Introduction

Sickle cell disease (SCD) and thalassaemia are both inherited disorders of haemoglobin (Hb) associated with considerable morbidity and mortality. Patients with both conditions can present to critical care in a number of different ways. These fall into three distinct categories:

1. An acute complication of their underlying haemoglobinopathy requiring organ support
   
2. An unplanned admission for an illness unrelated to their underlying haemoglobinopathy
   
3. A planned admission for post-operative care

This chapter will describe the acute and chronic complications of both conditions that may result in admission or complicate an admission to the critical care unit. The peri-operative care of patients with SCD will also be discussed.

Sickle cell disease

Sickle cell disease is the most common inherited disorder worldwide, and there are approximately 12–15,000 affected individuals in the UK. It is an autosomal recessive condition, and patients with homozygous SCD (HbSS) have a severe disease phenotype. The carrier form (sickle trait, HbAS) is in essence an asymptomatic condition. The sickle Hb gene can be also inherited with HbC, β-thalassaemia, HbO-Arab, HbD or HbE, causing the compound heterozygote conditions HbSC, HbS β-thalassaemia and so forth. These tend to be associated with a more moderate phenotype.

The abnormal haemoglobin S (HbS) arises from a point mutation in the β-globin gene. HbS has decreased solubility and increased viscosity compared to HbA, with a tendency to form polymers, which is increased by hypoxia. The formation of Hb polymers irreversibly distorts the cell, disrupting its membrane and producing the classical sickle cell shape on a blood film. These distorted cells break down more rapidly, leading to anaemia. They can also become trapped in the microvasculature and cause downstream ischaemia.

Diagnosis of SCD

In the majority of situations, patients will be aware of their diagnosis. Occasionally, the first presentation will be with an acute complication requiring critical care, or a patient may be admitted acutely unwell and unable to inform the medical team of their diagnosis. A suspicion of SCD may arise because of the clinical picture, and patients will usually have an Hb of 60–90 g/dL, although this can be higher in HbSC disease. There is a reticulocytosis, although the reticulocyte count can be low in some situations such as aplastic crisis due to parvovirus B19 infection. The blood film shows marked variation in red cell size with sickle cells and target cells; basophilic stippling is frequently seen in conjunction with Howell–Jolly bodies. A positive sickle solubility test indicates the presence of HbS but cannot differentiate between sickle cell trait and homozygous disease. However, the combination of positive solubility test and a consistent blood film allows a rapid diagnosis to be made. The diagnosis should be confirmed, promptly, with a definitive laboratory test such as high-performance liquid chromatography.