Symptoms of aHUS

The signs and symptoms of atypical hemolytic uremic syndrome (aHUS) usually appear suddenly, and often follow a trigger event, such as an infection.

aHUS is a disease characterized by the formation of blood clots in the body’s small blood vessels. These clots can cause damage to multiple organs, especially the kidneys. Often, people with aHUS will report a vague feeling of illness, with non-specific symptoms that may include paleness, nausea, vomiting, fatigue, drowsiness, high blood pressure, and swelling.

There are three hallmark symptoms that define aHUS: hemolytic anemia, thrombocytopenia, and kidney failure.

Symptoms can appear at any age, though it is slightly more common for them to first appear in childhood rather than later on in life. Adult-onset aHUS is more frequent in biological females than males, whereas childhood-onset disease affects both sexes equally.

Hemolytic anemia

Red blood cells are the cells that transport oxygen through the bloodstream to the body’s tissues and organs. Anemia occurs when red blood cells are destroyed faster than they are made, leading to a shortage of oxygen in tissues and organs.

“Hemolytic” anemia specifically refers to a type of anemia that is caused by the rupture or destruction of red blood cells, a process known as hemolysis. In aHUS, hemolysis is caused by red blood cells squeezing past clots inside small blood vessels.

Anemia commonly causes a series of symptoms, including pallor or unusually pale skin, jaundice — characterized by the yellowing of the eyes and skin — fatigue, headache, shortness of breath, dizziness, and chest pain. Cold hands and feet and an irregular heartbeat are other symptoms of anemia.


Thrombocytopenia refers to abnormally low levels of platelets, which are cell fragments found in the bloodstream that play a key role in blood clotting. In aHUS, platelet levels drop because they are being constantly used to form blood clots.

Most of the symptoms of thrombocytopenia are related to excessive bleeding, and include prolonged bleeding from cuts, nosebleeds or bleeding gums, blood in the urine or stool, and unusually heavy menstrual flow. Other main symptoms are fatigue and purpura, which is characterized by easy or excessive bruising.

Kidney failure

The kidneys are the organs that are responsible for filtering fluid and waste products from the blood. Kidney failure, as its name suggests, occurs when the kidneys are no longer able to perform their filtering function. In aHUS, damage to the kidneys resulting from clots in small blood vessels can lead to acute kidney failure.

Symptoms of such acute kidney failure may include decreased urine output and swelling in the legs, ankles, or feet. Shortness of breath, fatigue, confusion, nausea, weakness, an irregular heartbeat, and pain or pressure in the chest are other common symptoms. In severe cases, acute kidney failure can lead to seizures or coma.

Other symptoms

While the kidneys are the organs most affected by aHUS, other internal organs also may be damaged by the formation of clots in small blood vessels. Studies indicate that about one in five aHUS patients has symptoms associated with organ and tissue damage that does not involve the kidneys.

About 10% of aHUS patients have involvement of the central nervous system, comprised of the brain and spinal cord, which can be detected through an MRI scan. Nervous system involvement can cause symptoms like irritability, drowsiness, seizures, double vision, stupor, blindness, paralysis, and coma.

aHUS also can damage heart tissues and lead to a heart attack. This has been reported in about 3% of patients.

The disease also can affect multiple systems throughout the body simultaneously, including the kidneys, nervous system, heart, pancreas, and liver. Such multi-organ involvement has been observed in about 5% of cases.


Last updated: April 14, 2021


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