A 5-year-old boy is being evaluated for recurrent epistaxis and other abnormal bleeding episodes, including excessive bleeding from the umbilical cord at bih. Laboratory studies reveal the following: decreased hemoglobin (with microcytic hypochromic red cell indices), normal platelet count, markedly prolonged prothrombin time (PT) and paial thromboplastin time (PTT), and unmeasurable thrombin time (TT). Platelet aggregation studies reveal a normal platelet response to ristocetin, but with other substances (including collagen, ADP, and epinephrine), this patient's platelets exhibit a primary wave defect. Based on these findings, this patient most likely has

Platelet aggregation refers to platelets binding to other platelets. One mechanism for this involves fibrinogen, which can act as a molecular bridge between adjacent platelets by binding to GpIIb and GpIIIa receptors on the surface of platelets. Abnormalities of platelet aggregation (aggregation defects or primary wave defects) include Glanzmann's thrombasthenia and afibrinogenemia. Patients with Glanzmann's thrombasthenia have a deficiency of GpIIb-IIIa and defective platelet aggregation. Patients with low or no fibrinogen levels characteristically have prolonged PT, PTT, and TT values: in fact, they are so prolonged they are unmeasurable. In contrast to platelet aggregation, platelet secretion refers to the secretion of the contents of two types of granules within the platelet cytoplasm. a granules contain fibrinogen, fibronectin, and platelet-derived growth factor, while dense bodies contain ADP, ionized calcium, histamine, epinephrine, and serotonin. Decreased platelet secretion (activation defects) is seen with deficiencies of these granules; these diseases are called storage pool defects. They can involve either a granules (gray platelet syndrome) or dense bodies (Chediak-Higashi syndrome, Wiskott-Aldrich syndrome, or TAR). Wiskott-Aldrich syndrome is an X-linked disorder that is characterized by eczema, thrombocytopenia (small platelets), and immunodeficiency consisting of decreased levels of IgM and progressive loss of T cell function.These patients have recurrent infections with bacteria, viruses, and fungi. TAR refers to the combination of thrombocytopenia and absent radii. Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition.