Paroxysmal nocturnal haemoglobinuria is a rare blood disorder where part of the immune system mistakenly attacks and destroys healthy red blood cells and platelets, leading to serious complications including anemia, blood clots, and organ damage if left untreated.
Understanding Paroxysmal Nocturnal Haemoglobinuria
Paroxysmal nocturnal haemoglobinuria, commonly known as PNH, takes its name from one of its most distinctive symptoms. The term “paroxysmal” means sudden and irregular, “nocturnal” refers to nighttime, and “haemoglobinuria” describes the presence of hemoglobin (the oxygen-carrying protein in red blood cells) in the urine. This combination reflects how affected individuals may notice dark, reddish-brown, or tea-colored urine, particularly in the morning after overnight accumulation in the bladder.[1]
However, the name can be misleading because not everyone with PNH experiences this telltale symptom. Many people with the condition never notice dark urine at all, with fewer than 40% having this classic sign. Instead, they may primarily experience other symptoms related to the breakdown of red blood cells and its cascading effects throughout the body.[4][5]
PNH is classified as an acquired disorder rather than an inherited one. This means it develops during a person’s lifetime due to changes that occur in certain cells, rather than being passed down from parents to children. The condition affects the blood-forming stem cells in the bone marrow, which are responsible for creating all types of blood cells that circulate throughout the body.[3]
Epidemiology
Paroxysmal nocturnal haemoglobinuria is an extremely rare condition. According to medical research, approximately 6 out of every 1 million people are diagnosed with PNH each year. Globally, it’s estimated that around 20,000 people live with this disease worldwide. Due to its rarity, PNH is classified as an ultra-rare disorder, which can make it challenging for healthcare providers to recognize and diagnose.[1][5]
The condition can affect people of any age, race, or gender. However, it is most commonly diagnosed in young adults, particularly those between 30 and 40 years old. Some studies suggest the diagnosis occurs most frequently in people around their mid-thirties. Women are slightly more likely than men to develop PNH, though the difference is modest.[1][3][6]
The rarity of PNH creates significant challenges in the diagnostic process. Research shows that fewer than 40% of people with PNH receive a diagnosis within 12 months of when their symptoms first appear. Even more concerning, approximately 24% of all PNH diagnoses take five years or longer to confirm. This diagnostic delay occurs partly because symptoms vary greatly from person to person and are often common to many other diseases, causing the condition to be easily overlooked.[5]
Causes
Paroxysmal nocturnal haemoglobinuria occurs when a genetic change, called a mutation, develops in a specific gene within blood-forming stem cells in the bone marrow. The bone marrow is the spongy tissue inside certain bones where the body produces stem cells that eventually mature into red blood cells, white blood cells, and platelets. This mutation happens during a person’s lifetime rather than being inherited from parents.[1][6]
The genetic mutation in PNH affects a gene called PIGA, which is located on the X chromosome. The PIGA gene is responsible for producing a crucial component called glycophosphatidylinositol (GPI) anchors. These GPI anchors act like attachment points on the surface of cells, allowing important protective proteins to stick to the cell membrane. When the PIGA gene mutates in a single stem cell, that cell can no longer produce functional GPI anchors.[2][3]
This mutated stem cell then divides and creates copies of itself, forming a population of abnormal stem cells. These abnormal stem cells mature into red blood cells and platelets that lack the protective proteins normally anchored to their surfaces. Two particularly important missing proteins are CD55 and CD59, which normally protect blood cells from being attacked by the complement system — a part of the body’s immune system designed to fight infections.[2][5]
Without these protective proteins, the complement system mistakes the body’s own red blood cells for foreign invaders and destroys them. This destruction happens within the bloodstream itself, a process called intravascular hemolysis. When red blood cells break apart, they release their hemoglobin into the bloodstream, creating what healthcare providers call “free hemoglobin.”[1][4]
The body has mechanisms to clean up free hemoglobin, including a substance called haptoglobin that normally sweeps in to remove it. However, in PNH, the destruction of red blood cells happens so quickly that the cleaning system cannot keep up. The body also uses nitric oxide to help manage the excess hemoglobin, but this depletes the body’s supply of this important molecule. Without sufficient nitric oxide, people may experience sudden, painful spasms in their stomach muscles, esophagus, and back muscles.[1]
It remains unclear exactly why the PIGA mutation occurs in the first place or why the mutated stem cells sometimes gain a growth advantage over normal stem cells, allowing them to multiply and dominate blood cell production. This is an area of ongoing research in understanding PNH.[2]
Risk Factors
Unlike many diseases, paroxysmal nocturnal haemoglobinuria does not have traditional risk factors related to lifestyle choices, environmental exposures, or family history. Because the genetic mutation occurs spontaneously during a person’s lifetime in their bone marrow stem cells, there are no known preventable behaviors or conditions that increase the likelihood of developing PNH.[3]
However, there is a notable connection between PNH and certain other bone marrow disorders. People who have conditions like aplastic anemia (where the bone marrow fails to produce enough blood cells) or myelodysplastic syndrome (where the bone marrow produces abnormal blood cells) are more likely to develop paroxysmal nocturnal haemoglobinuria. This relationship works in both directions — PNH can occur as a secondary condition in people already diagnosed with these bone marrow disorders, or these disorders can develop in people who already have PNH.[1][4]
