Diagnostic Approach for Recurrent Angioedema with Steroid Response

Initial Screening and Diagnosis

The American Academy of Allergy, Asthma, and Immunology recommends measuring both C4 level and C1 esterase inhibitor (antigenic and functional levels) to diagnose C1 inhibitor deficiency in patients with recurrent angioedema 1, 2
C4 level is the best initial screening test for C1 inhibitor deficiency, as at least 95% of patients with C1INH deficiency will have a reduced C4 level even between attacks, increasing to virtually 100% during active angioedema episodes 1, 2
A normal C4 level during an attack of angioedema strongly suggests an alternative diagnosis rather than C1INH deficiency 1, 3

After confirming low C4, measuring both C1INH antigenic level AND C1INH functional level can distinguish between types of C1 inhibitor deficiency, with a low antigenic level confirming Type I hereditary angioedema (HAE) and a normal or elevated antigenic level with low functional level confirming Type II HAE 1, 3, 4, 5
If C1INH deficiency is confirmed and the patient is over 40 years old or has late-onset symptoms, measuring C1q level can distinguish acquired from hereditary forms, as C1q is normal in hereditary angioedema but low in most cases of acquired C1INH deficiency 3, 4

Angioedema caused by C1 inhibitor deficiency does not reliably respond to corticosteroids, antihistamines, or epinephrine, and the fact that a patient's swelling improved with steroids suggests the possibility of mast cell-mediated angioedema or other histaminergic causes rather than C1INH deficiency 1, 4
If C4 and C1INH levels are normal, consider HAE with normal C1 inhibitor (HAE-nC1INH), which requires exclusion diagnosis based on family history and lack of response to antihistamines, and targeted gene sequencing for known HAE pathogenic variants may be helpful if available 1, 4, 5