Atypical Haemolytic Uraemic Syndrome (aHUS)

Find out about atypical Haemolytic Uraemic Syndrome (aHUS): symptoms, diagnosis, treatment and sources of further information and support.

What is atypical Haemolytic Uraemic Syndrome (aHUS)?

Atypical Haemolytic Uraemic Syndrome (aHUS) is an extremely rare kidney condition.

Atypical – it’s a rare form of the disease

Haemolytic – it involves the blood

Uraemic – it involves the kidneys

Syndrome – there are several different things going on at the same time

aHUS occurs due to a fault in the complement system, which is part of the body’s immune system and helps it to fight infections. There is normally a system of protector proteins, which stop complement from attacking the body’s own cells. In aHUS, a fault in these proteins causes the complement to attack the body’s cells. This can cause tiny clots to form in small blood vessels. If left untreated, this can eventually lead to kidney damage.

There are about 20 new cases of aHUS a year in the UK.

What are the signs and symptoms of aHUS?

The first sign of aHUS is usually increasing tiredness and generally feeling unwell, like a bug that won’t go away. Other symptoms may include:

  • headache
  • blurred vision
  • nausea and/or vomiting
  • stomach pain
  • chest pain
  • rash
  • ulcers
  • blood in the urine (haematuria)
  • protein in the urine (proteinuria)

These symptoms may be mild at first but get worse and more noticeable over time.

What causes aHUS?

aHUS is a genetic condition, so the fault in the complement system is present from birth.

An aHUS episode can occur at any age and can be triggered by:

  • gastrointestinal illness – vomiting and/or diarrhoea
  • respiratory illness – a cold or flu
  • certain medications
  • pregnancy – especially in the third trimester and post-delivery

How is aHUS diagnosed?

There is currently no definitive test for aHUS so it is usually diagnosed by blood and urine tests and by ruling out other similar conditions.

Tests may include:

  • Full blood count – to examine the haemoglobin (red blood cell) and platelet levels in the blood
  • Complement studies – to measure the amount of certain proteins in the blood
  • Urinalysis – to check for blood or protein in the urine, which may be a sign of kidney damage
  • Stool/rectal swab – to exclude ‘typical’ HUS, also known as STEC HUS or diarrhoea positive HUS

In some cases, a kidney biopsy and/or an ultrasound is performed to check the function and structure of the kidneys.

Does aHUS affect other parts of the body?

aHUS mainly affects the kidneys as they contain lots of tiny blood vessels, which are more likely to be blocked by the blood clots. However, blood clots can occur in other organs including the liver, lungs, heart, brain and digestive tract.

Does aHUS run in families?

aHUS can run in families and its genetic basis was first identified in 1998. Several genetic faults or mutations have since been discovered, but so far these appear to merely increase the likelihood of aHUS rather than directly causing it.

Around 65% of people with one of these genetic mutations will develop aHUS by the age of 70 but it needs an environmental ‘trigger’ such as an illness to actually occur.

Genetic testing is available through the National aHUS Service in Newcastle. A blood sample may be sent to the lab to confirm the diagnosis of aHUS.

How is aHUS treated?

aHUS may be treated by a procedure called plasma exchange. Plasma is the straw-coloured liquid part of the blood that contains minerals, proteins and antibodies. The plasma that contains the faulty proteins is removed from the blood and replaced with fresh plasma from donated blood that contains functioning proteins. This takes place through a line that is inserted into a blood vessel in the arm.

In recent years, new treatments for aHUS have been discovered.

Eculizumab (Soliris©) and ravulizumab are medications that are given as regular infusions lasting around 45 minutes. They work by blocking the action of complement so it does not damage the body’s cells.

The treatment is very effective but is (currently) lifelong, usually given every two weeks by injection. Ongoing follow-ups are also needed, with blood and urine samples being taken to make sure that the treatment is working and there is no sign of the aHUS returning.

Without treatment, aHUS can lead to end stage kidney failure. A short-term course of dialysis may be needed to improve kidney function. If the kidney damage is severe, lifelong dialysis may be needed. A transplant may be offered in the future, but there is a risk that aHUS can reoccur in the new kidney.

All suspected cases of aHUS in England are assessed by the National aHUS Service in Newcastle, which determines if eculizumab is an appropriate treatment, based on the guidelines set by NHS England.

Where can I get more information or support about aHUS?

For more information on aHUS, including its genetics, diagnosis, symptoms and treatment, visit the National Renal Complement Therapeutics Centre.

Publication date: 11/2023

Review date: 11/2026

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