Resistance to activated protein C--frequent etiologic factor for venous thrombosis.

The discovery of the resistance to activated protein C (APCR) has provoked a new insight into the etiopathogenesis of venous and arterial thrombosis. APCR is determined in 95% genetically by point mutation in the gene for factor V resulting in substitution of arginine in the position 506 by glutamine. This change makes the activated form of factor V (factor Va) resistant to the cleavage by protein C in the place, where the cleavage takes place most quickly under normal conditions. The mutant factor V is known as factor V Leiden. Factor V Leiden preserves its procoagulation activity for a longer period, resulting thus into thrombophilia with all its negative consequences. The inherited deficiencies of antithrombin III, protein C and protein S occur in 10% of patients suffering from venous thrombosis, whereas factor V Leiden is present in as many as 20 to 60%. Thus, it seems that factor V Leiden is the most important inherited risk factor of venous thrombosis. The results of several trials did not indicate the participation of APCR in the development of myocardial infarction. On the other hand, APCR seems to be a risk factor of cerebrovascular accidents, especially of stroke and transitory brain ischemia. Factor V Leiden is an important risk factor of abortions, especially those occurring in the second trimester of pregnancy. According to recent results, factor V Leiden is considered to play a role in the pathogenesis of venous and arterial thromboses in children. The significant risk potential of factor V Leiden with respect to venous thrombosis development and relatively simple diagnosis of this mutation predispose the investigation of this disorder to become the screening method in indicated groups of patients. The investigation of APCR is recommended in all patients with either first or reoccurring attacks of venous thrombosis or thromboembolism, in patients with positive family history of thrombosis and thromboembolism and in women with repeated abortions, particularly in the second trimester of pregnancy. The investigation of APCR in selected groups of patients and early prophylactic anticoagulation therapy may be important in thrombosis prevention in situations with an increased thrombotic potential. (Tab. 1, Ref. 78.)