerged.

The good news is that PNH today is very different from PNH a generation ago. The arrival of targeted medicines that calm the part of the immune system responsible for the damage has changed how the condition is managed and how people live with it. Many patients who were once at high risk of severe anaemia and life-threatening blood clots now lead stable lives with ongoing treatment.

This guide walks through what PNH is, why it happens, how it is diagnosed, the current treatment options, what long-term care looks like, and what to expect over time. It is written for adults who already have the diagnosis (or are being investigated for it) and for the families who support them.

What Is Paroxysmal Nocturnal Hemoglobinuria?

Paroxysmal nocturnal hemoglobinuria is a rare, acquired disorder of the bone marrow and blood. Three words in the name help explain what is happening:

• Paroxysmal means “in episodes” or “in flares.” The condition can have ups and downs.
• Nocturnal means “at night.” The condition was first noticed because some patients passed dark urine in the morning. In reality, the underlying process happens around the clock; it is just sometimes more visible after a night’s sleep.
• Hemoglobinuria means “haemoglobin in the urine.” When red blood cells break apart in the bloodstream, the red pigment inside them (haemoglobin) is released and can pass into the urine, sometimes turning it dark.

In PNH, a change in the bone marrow stem cells means the blood cells they produce are missing certain protective proteins on their surface. These proteins normally act like a shield that prevents a part of the immune system called the complement system from attacking the body’s own cells. Without that shield, red blood cells are broken down inside the blood vessels — a process called intravascular haemolysis. The same missing proteins also affect platelets and white blood cells, which is why PNH can cause more than just anaemia.

PNH is not a cancer and it is not contagious. It is also not inherited — you cannot pass it to your children, and your parents did not pass it to you. The change occurs during a person’s lifetime in a single bone marrow stem cell, which then multiplies over time.

Types and Clinical Patterns of PNH

Doctors often describe PNH in three broad clinical patterns. These are not rigid categories; many patients move between them over time.

Classical PNH

In classical PNH, haemolysis (red cell breakdown) is the dominant feature. The bone marrow is generally still working and producing cells, but the cells it makes are vulnerable. Patients typically have anaemia, fatigue, episodes of dark urine, and a higher risk of blood clots.

PNH in the Setting of Another Bone Marrow Disorder

PNH can occur alongside aplastic anaemia (where the bone marrow does not make enough blood cells) or myelodysplastic syndromes (MDS, a group of conditions where the marrow makes abnormal cells). When PNH appears with one of these conditions, the marrow failure features often dominate the picture, and treatment must address both.

Subclinical PNH

Some patients have a small population of PNH cells detected on testing but do not have significant haemolysis or symptoms. This pattern is usually picked up when testing is done for another reason, such as the work-up of aplastic anaemia. These patients are monitored over time rather than treated immediately.

Causes and Risk Factors

PNH begins with an acquired change (mutation) in a gene called PIG-A, located on the X chromosome of a single bone marrow stem cell. This gene is needed to make a molecular anchor (called GPI) that holds the protective proteins onto the surface of blood cells. Without a working PIG-A gene, the cells produced by that stem cell cannot place those protective proteins where they belong.

Why this mutation happens, and why it then expands into a large population of cells, is not fully understood. Researchers believe a combination of two events is usually needed:

• The mutation occurs in a stem cell.
• The bone marrow environment then gives that mutated cell a growth advantage — often because of an immune attack on the normal stem cells. This is why PNH is so closely linked to aplastic anaemia.

Known associations and risk factors include:

• A history of aplastic anaemia or other bone marrow failure
• A diagnosis of myelodysplastic syndrome
• Previous immune-related damage to the bone marrow

PNH can be diagnosed at any age but is most often picked up in young to middle-aged adults. It affects men and women in roughly equal numbers. There is nothing a person did, or failed to do, that caused the condition.

Signs and Symptoms

If you already have a diagnosis, you may recognise some of the patterns below from your own experience before testing. Knowing them is also useful because they can help you and your care team identify a flare or new complication early.

