# Haemolytic Uraemic Syndrome and Atypical HUS

> Haemolytic uraemic syndrome for NEET-PG Medicine: Shiga toxin-mediated typical HUS, complement-driven atypical HUS, TTP differences and eculizumab use.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/haemolytic-uraemic-syndrome-and-atypical-hus
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Harrison's Principles of Internal Medicine; Nelson Textbook of Pediatrics; Oxford Textbook of Medicine
- How to cite: "Haemolytic Uraemic Syndrome and Atypical HUS", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/haemolytic-uraemic-syndrome-and-atypical-hus

## Direct answer

Haemolytic uraemic syndrome is a thrombotic microangiopathy with microangiopathic haemolytic anaemia, thrombocytopenia and acute kidney injury. Typical HUS follows infection with Shiga toxin-producing organisms and bloody diarrhoea, whereas atypical HUS results from uncontrolled activation of the alternative complement pathway. Typical disease is managed supportively, and complement blockade with eculizumab is reserved for atypical or refractory forms.

## What you must remember

- **Triad:** microangiopathic haemolytic anaemia with schistocytes, thrombocytopenia and acute kidney injury define the syndrome.
- **Typical HUS:** follows bloody diarrhoea from Shiga toxin-producing Escherichia coli, most often serotype O157:H7, and is common in children.
- **Shigella dysenteriae type 1:** produces Shiga toxin and is an important cause of HUS in India.
- **Atypical HUS:** caused by complement dysregulation, often through genetic variants in complement regulators or anti-factor H antibodies, with no preceding diarrhoea.
- **Lab clues:** raised LDH, low haptoglobin, a high reticulocyte count and a negative direct Coombs test.
- **ADAMTS13:** normal in typical HUS and in most atypical HUS, whereas severe deficiency points to thrombotic thrombocytopenic purpura.
- **Eculizumab:** a monoclonal antibody against complement C5, used for atypical HUS.
- **Antibiotics in STEC:** avoided where E. coli O157 infection is suspected, as they may increase toxin release; Shigella dysentery is still treated with antibiotics.

## Typical and atypical HUS compared

Both forms share the triad of haemolysis, low platelets and kidney injury, but they differ in trigger, mechanism and treatment. The distinction matters because one form is usually self-limiting with supportive care, while the other may need complement blockade to prevent loss of kidney function.

| Feature | Typical HUS (STEC) | Atypical HUS |
|---|---|---|
| Preceding event | Bloody diarrhoea, often in children | Often none; infection, pregnancy or drugs may trigger |
| Mechanism | Shiga toxin injures endothelium | Alternative complement pathway dysregulation |
| Complement tests | Typically normal | Often abnormal, with genetic or antibody causes |
| Main treatment | Supportive care and dialysis when needed | Eculizumab, with plasma exchange in some cases |
| Diarrhoea drugs | Antibiotics and antimotility agents avoided in STEC | Not governed by the same restriction |

Typical HUS usually follows a self-limiting course in most children, although dialysis is needed in some. Atypical HUS carries a higher risk of persistent renal damage, so a complement workup and early specialist review are important.

## Where the examiner tries to catch you

HUS questions reward the student who separates the infective trigger from the complement trigger, and who knows that TTP shares the haemolysis picture.

- **Trap:** giving eculizumab to every child with diarrhoea-associated HUS. — **Correct:** typical STEC-HUS is managed supportively, and complement blockade is reserved for atypical or refractory cases.
- **Trap:** giving an antibiotic for bloody diarrhoea in a child with suspected STEC infection. — **Correct:** antibiotics and antimotility agents may increase Shiga toxin release, so they are avoided.
- **Trap:** labelling every microangiopathy as HUS without testing ADAMTS13. — **Correct:** severe ADAMTS13 deficiency, usually below about 10 per cent, identifies TTP, which has more prominent neurological features and fever.

## Frequently asked questions

### Which organism produces Shiga toxin in typical haemolytic uraemic syndrome?

Shiga toxin-producing Escherichia coli, most often serotype O157:H7, causes typical haemolytic uraemic syndrome after bloody diarrhoea. Shigella dysenteriae type 1 also produces Shiga toxin and is an important cause of the same syndrome in India.

### Why are antibiotics avoided in STEC-associated haemolytic uraemic syndrome?

Antibiotics and antimotility agents are avoided in suspected STEC infection because they may increase the release of Shiga toxin. Supportive care, fluid management and dialysis when needed form the core treatment for typical haemolytic uraemic syndrome.

### What is the role of eculizumab in haemolytic uraemic syndrome?

Eculizumab is a monoclonal antibody against complement component C5, used in atypical haemolytic uraemic syndrome driven by complement dysregulation. It is not standard for typical Shiga toxin-associated HUS, where supportive care is the main treatment.

### How does thrombotic thrombocytopenic purpura differ from haemolytic uraemic syndrome?

Thrombotic thrombocytopenic purpura is distinguished by severe deficiency of ADAMTS13, usually below about 10 per cent, and by prominent neurological features with fever. Haemolytic uraemic syndrome is dominated by acute kidney injury, and ADAMTS13 is normal in typical and most atypical forms.

### What triggers atypical haemolytic uraemic syndrome?

Atypical haemolytic uraemic syndrome is caused by dysregulation of the alternative complement pathway, often through genetic variants in complement regulators or anti-factor H antibodies. Infection, pregnancy and certain drugs can act as triggers in susceptible patients.