Atypical haemolytic uraemic syndrome is a rare and serious condition where the immune system mistakenly attacks blood vessel walls, leading to dangerous blood clots that can damage the kidneys and other vital organs throughout the body.

Understanding Atypical Haemolytic Uraemic Syndrome

Atypical haemolytic uraemic syndrome, often shortened to aHUS, is a potentially life-threatening disease that affects how blood flows through tiny vessels in the body. When someone has aHUS, parts of their immune system go wrong and begin attacking the cells that line blood vessels. This attack triggers the formation of small blood clots throughout the body, particularly affecting the kidneys and sometimes other organs like the heart, brain, liver, and lungs^[1].

Healthcare providers typically identify aHUS when three specific conditions occur together. The first is microangiopathic haemolytic anaemia, which means red blood cells are being destroyed faster than the body can make new ones to replace them. The second is thrombocytopenia, meaning there are too few platelets in the blood—platelets are the cells that help blood clot when you’re injured. The third condition is acute kidney injury, a type of sudden kidney failure that may be reversible with proper treatment^[1].

What makes aHUS “atypical” is that it differs from typical haemolytic uraemic syndrome. The typical form is caused by certain strains of E. coli bacteria and usually causes severe diarrhoea, especially in children. In contrast, aHUS does not cause diarrhoea and is often linked to genetic changes rather than bacterial infection. Healthcare providers sometimes use another term for aHUS: complement-mediated thrombotic microangiopathy, which describes how the disease affects the complement system, a crucial part of the immune response^[1].

How Common Is This Disease?

Atypical haemolytic uraemic syndrome is extremely rare. Studies estimate that only about 2 people out of every million in the United States develop aHUS each year^[3][8]. This means that in a city of one million people, only two individuals would be expected to develop this condition in any given year. The atypical form is approximately ten times less common than typical haemolytic uraemic syndrome caused by bacterial infection^[6].

The disease can affect people of any age, from newborns to elderly adults. In children, boys and girls are equally likely to develop aHUS. However, as people get older, women appear to be more susceptible than men, partly because pregnancy can trigger the disease^[8]. The rarity of aHUS means that many healthcare providers may never encounter a case during their careers, which can sometimes lead to delays in diagnosis.

Because aHUS is so uncommon, exact numbers for different populations and regions around the world are difficult to establish. Most of what researchers know comes from specialized medical centers that track rare diseases and from patient registries that collect information from multiple countries.

What Causes Atypical Haemolytic Uraemic Syndrome?

The root cause of aHUS lies in how the body’s immune system functions. In about half of all cases, the condition stems from genetic mutations—permanent changes in a person’s DNA that affect how certain proteins work^[6]. These genetic changes can be inherited from a biological parent, or they can occur spontaneously without any family history of the disease.

The genes most commonly affected in aHUS provide instructions for making complement factors, which are proteins that play a vital role in the immune system’s ability to identify and destroy harmful invaders like bacteria and viruses. Think of complement proteins like seasoning on food—they coat dangerous germs to make them recognizable and “tasty” to other immune cells that will then destroy them^[1].

The mutations in aHUS most frequently affect complement factors called H, I, 3, and B^[1]. When these factors don’t work properly, the complement system loses its normal controls and becomes overactive. For example, complement factor H normally protects the body’s own cells from being attacked by other complement proteins. When the gene for factor H is mutated, this protection fails. The result is that immune cells mistakenly attack and damage the cells lining blood vessels, called endothelial cells. Platelets then rush to the site of damage to form clots, but these clots block blood flow to vital organs, leading to the serious complications seen in aHUS^[1].

In some cases, aHUS is caused not by inherited genetic mutations but by autoantibodies—proteins that the body mistakenly produces against its own complement factors. These autoantibodies interfere with the normal function of complement proteins, creating the same kind of uncontrolled immune activation seen with genetic mutations^[1][7].