Symptoms vary widely. Some people have mild fatigue only; others have severe episodes that bring them to hospital. Common features include:

• Fatigue and weakness — often disproportionate to the level of anaemia, because free haemoglobin in the bloodstream also affects blood vessel tone and oxygen delivery
• Shortness of breath on exertion
• Dark or cola-coloured urine, often most noticeable in the morning or during flares triggered by infection, surgery, or stress
• Abdominal pain, sometimes severe, due to spasm of small blood vessels and clots
• Difficulty swallowing or spasm of the oesophagus
• Headaches and dizziness
• Erectile dysfunction in men
• Yellowing of the skin or eyes (jaundice) when haemolysis is brisk
• Easy bruising or bleeding, if platelets are low
• Frequent or unusual infections, if white cells are low

The most serious complications, and the leading cause of harm in untreated PNH, are blood clots (thrombosis). These can occur in unusual places — the veins of the liver (Budd–Chiari syndrome), the brain, the abdomen — as well as in the legs and lungs. Kidney problems, both acute and chronic, can also develop from repeated exposure to free haemoglobin.

Because PNH is rare and its symptoms overlap with many other conditions, diagnosis often follows a period of unexplained anaemia or unusual clotting. The key tests are described below.

Blood Tests

• Complete blood count (CBC) shows anaemia, and sometimes low platelets and low white cells.
• Reticulocyte count is usually raised, reflecting the marrow’s effort to replace destroyed red cells.
• Lactate dehydrogenase (LDH) is typically markedly elevated. LDH is released when red cells break apart and is one of the most useful markers of how active the haemolysis is.
• Bilirubin may be raised, while haptoglobin (a protein that mops up free haemoglobin) is usually very low.
• Direct antiglobulin test (Coombs test) is negative in PNH — an important clue, because most other immune causes of haemolysis are Coombs-positive.
• Kidney and liver function tests help assess organ involvement.
• D-dimer and other clotting markers may be checked if thrombosis is suspected.

Flow Cytometry — the Confirmatory Test

The diagnosis of PNH is confirmed by high-sensitivity flow cytometry on a blood sample. This test looks at the surface of your red and white blood cells and identifies the proportion that lack the GPI-anchored proteins. Modern flow cytometry, often using a reagent called FLAER, can detect even small PNH populations.

The test reports the PNH clone size — the percentage of your blood cells that are affected. Clone size, along with how active the haemolysis is and what symptoms you have, helps guide treatment decisions.

Bone Marrow Examination

A bone marrow biopsy is often performed, particularly to look for associated aplastic anaemia or myelodysplastic syndrome. It helps the care team understand whether the marrow is producing enough cells and whether there are other abnormalities to address.

Imaging

If clots are suspected, ultrasound, CT, or MRI of the relevant area may be used. Imaging of the abdomen is sometimes done at baseline because clots in the liver veins can be silent in early stages.

Treatment and Management

Treatment for PNH is personalised. Decisions depend on the size of the PNH clone, how active haemolysis is, whether there is significant marrow failure, whether clots have already occurred, and your overall health. The main pillars of treatment are described below.

Supportive Care

For patients with small PNH clones and few symptoms, observation with regular monitoring may be appropriate. Supportive measures that may be used at any stage include:

• Folic acid supplementation to support the increased red cell production needed when haemolysis is ongoing
• Iron supplementation, which is sometimes needed because iron is lost in the urine in PNH
• Blood transfusions for severe or symptomatic anaemia
• Vaccinations and infection prevention — particularly important before starting complement inhibitor therapy (see below)

Complement Inhibitor Therapy

The arrival of complement inhibitor medicines has transformed PNH care. These medicines block the part of the immune system (the complement cascade) that destroys PNH red cells. Major hematology societies describe complement inhibition as the standard of care for symptomatic classical PNH.

• C5 inhibitors (such as eculizumab and ravulizumab) block the complement system at a late step. They are given by intravenous infusion at intervals that depend on the specific drug.
• Proximal complement inhibitors (such as pegcetacoplan, and newer agents acting on complement factors B or D) block the cascade at an earlier step and can also reduce a separate form of red cell breakdown that can persist on C5 inhibitors. Some of these are given by injection under the skin or by mouth.

