Haemolytic uraemic syndrome

Published on 23/06/2015 by admin

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16.5 Haemolytic uraemic syndrome

Karen McCarthy

Essentials

1 Diagnosis is based on the presence of microangiopathic haemolytic anaemia, thrombocytopenia and renal impairment.
2 Diarrhoea-associated haemolytic uraemic syndrome accounts for 90% of cases.
3 The cornerstone of treatment is careful attention to fluid and electrolyte balance.
4 Most patients with haemolytic uraemic syndrome have some degree of encephalopathy.
5 Hypertension is present in up to 50%.
6 Long-term follow up is necessary.
7 Use of antibiotics, antimotility agents and non-steroidal anti-inflammatory drugs should be avoided in children acutely infected with Shigatoxin Escherichia coli.

Introduction

Definition

Haemolytic uraemic syndrome (HUS) is the commonest cause of acute renal failure in infants and young children <5 years of age in developed countries. There is multisystem involvement in this syndrome, which is characterised by sudden onset of non-immune microangiopathic haemolytic anaemia, thrombocytopenia and acute renal failure. Various degrees of severity occur. One-third of patients are anuric at presentation.

An inherited form, which is transmitted in an autosomal dominant manner, may present in the newborn period.

Epidemiology

The commonest form of HUS in children is the one that occurs following a prodromal illness of acute gastroenteritis with bloody diarrhoea. This diarrhoea-associated haemolytic uraemic syndrome (D+HUS) accounts for 90% of cases. D+HUS is most often caused by Shiga toxin (verotoxin) producing Escherichia coli (STEC, also called VTEC). Other cytotoxin-producing bacteria such as Shigella dysenteriae type 1, Salmonella typhi and Campylobacter are less common causes of HUS. Streptococcus pneumoniae also causes HUS but does so via a completely different mechanism, which involves neuraminidase rather than cytotoxin production.

D+HUS occurs in epidemics as well as sporadically. Outbreaks can be traced to contaminated food, especially undercooked hamburger and contaminated water.

There are over 100 different serotypes of STEC with different phage typing and subtyping. STEC O157:H7 is the commonest subtype producing disease in North America, the British Isles and Japan. It is rare as a causative agent of HUS in Australia, where other types, including O111:H, are pathogenic. E. coli can be spread by person to person contact. The incubation period to onset of diarrhoea is 1–8 days.

Non-diarrhoeal-associated HUS (D– HUS) cases account for 10% of cases. These are secondary to:

• non-enteric infections, e.g. streptococcal pneumonia;
• immunosuppression: malignancy-associated, following bone-marrow transplant, and drug-related, e.g. mitomycin, ciclosporin and FK-506 (tacrolimus);
• immunodeficiency and pregnancy may also be associated with HUS.

Pathophysiology

The primary process in the pathogenesis of HUS is vascular endothelial cell injury. E. coli is ingested and colonises the gut. These bacteria attach to the epithelium of the distal ileum and large intestine by complex mechanisms. Cytotoxins are produced and they damage the microvasculature of the intestinal wall, causing haemorrhagic and ulcerative intestinal lesions. Once the intestinal–blood barrier has been compromised, the toxins gain entry to the circulation. It is presumed that the toxin then initiates microvascular damage in the glomerular endothelium of the kidneys. Intravascular and intraglomerular fibrin clot is formed and at the same time there is activation of the coagulation system, which leads to occlusion of the vessels and glomerular loss. Erythrocytes passing through these vessels become damaged. Microvascular endothelial cell injury is central to the renal thrombotic microangiopathy that is characteristic of HUS. Thrombocytopenia results from intrarenal platelet adhesion and platelet survival is shortened. Renal dysfunction results from this process.

History

A child who has previously been healthy presents with a history of gastroenteritis within the previous 2 weeks. There is subsequent development of vomiting, bloody diarrhoea and crampy abdominal pain. The child is usually afebrile at presentation. The presentation may resemble an acute surgical abdomen. The child then becomes pale and develops haematuria and oliguria and is noted to become lethargic. Not uncommonly, the patient appears to be improving from his/her gastroenteritis but then deteriorates suddenly, becoming quite ill.

Examination

The child appears pale, lethargic and dehydrated. Petechiae may be noted on the skin, which may also appear jaundiced.

Most patients have some degree of encephalopathy. Central nervous system involvement can range from irritability and restlessness to seizures or coma. Seizures can be present in up to 40%. Hemiparesis and cranial nerve dysfunction may be seen.

Hypertension is present in up to 50% and is likely secondary to increased renin levels. Evidence of cardiac involvement may present as myocarditis, cardiomyopathy or high-output failure. Oliguria occurs often. Hepatomegaly and oedema develop (reflecting fluid overload) if the oliguria is not recognised and fluid intake is not restricted. Splenomegaly is not as common as hepatomegaly.

Investigations

Diagnosis of HUS is based on the presence of microangiopathic haemolytic anaemia, thrombocytopenia and renal failure.

• Anaemia is usually severe and is normochromic normocytic in type with a haemoglobin level of 50–90 g L–1.
• White cell count is non-specific and often raised with increased number of immature forms.
• Platelet count is low in 95% and can be as low as 20 × 109 L–1. Subsequent increase in platelet count may be the first indication of resolution of the microangiopathic process.
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