In up to half of people with aHUS, doctors cannot find a specific genetic mutation or autoantibody causing the disease. This type is called “idiopathic,” meaning the cause remains unknown. Researchers believe these cases may involve genetic changes that haven’t been identified yet or involve combinations of factors not yet understood^[8].

Risk Factors and Triggers

Having a genetic mutation alone often isn’t enough to cause aHUS symptoms. Most people with these genetic changes need something to trigger the disease before symptoms appear. This is known as incomplete penetrance, meaning not everyone with the genetic risk will develop the disease^[3].

Several common health events can trigger aHUS episodes in people with underlying genetic susceptibility. Pregnancy is one of the most significant triggers, which explains why more women than men develop aHUS in adulthood^[1]. The physical stress and immune changes that occur during pregnancy and childbirth can unmask a previously silent genetic defect.

Infections are another major trigger. These can include respiratory infections, diarrhoeal illnesses not caused by Shiga toxin, and various viral infections. Even common childhood illnesses have been known to trigger aHUS in susceptible individuals^[1][3]. Cancer can also act as a trigger, as can certain medical treatments.

Medications represent an important group of triggers that people can potentially avoid. Certain cancer chemotherapy drugs, immunosuppressive medications used after organ transplantation, blood thinners, oral contraceptive pills, and anti-inflammatory medications have all been associated with triggering aHUS episodes^[1]. If you have a known genetic risk for aHUS, discussing these medication risks with your healthcare provider before starting any new treatment is essential.

Organ transplantation itself can trigger aHUS, creating a challenging situation for people who have developed kidney failure from the disease. Even chronic diseases like certain autoimmune conditions can sometimes trigger aHUS episodes^[8].

Recognizing the Symptoms

The symptoms of aHUS can vary considerably from person to person, and they may come on suddenly or develop gradually over time. Many people describe feeling generally unwell for a while, as if they’ve been sick with something they just can’t shake^[1]. This vague feeling of illness can make early diagnosis challenging.

Fatigue is one of the most common symptoms, often severe enough to interfere with daily activities. This exhaustion comes from anaemia, the condition where there aren’t enough healthy red blood cells to carry oxygen throughout the body. People may also notice their skin looks unusually pale, a condition called pallor^[1].

Changes in urination patterns often signal kidney problems. People with aHUS may urinate less frequently than usual, or they might notice blood in their urine, which can make it appear pink, red, or brown. Some people develop swelling, called edema, particularly in the legs, feet, ankles, or around the eyes. This swelling happens because damaged kidneys can’t remove excess fluid from the body properly^[1].

High blood pressure, known as hypertension, commonly develops with aHUS because the kidneys play a crucial role in regulating blood pressure. When the kidneys are damaged, blood pressure can rise to dangerous levels^[7].

Digestive symptoms may include nausea, vomiting, and abdominal pain. Some people experience fever. Shortness of breath, called dyspnea, can occur when anaemia becomes severe or when fluid builds up in the lungs^[1].

Less commonly, aHUS affects the brain and nervous system, causing neurological symptoms such as confusion, seizures, or even coma in severe cases. These symptoms indicate that blood flow to the brain has been significantly reduced by the formation of small clots^[1][7]. Heart attacks, strokes, liver damage, and pancreas inflammation can also occur, though these complications are less frequent^[7].

The symptoms often appear suddenly after a triggering event and can be mild or severe. Severe episodes can cause rapid kidney failure requiring immediate medical attention. Some people may experience only a few symptoms, while others develop multiple problems affecting several organ systems simultaneously.

Prevention Strategies

Because aHUS is primarily caused by genetic factors, there is no way to completely prevent the disease if you carry the genetic mutations. However, understanding your genetic risk and taking steps to avoid triggers can significantly reduce the likelihood of developing symptoms or experiencing disease flare-ups.

Genetic testing and family counseling are important for families where aHUS has been diagnosed. If genetic testing reveals that you carry mutations associated with aHUS but haven’t developed symptoms, you can work with your healthcare team to create a prevention plan. This might include avoiding medications known to trigger the disease, being vigilant about treating infections promptly, and carefully planning decisions around pregnancy^[16].

