Notes:
Acquired C1 inhibitor deficiency can occur rarely in SLE and B cell lymphomas. Acquired C1 inhibitor deficiency can be distinguished from the hereditary form by measuring C1 q levels which are low in the acquired form but normal in the inherited.
Patients who present with angioedema may have C1 inhibitor deficiency. The angioedema associated with this condition is not associated with uticaria. This clinical feature helps to distinguish it from the very much more common allergic or idiopathic angioedema.
Hereditary angioedema is a rare disorder resulting from an autosomal dominant deficiency of C1 inhibitor. This deficiency produces an uncontrolled activation of the early components of the classical complement system, with generation of a kinin-like substance that causes recurrent angioedema of the gastrointestinal and genitourinary tracts and the larynx. The subcutaneous and submucosal oedema is usually harmless, but when the larynx is involved, death may occur due to respiratory obstruction and asphyxiation.
Depressed levels of total haemolytic complement activity, C4 and C2 may be manifestations of an underlying deficiency of C1 inhibitor. C2 is low during acute attacks.
Clinical attacks of oedema are accompanied by the appearance of free active C1 in the plasma and by falls in the levels of C4 (which are subnormal even during asymptomatic intervals) to undetectable. Diagnosis is most simply and reliably made by the C4 level, a normal value virtually excludes the diagnosis. Some people synthesise an abnormal C1 inhibitor protein that is reactive in immunoassays, but devoid of the ability to inhibit the C1 enzyme, so that direct immunoassays for C1 inhibitor may not detect all cases.