Haematology Vignettes – 11

When is ITP not ITP?

Idiopathic thrombocytopenic purpura (ITP) is a disorder characterised by the destruction of antibody-sensitised platelets by macrophages, most notably in the spleen. It is primarily a diagnosis of exclusion as it is made after conditions associated with secondary thrombocytopenia have been excluded. There are 2 forms of ITP, acute and chronic. Acute ITP usually occurs in children between the age of 2 and 6 years. It is often preceded by vaccination or by a viral infection. Chronic ITP usually occurs in children more than 10 years of age, as well as in adults.

ITP is caused by autoantibodies interacting with platelet membrane glycoproteins and resulting in accelerated destruction of platelets. As these autoantibodies diminish in strength and finally disappear, the platelet count returns to normal. This process takes up to 6 months in children with acute ITP and takes even longer in children with chronic ITP.

Acute ITP is characterised by platelet counts less than 20 x 109/L and often less than 10 x 109/L. The clinical presentation is bruising with a petechial rash in an otherwise healthy child. Acute ITP affects males and females equally. Chronic ITP is characterised by a somewhat higher platelet count, less than 150 x 109/L and occurs predominantly in females by a 2:1 ratio.

A full blood count, including platelet count, should be performed on all children suspected of having ITP. Features inconsistent with ITP should prompt further investigations. A bone marrow biopsy reveals normal or increased numbers of megakaryocytes in both acute and chronic ITP.

Acute ITP is a self-limiting disorder requiring minimal or no therapy in the majority of cases. Corticosteroids or Intragam (IgG) may be given at the discretion of the clinician. Chronic ITP is also treated with corticosteroids or Intragam and may, in some cases, ultimately require splenectomy.

An 8-year-old boy presented at the Paediatric Casualty Department with multiple bruises and mucocutaneous bleeding. He is said to have had a history of ITP. A full blood count was performed with the following results:

Hb 125 g/L, WCC 5.9 x 109/L and platelet count 52 x 109/L.

Examination of the blood film revealed the presence of large and giant platelets. The low platelet count and abnormal platelet morphology prompted further investigation including platelet aggregation studies. The result of these studies revealed normal platelet aggregation with ADP, collagen, arachidonic acid and adrenaline and absent platelet aggregation with ristocetin. Monoclonal antibody typing of platelet glycoprotein analysed by flow cytometry revealed a reduction in the number of Transient erythroblastopenia of childhood GpIb/IX complexes. These results are consistent with the disorder known as Bernard Soulier Syndrome (BSS).

Bernard Soulier Syndrome

Peripheral blood film showing giant platelets

Peripheral blood film showing giant platelets

BSS is a rare autosomal recessive disorder characterised by a moderate thrombocytopenia. The platelets are giant sized and morphologically abnormal. Platelet aggregation is normal with ADP, collagen and adrenaline. There is a delayed response with thrombin and absent platelet aggregation with human VWF and ristocetin. The platelet membrane shows deficiency in the GpIb/IX complex when analysed by flow cytometry.

Patients with BSS usually present early in life. Clinically, they present with bruising, bleeding from mucous membranes and epistaxis. Adult females suffer from menorrhagia. The platelet count is only moderately reduced with total counts between 50 and 100 x 109/L. The bleeding time is prolonged.

Forms of treatment include hormonal management in females, administration of DDAVP (1-deamino-8-D-arginine vasopressin) to increase the level of circulating VWF and the administration of activated factor VIIa in emergency situations. Platelet transfusions have proved unsuccessful as alloantibodies to components of the GpIb/IX complex often lead to refractive platelet therapy.

This case study demonstrates the importance of assessing platelet numbers and morphology as well as performing other relevant tests prior to making a diagnosis of ITP.

When is ITP not ITP!