In clinical studies, complement inhibitors have been shown to reduce haemolysis, lower the need for transfusions, significantly reduce the risk of blood clots, and improve fatigue and quality of life. They do not cure PNH — the underlying bone marrow change remains — but they control its most dangerous consequences.

Because complement inhibitors slightly increase the risk of infection with certain bacteria, particularly Neisseria meningitidis (which causes meningococcal disease), vaccination against meningococcus is given before treatment begins, and patients are often advised to carry an alert card and to seek urgent medical attention for any unexplained fever.

Anticoagulation Therapy

Blood thinners may be used to treat or prevent thrombosis. The decision is individualised: who needs anticoagulation, for how long, and at what intensity depends on clinical history, clone size, and whether complement inhibitor therapy is being used. With effective complement inhibition, the very high clot risk of untreated PNH is substantially reduced.

Treatment of Associated Bone Marrow Failure

When PNH occurs alongside aplastic anaemia, treatment of the marrow failure — typically with immunosuppressive therapy (such as antithymocyte globulin and ciclosporin) or with transplant — is central. The hematology team will balance treatment of haemolysis with treatment of the underlying marrow problem.

Allogeneic Bone Marrow Transplant

An allogeneic bone marrow (stem cell) transplant — using donor stem cells — is currently the only treatment that can cure PNH. Because transplant carries significant short-term and long-term risks, it is reserved for selected patients, typically those with:

• Severe bone marrow failure not responding to immunosuppressive treatment
• Life-threatening complications not controlled by medical therapy
• Limited access to long-term complement inhibitor therapy in situations where ongoing medication is not feasible
• A suitable matched donor and good overall fitness for the procedure

1. A full pre-transplant assessment of heart, lung, liver, and kidney function
2. Identification of a suitable donor (sibling, unrelated registry donor, or in some cases a half-matched family donor)
3. Conditioning treatment (chemotherapy, sometimes with low-dose radiation) to prepare the marrow
4. Infusion of donor stem cells
5. A weeks-to-months period of engraftment and immune recovery, with intensive supportive care
6. Long-term follow-up for graft-versus-host disease, infections, and late effects

The decision to pursue transplant is made jointly between the patient, family, hematologist, and transplant team after careful discussion of risks and benefits.

New and Emerging Treatments

The treatment landscape in PNH is changing quickly. Newer oral and subcutaneous complement inhibitors continue to be studied and approved in various regions. Patients in active care today often have more options than were available even a few years ago, and an experienced PNH team can discuss what is currently appropriate and accessible.

Lifestyle and Self-Management

While PNH cannot be prevented or controlled through lifestyle alone, several habits support overall health and reduce the risk of complications.

• Stay well hydrated. Adequate fluid intake helps protect the kidneys, particularly during a haemolytic flare.
• Avoid smoking. Smoking increases the risk of blood clots, which is already elevated in PNH.
• Be cautious with alcohol, particularly if liver function or anticoagulation is a concern.
• Eat a balanced diet with adequate protein, iron, folate, and vitamin B12 — the building blocks of healthy blood. Specific supplements should be guided by blood tests.
• Stay reasonably active as tolerated. Anaemia and fatigue may limit exercise, but light to moderate activity supports cardiovascular and mental health.
• Pay attention to infections. Vaccinations recommended by your team (including meningococcal, pneumococcal, Haemophilus influenzae type B, COVID-19, and seasonal influenza) are important. Treat fevers seriously, especially if you are on a complement inhibitor.
• Plan around stressors. Surgery, pregnancy, severe infections, and major dental procedures can trigger flares; coordinate these in advance with your hematology team.

Many patients also find it helpful to keep a simple symptom and treatment diary, which can be useful at follow-up appointments and during travel.