When PNH develops on its own without any other bone marrow condition, it is referred to as “primary PNH.” When it occurs alongside or following another bone marrow disorder, it is called “secondary PNH.” The presence of an underlying bone marrow disorder may influence how PNH manifests and progresses, as well as which treatment approaches might be most appropriate.[4]
Understanding these associations is important for healthcare providers when monitoring patients with bone marrow disorders. Regular screening may be recommended for people with conditions like aplastic anemia to detect the development of PNH early, when treatment can be most effective.[11]
Symptoms
The symptoms of paroxysmal nocturnal haemoglobinuria can vary dramatically from one person to another, and not everyone experiences every symptom. This variation makes PNH somewhat of a “great impersonator” in medicine, as its diverse symptoms can resemble many other conditions. The severity of symptoms often relates to how extensively the disease affects blood cell production and destruction.[5][7]
The breakdown of red blood cells leads to hemolytic anemia, which is the shortage of healthy red blood cells in the bloodstream. This causes one of the most common and debilitating symptoms: profound fatigue and weakness. People with PNH often describe feeling exhausted even after rest, which can significantly impact their ability to work, care for their families, or participate in daily activities. Along with fatigue, many experience shortness of breath, particularly during physical activity, and an increased heart rate as the body struggles to deliver oxygen to tissues with fewer functional red blood cells.[2][3]
The classic symptom that gives the disease its name — dark, reddish-brown, or cola-colored urine — occurs when free hemoglobin from destroyed red blood cells is filtered through the kidneys and appears in the urine. This discoloration is often most noticeable in the morning because urine has been concentrating in the bladder overnight. However, hemolysis actually occurs throughout the day, not just at night. Some people notice their urine clears as the day progresses.[1][7]
Many people with PNH experience episodes of abdominal pain, which can range from mild discomfort to severe cramping. Some also report difficulty swallowing or pain during swallowing, particularly when consuming certain foods or liquids. These symptoms occur mainly when red blood cell breakdown is happening rapidly and are related to muscle spasms caused by depletion of nitric oxide in the body. The smooth muscles in the digestive system and blood vessels require nitric oxide to function properly, and its absence causes them to contract painfully.[1][4]
Headaches are another frequent complaint among people with PNH. These may be related to reduced oxygen delivery to the brain or to other effects of hemolysis on blood vessel function. People may also notice they appear abnormally pale, a sign called pallor, which reflects the reduced number of red blood cells in circulation.[3]
Because PNH also affects platelets (the blood cells responsible for clotting), some people experience unusual bruising or bleeding. They may notice they bruise easily from minor bumps or have difficulty controlling bleeding from small cuts or injuries. Conversely, and somewhat paradoxically, about 40% of people with PNH develop blood clots, which represent one of the most serious and life-threatening complications of the disease.[3][4]
Blood clots in PNH can occur in common locations like the deep veins of the legs (causing deep vein thrombosis) or the lungs (causing pulmonary embolism when clots break off and travel). However, PNH has a particular tendency to cause clots in unusual locations, such as the veins draining the liver or other abdominal organs. Before modern treatments became available, blood clots were the leading cause of death in people with PNH.[4][7]
The shortage of white blood cells that can occur with PNH makes some people more prone to infections. They may experience recurring infections or develop fevers more frequently than usual. Additionally, men with PNH may experience erectile dysfunction, another consequence of nitric oxide depletion affecting blood vessel function.[3][4]
In rare cases, PNH can present with unusual inflammatory symptoms. A very small number of affected individuals experience recurrent aseptic meningitis (inflammation of the membranes surrounding the brain and spinal cord not caused by infection), itchy red rashes called hives, joint pain, or inflammatory bowel disease. These inflammatory symptoms usually begin earlier than the blood cell problems.[3]
Prevention
Because paroxysmal nocturnal haemoglobinuria results from a spontaneous genetic mutation that occurs during a person’s lifetime in their bone marrow cells, there are no known methods to prevent the disease from developing. Unlike conditions influenced by diet, exercise, environmental exposures, or inherited genetic factors, PNH arises unpredictably without identifiable triggers that could be avoided.[3]
However, for people who have already been diagnosed with PNH, certain measures can help prevent complications and manage symptoms. One crucial preventive strategy involves vaccination. People with PNH who are treated with certain medications that block part of the immune system (complement inhibitors) become more vulnerable to specific bacterial infections, particularly those caused by Neisseria meningitidis, which can cause meningitis and other serious infections.[14]
Before starting treatment with complement inhibitors, patients must receive meningococcal vaccines that protect against several strains of this bacteria, including types A, C, W, and Y. Vaccination against serotype B, which is the most common strain in some countries, is also recommended. These vaccines should be administered at least two weeks before beginning treatment to ensure protection has developed. In addition to vaccination, some healthcare providers recommend that patients take prophylactic antibiotics (such as penicillin or erythromycin) regularly to further reduce infection risk.[11][14]
