Thrombotic Microangiopathy

Thrombotic microangiopathy is a rare but life-threatening group of blood disorders where tiny blood clots form in the smallest blood vessels throughout the body, leading to organ damage and requiring urgent medical care.

Table of contents

• What is thrombotic microangiopathy?
• Main types of thrombotic microangiopathy
• Signs and symptoms
• Causes and risk factors
• How is it diagnosed?
• Treatment approaches
• Living with thrombotic microangiopathy

What is thrombotic microangiopathy?

Thrombotic microangiopathy, often shortened to TMA, is a group of disorders where small blood clots form in the body’s tiniest blood vessels, called capillaries and arterioles.^[1] These small clots can block blood flow to important organs and cause serious damage.

When these clots form, they trap blood cells called platelets, causing the platelet count to drop dangerously low. This condition is known as thrombocytopenia.^[1] At the same time, red blood cells get damaged as they try to squeeze through the blocked vessels, breaking apart into fragments. This destruction of red blood cells is called microangiopathic hemolytic anemia.^[2]

The combination of low platelet counts, damaged red blood cells, and blocked small vessels leads to reduced oxygen delivery to organs, potentially causing kidney failure, brain problems, heart damage, or injury to other parts of the body.^[1] Though rare, affecting only about 1 to 3 people per million, thrombotic microangiopathy is a medical emergency that requires immediate treatment.^[14]

Main types of thrombotic microangiopathy

There are several different types of thrombotic microangiopathy, each with distinct causes and characteristics. The two most common primary forms are thrombotic thrombocytopenic purpura and hemolytic uremic syndrome.^[1]

Thrombotic thrombocytopenic purpura (TTP)

Thrombotic thrombocytopenic purpura, or TTP, occurs when there is a problem with a protein called ADAMTS13.^[5] This enzyme is made by the liver and normally helps regulate blood clotting. When someone has a blood vessel injury, the body produces a clotting protein called von Willebrand factor to form a clot at the injury site. ADAMTS13 acts like scissors to cut this clotting protein into smaller pieces once enough clotting has occurred, preventing excessive clot formation.^[5]

In people with TTP, ADAMTS13 activity is severely low or absent. Some people are born with a genetic mutation affecting the ADAMTS13 gene, while others develop an autoimmune condition where the body makes antibodies that block ADAMTS13 from working properly.^[5] Without enough ADAMTS13, clotting continues unchecked, leading to widespread small blood clots.

TTP affects about 3 people per 1 million adults each year.^[1] It primarily causes neurological symptoms such as headache, confusion, and signs of stroke, along with blood abnormalities.^[5]

Hemolytic uremic syndrome (HUS)

Hemolytic uremic syndrome, or HUS, primarily affects the small blood vessels of the kidneys, leading to kidney damage.^[5] There are two main categories of HUS.

The first is Shiga toxin-associated HUS, which typically follows a serious bowel infection, most often caused by E. coli bacteria that produce Shiga toxin.^[2] This toxin damages the cells lining blood vessels in the kidneys, triggering clot formation. This type usually occurs after diarrhea, particularly bloody diarrhea, and mainly affects children under five years old.^[15] The incidence of HUS in children is about 3 per 100,000.^[1]

The second category is complement-mediated thrombotic microangiopathy (previously called atypical HUS), which results from problems with the body’s complement system—a part of the immune system that helps fight infections.^[10] This can be caused by genetic mutations in complement proteins or by autoantibodies that interfere with complement regulation. About 50% to 70% of patients with this form have identifiable genetic mutations.^[4]

Secondary thrombotic microangiopathy

Thrombotic microangiopathy can also occur as a secondary condition triggered by various factors including pregnancy, severe high blood pressure, certain medications (particularly some cancer drugs and immunosuppressants), infections, autoimmune diseases, or following organ transplantation.^[2] When the underlying cause is identified and treated, secondary TMA often resolves.^[1]

Signs and symptoms

The early symptoms of thrombotic microangiopathy can be quite vague and nonspecific. People may experience general feelings of being unwell, weakness, fatigue, and headaches.^[5] Many patients are first hospitalized when routine blood work reveals severely low platelet counts, sometimes reaching dangerously low levels.^[5]

The specific symptoms depend largely on which organs are affected by the blocked blood vessels. When TMA affects the brain, symptoms may include confusion, seizures, vision problems, and signs of stroke.^[2] Kidney involvement can cause decreased urine output and signs of kidney failure. Heart involvement may lead to chest pain or abnormal heart rhythms. Some people develop severe high blood pressure.^[2]

Because platelets help stop bleeding, people with very low platelet counts may bruise easily or have bleeding problems, including small purple spots on the skin called purpura.^[5] The destruction of red blood cells causes anemia, leading to pale skin, tiredness, and shortness of breath.