Monitoring and Long-Term Care

A typical follow-up plan may include:

• Full blood count, LDH, reticulocytes, kidney and liver function at regular intervals (often every 1–3 months, depending on stability)
• Repeat flow cytometry periodically to track clone size
• Iron and vitamin levels as clinically indicated
• Assessment for clot risk, including review of any new symptoms
• Imaging if there is suspicion of thrombosis or organ involvement
• Bone marrow review if there are changes in counts or new concerns about marrow failure or progression
• Review of medications, including timing of complement inhibitor doses and vaccination status

Care is most effective when delivered by a hematology team with experience in PNH and access to multidisciplinary support — including transplant specialists, kidney doctors, and, where relevant, obstetricians.

Complications

Untreated PNH carries a meaningful risk of serious complications. With modern treatment, many of these are now far less common, but awareness remains important.

Thrombosis (Blood Clots)

Thrombosis is the most feared complication of PNH and historically the leading cause of death. Clots can occur in unusual sites, including the hepatic veins (Budd–Chiari syndrome), the abdominal veins, and the cerebral veins, as well as in standard locations such as the legs and lungs. Effective complement inhibition substantially reduces this risk.

Kidney Disease

Repeated exposure to free haemoglobin can damage the kidneys. Some patients develop chronic kidney disease over time. Adequate hydration, blood pressure control, and avoiding medications that can harm the kidneys (when possible) all matter.

Pulmonary Hypertension

Free haemoglobin in the bloodstream affects nitric oxide signalling and can contribute to high blood pressure in the lungs, leading to breathlessness. This often improves with effective treatment of haemolysis.

Bone Marrow Failure and Evolution

Some patients develop progressive marrow failure or, less commonly, transformation to myelodysplastic syndrome or acute leukaemia. Regular monitoring is designed to pick these changes up early.

Anaemia and Transfusion-Related Issues

Chronic anaemia affects energy and quality of life. Repeated transfusions can lead to iron overload, which is monitored and managed when it occurs.

Infections

Low white cell counts and complement inhibitor therapy both increase infection risk. Vaccination and prompt treatment of fevers are central to safe long-term care.

Living with PNH

Receiving a diagnosis of a rare, lifelong condition can be emotionally heavy. Fatigue, uncertainty about flares, the routine of regular blood tests and infusions, and the rarity of the condition itself (which often means friends and family have not heard of it) all add to the load. The following points come up often in PNH care.

Work, Travel, and Daily Life

Many people with well-controlled PNH continue to work, study, and travel. Practical planning helps: keeping treatment schedules in mind when travelling, carrying a medical summary and meningococcal alert card if on complement inhibitor therapy, and identifying a hematology centre at your destination for longer trips.

Pregnancy and PNH

Pregnancy is possible in PNH but carries higher risks of clots and complications and must be planned with both a hematologist and an obstetrician experienced in high-risk pregnancy. Adjustments to treatment, close monitoring, and anticoagulation are often needed. Fertility preservation discussions may also be relevant before treatments that affect fertility, such as conditioning chemotherapy for transplant.

Mental Health and Support

Fatigue, anxiety, and low mood are common in chronic blood disorders. Psychological support, patient organisations focused on rare blood diseases, and peer connections with other PNH patients can be very helpful. Speaking openly with your care team about how you are coping is part of good care, not a separate issue.

Family and Genetic Concerns

Because PNH is acquired, not inherited, your children and siblings are not at increased risk of developing PNH because of your diagnosis. Family members do not need to be tested.

PNH in Children

PNH is uncommon in children and adolescents, but it does occur. In children, PNH is more often associated with aplastic anaemia than it is in adults. The general principles of diagnosis and treatment are similar flow cytometry confirms the diagnosis, complement inhibitor therapy can be used in symptomatic disease, and allogeneic bone marrow transplant is considered for severe marrow failure or refractory disease.

Important differences in paediatric care include:

• Greater emphasis on the underlying marrow disorder. Many paediatric PNH cases are managed primarily as marrow failure with PNH as a feature.
• Transplant outcomes in children with bone marrow failure are generally better than in adults, which may shift the balance of decision-making.
• Growth, schooling, and development need protection alongside treatment.
• Vaccination and infection precautions around complement inhibitor therapy require careful planning in school-age children.
• Family support and age-appropriate explanation of the condition matter a great deal.