Vaccination is a crucial preventive measure. Anyone at risk for aHUS or diagnosed with the condition should receive all recommended vaccinations to prevent infections that could trigger disease episodes. This is especially important because some of the treatments for aHUS can increase susceptibility to certain infections^[11][12].

If you’re taking medications for aHUS, such as complement inhibitors, you’ll need vaccination against meningococcal bacteria at least two weeks before starting treatment. This vaccination is critical because these medications increase the risk of severe meningococcal infections, which can be life-threatening. The U.S. Food and Drug Administration requires this vaccination for anyone taking eculizumab or ravulizumab, the two approved complement inhibitors for aHUS^[11][12].

Maintaining overall good health can help reduce the risk of infections that might trigger aHUS. This includes practicing good hygiene, such as regular handwashing, eating a balanced diet, getting adequate sleep, and managing stress. For people already diagnosed with aHUS, regular medical monitoring is essential for catching any early signs of disease activity before they progress to serious complications.

Women with aHUS or genetic risk factors should have detailed discussions with their healthcare providers before becoming pregnant. Pregnancy carries significant risk for triggering or worsening aHUS, so careful planning and close monitoring throughout pregnancy and after delivery are essential^[16].

How the Disease Affects the Body

Understanding what happens inside the body during aHUS helps explain why the disease causes such serious problems. The process begins with the complement system, a network of proteins that normally protects the body from infections. This system works like a carefully controlled chain reaction—one protein activates the next in sequence, ultimately destroying harmful bacteria or viruses.

In healthy people, regulatory proteins keep this chain reaction under control, ensuring it only activates when needed and doesn’t damage the body’s own cells. These regulatory proteins act like brakes on a car, stopping the complement system from running wild. In aHUS, genetic mutations or autoantibodies disable these brakes, causing the complement system to activate uncontrollably^[6].

When the complement system becomes overactive, it attacks the endothelial cells that line the inside of blood vessels. These cells normally provide a smooth, protective surface that allows blood to flow freely. The complement attack damages this protective lining, creating rough, injured areas inside the vessels. The body perceives this damage as an injury that needs repair, so platelets rush to the site and begin forming clots to seal the breach^[1].

However, because the complement attack is widespread and continuous, clots form throughout the body’s small blood vessels rather than just at a single injury site. These numerous small clots create a condition called thrombotic microangiopathy, where tiny blood vessels become blocked throughout the body^[7].

As red blood cells try to squeeze through narrowed, clot-filled vessels, they get damaged and break apart. This destruction of red blood cells is called haemolysis, and it happens faster than the bone marrow can produce new cells to replace them. The result is severe anaemia, leaving the body without enough oxygen carriers in the blood^[6].

Meanwhile, platelets are being consumed rapidly in forming all these clots, leading to thrombocytopenia—too few platelets remaining in circulation. This creates a paradox: the body is forming too many clots inside blood vessels while simultaneously lacking enough platelets to clot normally when needed, such as after an injury.

The kidneys are particularly vulnerable to damage in aHUS because they contain vast networks of tiny blood vessels that filter blood to remove waste products. When these vessels become clogged with clots, the kidneys can’t filter properly, and waste products build up in the blood. The kidneys may fail suddenly or gradually lose function over time. In severe cases, approximately 33 to 40 percent of people developed end-stage kidney failure or died during their first episode of aHUS before modern treatments became available^[7].

Other organs can also be affected when their blood supply becomes compromised. The brain may not receive adequate oxygen, leading to confusion, seizures, or stroke. The heart can be damaged, potentially causing heart attacks. The digestive system, liver, and pancreas may all suffer damage when their blood supply is reduced^[7].

The severity and pattern of organ damage vary considerably among individuals with aHUS. Some people experience primarily kidney problems, while others may have multi-organ involvement. The specific genetic mutation involved can influence which organs are most affected and how severe the disease becomes, though this relationship isn’t completely predictable^[3].