Older medical descriptions mentioned a “pentad” of five symptoms—fever, low platelet count, neurological symptoms, kidney injury, and anemia—but recent studies show that less than five percent of TTP patients actually have all five features.^[5] A person can have TTP or another form of TMA with just one or two of these symptoms.

Causes and risk factors

The causes of thrombotic microangiopathy vary depending on the specific type. The central event in all forms is injury to the cells lining blood vessels, called endothelial cells.^[2] This injury reduces the production of substances that normally prevent clotting, leading to excessive clot formation in small vessels.

For TTP, the cause is insufficient ADAMTS13 enzyme activity. This can result from inherited genetic mutations or, more commonly, from acquired autoimmune antibodies that attack ADAMTS13.^[7]

For Shiga toxin-associated HUS, bacterial toxins from E. coli or Shigella infections are the primary cause. These toxins inhibit protein production in cells, damaging the blood vessel walls in the kidneys and triggering the clotting cascade.^[2]

Complement-mediated TMA results from dysregulation of the alternative complement pathway. Mutations in genes that control complement proteins have been identified in many patients.^[2] These genetic changes often have incomplete penetrance, meaning that people with the mutations may not develop disease unless exposed to a triggering factor or “second hit” such as infection, pregnancy, or certain medications.^[4]

Secondary TMA can be triggered by pregnancy, severe uncontrolled high blood pressure (malignant hypertension), certain chemotherapy drugs, immunosuppressive medications used after transplants (particularly calcineurin inhibitors), autoimmune diseases, and infections.^[2] More recently, researchers have linked some targeted cancer therapies and anti-VEGF medications to TMA development.^[2]

How is it diagnosed?

Diagnosing thrombotic microangiopathy requires a thorough evaluation because presenting symptoms are often nonspecific, but prompt diagnosis is critical for starting appropriate treatment.^[1]

Basic laboratory tests reveal the characteristic pattern of TMA: low platelet count (thrombocytopenia) and anemia with evidence of red blood cell destruction.^[1] When a blood sample is examined under a microscope, fragmented red blood cells called schistocytes can be seen.^[6] Blood tests also show elevated levels of lactate dehydrogenase (LDH), an enzyme released when cells are damaged, and low levels of haptoglobin, a protein that decreases during hemolysis.^[6]

The Coombs test, which detects antibodies attached to red blood cells, is typically negative in TMA (except in rare cases of pneumococcal HUS), helping distinguish TMA from other causes of red blood cell destruction.^[6] Importantly, standard blood clotting tests (PT, aPTT, and fibrinogen) are usually normal in TMA, which helps differentiate it from a similar condition called disseminated intravascular coagulation.^[6]

The key specialized test is measurement of ADAMTS13 activity.^[7] If ADAMTS13 activity is severely reduced (typically less than 10%), this confirms TTP and distinguishes it from other forms of TMA. Testing should also check for ADAMTS13 inhibitors (antibodies) to determine if the condition is acquired or hereditary.

For suspected Shiga toxin-associated HUS, stool samples are tested for the presence of Shiga toxin-producing bacteria or toxins.^[6] Genetic testing for complement system abnormalities is recommended to identify underlying susceptibilities in complement-mediated TMA.^[10]

Additional testing may include kidney function tests, urine analysis, imaging studies, and evaluation for potential secondary causes such as pregnancy, medications, autoimmune diseases, or infections.^[3] A complete medical history, including recent infections, medication use, and family history, is essential for accurate diagnosis.