For individuals with bone marrow disorders like aplastic anemia or myelodysplastic syndrome, which are associated with increased risk of developing PNH, regular screening can enable early detection. While this doesn’t prevent PNH, it allows treatment to begin before severe complications develop. Healthcare providers may recommend periodic blood tests to check for PNH cells in patients with these conditions.[11]
People living with PNH can take steps to reduce triggers that might worsen their symptoms. Avoiding situations that activate the complement system — such as infections, surgery, or significant physical stress — when possible may help minimize hemolysis episodes. However, many of these triggers are unavoidable parts of life, which is why medical treatment remains the primary approach to managing the disease.[2]
Pathophysiology
Understanding the pathophysiology of paroxysmal nocturnal haemoglobinuria requires examining how normal blood cells are produced and protected, and what happens when this process goes wrong. The disease represents a complex interplay between genetic changes, immune system function, and blood cell survival.[7]
In healthy individuals, blood-forming stem cells in the bone marrow produce all types of blood cells. These stem cells possess a normally functioning PIGA gene that creates GPI anchors — specialized molecules that act as attachment points on the cell surface. Various protective proteins use these anchors to attach themselves to the outside of blood cells. Among these are CD55 and CD59, which are complement regulatory proteins that serve as shields against the complement system.[2][5]
The complement system is part of the body’s innate immune system, designed to identify and destroy invading pathogens like bacteria and viruses. It consists of a cascade of proteins that, when activated, can create holes in cell membranes, causing cells to burst and die. This process is essential for fighting infections, but it needs to be carefully controlled to prevent it from attacking the body’s own cells.[4]
When the PIGA gene mutates in a stem cell, that cell loses the ability to produce GPI anchors. As this mutated stem cell divides and matures into red blood cells and platelets, these cells emerge without the protective complement regulatory proteins CD55 and CD59 on their surfaces. Without this protection, they become vulnerable to attack by the complement system, which mistakenly identifies them as foreign cells and destroys them.[1][2]
The destruction occurs through a process where complement proteins assemble on the cell surface and form a structure called the membrane attack complex or terminal complement complex. This complex creates pores in the cell membrane, allowing water and ions to flood in, causing the cell to swell and burst. Because this happens within the blood vessels themselves (intravascular hemolysis), the contents of the red blood cells spill directly into the bloodstream.[8]
When red blood cells rupture, they release their hemoglobin into the plasma. Normally, a protein called haptoglobin quickly binds to free hemoglobin and carries it to the liver for processing. However, in PNH, the continuous destruction of red blood cells produces more free hemoglobin than haptoglobin can handle, leading to a state of haptoglobin depletion. The excess free hemoglobin circulates in the bloodstream and eventually passes through the kidneys, appearing in the urine and causing its characteristic dark color.[1]
Free hemoglobin has another important effect: it binds to and consumes nitric oxide, a crucial molecule that helps blood vessels relax and maintain proper blood flow. Nitric oxide is essential for smooth muscle function throughout the body, including in blood vessels, the digestive tract, and erectile tissue. The depletion of nitric oxide leads to several PNH symptoms, including painful muscle spasms, difficulty swallowing, abdominal pain, and erectile dysfunction.[4][9]
The loss of nitric oxide and the presence of free hemoglobin also contribute to the high rate of blood clot formation in PNH. Platelets become activated, and the normal balance between clotting and anti-clotting mechanisms is disrupted. Additionally, PNH platelets lack the same protective proteins as red blood cells, making them more susceptible to complement-mediated activation, which triggers clotting. This explains why people with PNH face increased risk of thrombosis despite also having fewer platelets overall.[4]
The bone marrow attempts to compensate for the ongoing destruction of red blood cells by increasing production. However, in many cases, the bone marrow in people with PNH is already compromised, functioning below normal capacity. This leads to a state where the bone marrow cannot keep up with the demand, resulting in persistent anemia. Some people develop complete bone marrow failure over time, a serious complication that requires different treatment approaches.[1][7]
The severity of symptoms in PNH often relates to the proportion of blood cells affected. Flow cytometry testing can determine what percentage of a person’s blood cells lack GPI-anchored proteins. People with a larger “PNH clone” (higher percentage of abnormal cells) typically experience more severe hemolysis and symptoms. However, even people with relatively small clones can develop serious complications like blood clots.[7]
An interesting aspect of PNH pathophysiology is why the mutated stem cells sometimes expand to dominate blood cell production. One theory suggests that in the context of bone marrow disorders like aplastic anemia, where the immune system attacks stem cells, PNH cells may have a survival advantage because they lack certain surface proteins that are targets of the immune attack. This allows them to survive and proliferate while normal stem cells are being destroyed.[2]
