HEREDITARY DISORDERS OF PLATELET-FUNCTION

The most common hereditary disorders of platelet function are those in which there is decreased platelet aggregation in response to more than one of the commonly used aggregating agents, collagen, ADP, adrenaline and arachidonic acid. When measured, there is usually also a reduction in the extent of the platelet release reaction and often also thromboxane production. In a proportion of these cases it is possible to demonstrate that there is a decrease in or absence of the contents of the dense storage granules which accounts for the decrease in platelet responsiveness. In most other cases the primary cause of the decreased responsiveness has not been determined, although in some cases deficiency of cyclooxygenase or thromboxane synthetase has been demonstrated. Investigation of patients with these disorders is often difficult because the tests involved are difficult to subject to adequate quality control, their sensitivity and specificity has not been adequately defined, and lack of reproducibility renders it difficult in less severe cases to be certain of abnormality even after repeat tests. Much less common but of great interest are the disorders in which the primary abnormality is in one of the glycoproteins on the platelet surface. In Glanzmann's thrombasthenia glycoprotein IIb.IIIa is absent or greatly decreased leading to failure of activated platelets to bind fibrinogen to their surface. In contrast to the decrease in aggregation seen in the above disorders, the platelets do not aggregate at all in response to the usual aggregating agents. In Bernard-Soulier syndrome there are severe deficiencies of three glycoproteins, particularly Ib, leading to inability to bind von Willebrand's factor and consequent inability of the platelets to aggregate in response to ristocetin. Study of the disorders of platelet function will continue to contribute to our ability to detect and treat these disorders and to our knowledge of platelet physiology and biochemistry.