Platelet disorders

Published on 23/06/2015 by admin

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Last modified 23/06/2015

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11.4 Platelet disorders

Jane Cocks

Essentials

1 Abnormal bleeding can occur from disorders of total numbers of platelets or platelet function.
2 The normal platelet count is between 150 and 400 × 109 L−1.
3 Idiopathic thrombocytopenic purpura (ITP) is the commonest platelet disorder in childhood.

Introduction

The normal platelet count is between 150 and 400 × 109 L–1 and platelets usually exist within the circulation for 5–7 days. In normal haemostasis, blood loss is initially limited by the formation of a platelet plug and cross-linked fibrin at the site of injury. In platelet disorders this platelet plug is not formed or is ineffective, and abnormal bleeding occurs. Platelet abnormalities can be either quantitative or qualitative. Small petechial skin haemorrhages, less than 3 mm in diameter, are characteristic of platelet defects.

The commonest platelet disorder is thrombocytopenia (low platelet count). Significant thrombocytopenia has a characteristic bleeding profile of spontaneous bruising, petechiae, epistaxis and mucosal bleeding. Bleeding complications usually only occur at levels of less than 50 × 109 L–1, with spontaneous bleeding possible at levels below 10 × 109 L–1. As most platelet counts are performed by electronic particle counters, an inappropriately low platelet count can result from the spontaneous platelet clumping that can occur in EDTA collection tubes. This is confirmed by direct inspection of the peripheral blood smear for platelet clumping.

In qualitative disorders of platelet function the abnormal bleeding occurs despite a normal platelet count. Platelet dysfunction may occur in hepatic failure, chronic renal failure, myeloproliferative disorders, with some drugs (e.g. aspirin) and in rare inherited disorders of platelet function (Glanzmann, Portsmouth, Hermansky–Pudlak, May–Hegglin and Bernard–Soulier syndromes).

Thrombocytosis (elevated platelet count) is seen in inflammatory reactions including Kawasaki disease, patients with malignancy and polycythaemia rubra vera. Thrombocytosis of itself is rarely of clinical importance but platelet levels greater than 1000 × 109 L–1 can be associated with acute thrombosis or haemorrhage.

Idiopathic thrombocytopenic purpura

Introduction

Idiopathic thrombocytopenic purpura (ITP) is the commonest platelet disorder in children. It is an autoimmune disorder characterised by the development of platelet autoantibodies leading to decreased platelet survival. The incidence of ITP peaks in winter and spring due to the increased incidence of viral infections at those times. ITP in children is usually of the acute form, which lasts for less than 6 months, with a high rate of spontaneous resolution. ITP in children is most common between the ages of 2 and 6 years, but can occur in any age group. Up to 28% of children with ITP will develop a chronic form which lasts longer than 6 months.

Clinical presentation

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