Children with PNH are best cared for in a paediatric hematology centre experienced in bone marrow failure syndromes.

Preventing Complications

Because PNH cannot be prevented at the level of its cause, prevention efforts focus on avoiding its complications. The key elements are:

• Staying on prescribed therapy and not missing doses of complement inhibitor treatment, which can trigger a rebound flare
• Keeping vaccinations up to date, particularly against meningococcus before and during complement inhibitor therapy
• Reporting fevers promptly, especially if on complement inhibition
• Recognising symptoms of clots early — leg swelling, sudden severe headache, breathlessness, severe abdominal pain — and seeking urgent medical attention
• Coordinating surgery, dental work, and pregnancy with the hematology team in advance
• Attending routine follow-up even when feeling well

When to Seek Urgent Care

Some symptoms should not wait for a routine appointment. Seek urgent medical attention if you experience:

• Sudden severe headache, confusion, weakness on one side of the body, or trouble speaking
• Sudden shortness of breath or chest pain
• Severe abdominal pain, especially with vomiting
• Painful or severely swollen leg
• A significant drop in urine output, or dark urine that is much heavier than usual
• Fever (especially above 38°C / 100.4°F) while on complement inhibitor therapy — this should always be treated as a medical emergency until infection is ruled out
• Heavy or uncontrolled bleeding

If you are on a complement inhibitor, carry your alert card and tell any treating clinician about your medication.

Frequently Asked Questions

Is PNH a cancer?

No. PNH is a rare, acquired disorder of bone marrow stem cells, but it is not a cancer. A small proportion of patients may develop other marrow disorders over time, which is one reason long-term follow-up matters.

Will I pass PNH on to my children?

No. PNH is acquired during life in a single bone marrow stem cell. It is not inherited, and your children are not at increased risk because of your diagnosis.

Can PNH be cured?

Allogeneic bone marrow transplant is currently the only treatment that can eliminate the PNH clone. Because transplant has significant risks, it is reserved for selected patients. For most people, PNH is managed long-term with medication rather than cured.

Do I have to be on complement inhibitor therapy for life?

For patients with symptomatic classical PNH, treatment is typically long-term. Stopping medication usually allows haemolysis to return and can trigger a flare, so any change is planned carefully with the hematology team.

Will I always need transfusions?

Many patients on effective complement inhibitor therapy no longer require regular transfusions, although some still need them occasionally. The need for transfusion is one of the markers used to judge how well treatment is working.

Can I exercise with PNH?

Most patients can and should remain active, with intensity guided by how anaemia and fatigue are affecting them. Contact sports may need to be reviewed if platelets are low or anticoagulation is in use.

Is it safe to become pregnant?

Pregnancy is possible but higher risk and requires close planning with both hematology and obstetrics. Adjustments to treatment and the use of anticoagulation are common. This should be discussed with your team well before trying to conceive.

Why is the meningococcal vaccine so important?

Complement inhibitor medications slightly increase the risk of meningococcal infection, which can be severe. Vaccination greatly reduces this risk, and the alert card you are given allows any treating clinician to act quickly if you become unwell.

How often will I need follow-up?

This varies, but most patients see their hematology team every 1–3 months when stable, with more frequent visits during treatment changes, flares, or other medical events.

What does dark urine mean if I see it again?

Dark urine usually reflects an episode of haemolysis. It may be triggered by infection, surgery, stress, or a missed treatment dose. Persistent dark urine, especially with reduced urine output, pain, or fever, should be reviewed promptly by your team.

Conclusion

Paroxysmal nocturnal hemoglobinuria is rare, complex, and unfamiliar to most people on the day of diagnosis. It is also far more treatable than it once was. Targeted medicines that block the immune attack on red blood cells, careful prevention of clots, attention to associated bone marrow problems, and in selected cases curative transplant have together changed what living with PNH looks like.

For most patients, the path forward is a long-term partnership with a hematology team: regular monitoring, steady treatment, planning ahead for life events, and quick action when something changes. With that structure in place, many people with PNH live full and active lives, with the condition managed quietly in the background rather than dominating each day.