ABSTRACT
CI inhibitor deficiency causes angioedema as a result of excessive bradykinin production (see Chap. 3). Thus the pathways and control mechanisms for bradykinin formation and degradation are variables that one must consider in any patient with angioedema. Activated factor XII as well as factor XIIf are inhibited by Cl INH (1,2). Thus absence ofCI INH facilitates factor XII autoactivation, which augments the ability of factor XlIa to convert both prekallikrein to kallikrein and factor XI to factor Xla. rt is estimated that over 90% of plasma inhibition of factor XfIa and factor XlIfis due to Cl INH (2). The remainder is due to antithrombin III and the protein C, inhibitor The next enzyme in the cascade is kallikrein and it is inhibited by CI INH and O'~ macroglobulin in approximately equal proportions (3). Minor inhibitors of kallikrein are antithrombin In and 0'1 antitrypsin. Thus if there is any stimulus for activation of the plasma bradykinin-forming cascade, in the absence of functional C I INH, there is a marked augmentation of bradykinin formation with angioedema as the result. Urticaria is not seen in patients with CI INH deficiency, but at the inception of an episode of swelling they may have a rash resembling erythema marginatum. When patients present with both chronic urticaria and angioedema, assays for C I INH are generally unnecessary and the utility of a C4 determination depends whether an underlying vasculitis is suspected.
