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Phenotypic diversity within the β-thalassaemia syndromes has traditionally received considerable interest, with several molecular and environmental modifiers of disease severity so far described.1 Patients with transfusion-dependent β-thalassaemia major (TM) suffer the most severe form and show the highest mortality rates.1 Nevertheless, the introduction of safe transfusion practices and effective iron chelation therapy continues to improve patient survival, allowing for several clinical complications to have time to manifest.2 The term β-thalassaemia intermedia (TI) was first suggested to describe patients who have milder anaemia compared with patients with TM and who usually present to medical attention later in childhood and remain largely transfusion-independent. However, recent evidence suggests that the diagnosis of TI carries higher morbidity than previously recognised, especially in the transfusion-independent patient, where the mechanism of disease remains largely unbalanced.3
Three main factors highlight the pathophysiology of TI: ineffective erythropoiesis, chronic anaemia/haemolysis, and iron overload secondary to increased intestinal absorption.3 The extreme diversity in phenotypic expression within the diagnosis of TI itself led to a wide variation in observed clinical complications and management practices.3,4 Among the medical complications of TI that were found to occur at high rates, even more frequently than in patients with TM, are thromboembolic events (TEE).5 Here, we review current evidence on TEE in thalassaemia patients, with special emphasis on TI.
Hypercoagulability in patients with thalassaemia has been attributed to several factors (see Figure 1).5,6 It is often a combination of these factors that leads to TEE.
It is widely accepted that patients with thalassaemia have chronically activated platelets and enhanced platelet aggregation,7 as confirmed by the increased expression of CD62P (P-selectin) and CD63, markers of in vivo platelet activation.8,9 Splenectomised thalassaemia patients have high platelet counts,10,11 but with a shorter lifespan due to enhanced consumption.12 It has also been shown that splenectomised TM and non-splenectomised TI patients have four to 10 times higher levels of metabolites of prostacyclin (PGI2) and thromboxane A2, both markers of haemostatic activity, than controls. However, no significant difference was found between TM and TI patients.13