Treatment approaches

Treatment of thrombotic microangiopathy depends on the specific type and underlying cause, but because TMA is a medical emergency, treatment often begins immediately once the diagnosis is suspected, even before all test results are available.^[1]

Treatment for TTP

For thrombotic thrombocytopenic purpura, the main treatment is plasma exchange (also called plasmapheresis).^[7] This procedure removes the patient’s plasma, which contains the antibodies attacking ADAMTS13, and replaces it with fresh frozen plasma that contains normal ADAMTS13 enzyme. Plasma exchange should be started as soon as TTP is suspected, as it can be life-saving.^[1]

Recent advances have introduced targeted therapies for TTP. Medications that suppress the immune system may be used to reduce antibody production. Treatment is typically continued until laboratory values normalize and the patient’s condition improves.^[6]

Treatment for complement-mediated TMA

The introduction of C5 inhibitors, medications that block a specific part of the complement system, has transformed the management of complement-mediated TMA.^[10] These drugs, such as eculizumab, significantly improve outcomes compared to previous treatments. Before 2011, when these medications became available, plasma exchange was the primary therapy, but results were often poor.^[10]

An important consideration with C5 inhibitors is that they increase the risk of serious infections, particularly meningococcal disease. Therefore, patients must receive meningococcal vaccination before starting treatment, and prophylactic antibiotics may be recommended.^[10]

Treatment for Shiga toxin-associated HUS

Management of Shiga toxin-associated HUS is primarily supportive.^[15] This includes ensuring adequate hydration, controlling blood pressure, correcting electrolyte imbalances, and providing blood transfusions when needed. Many patients require temporary dialysis to support kidney function. The focus is on supporting the body while it recovers from the infection.^[15]

Treatment for secondary TMA

When TMA occurs secondary to another condition, treatment focuses on addressing the underlying cause. This might involve stopping a triggering medication, treating an infection, managing high blood pressure, or addressing pregnancy-related complications.^[1] Once the underlying condition is controlled, secondary TMA usually resolves.^[4]

All patients require close monitoring of blood counts, kidney function, and other organ systems. Supportive care includes blood transfusions for severe anemia, management of high blood pressure, and treatment of organ complications as they arise.^[6]

Living with thrombotic microangiopathy

Recovery from a thrombotic microangiopathy episode takes time, and each person’s experience is different. It’s important to take things slowly and listen to your body during recovery. Physical recovery often happens before mental and emotional recovery.^[9]

Recovery and ongoing challenges

During recovery, some people experience neurological difficulties such as memory problems, confusion, loss of concentration, dizziness, lack of balance, headaches, or vision changes.^[9] These symptoms might not seem related to TMA, especially if you’re feeling better otherwise, but they should be reported to your healthcare team.

It’s completely normal to experience anxiety and stress after a TMA episode. Many people worry about having another episode, which can affect mental health.^[9] Checking in about your mental and emotional health is just as important as monitoring physical symptoms. A healthy diet, adequate sleep, regular exercise when cleared by your doctor, and staying connected with loved ones can all support mental health. If you’re experiencing clinical depression or severe anxiety, your care team can ensure you get appropriate help.^[9]

Special considerations

People with TMA may wonder about future plans such as travel or starting a family. These things are possible with careful planning. Before traveling, discuss with your doctor the risk of a TMA episode during travel. They may check your ADAMTS13 levels or other blood tests to ensure it’s safe to travel.^[9] Bring contact information for your care team and documents explaining your condition in case you need medical care while away.

Pregnancy can be a trigger for TMA episodes, particularly for complement-mediated forms.^[9] If you’re planning to become pregnant, thorough discussion with your care team is essential to make the safest decisions and develop a monitoring plan.

Long-term follow-up is important because some people may experience more than one TMA episode. Regular monitoring of blood tests, kidney function, and overall health helps detect early signs of recurrence.^[9] The frequency of follow-up depends on the type of TMA and individual risk factors.

Despite the serious nature of thrombotic microangiopathy, many people return to their normal activities with appropriate treatment and monitoring. A TMA diagnosis doesn’t need to hold back your life, though you may need to make some adjustments and take extra precautions.^[9]

• Blood vessels (capillaries and arterioles)
• Kidneys
• Brain
• Heart
• Gastrointestinal tract
• Skin
• Lungs
• Liver