Chronic Spontaneous Urticaria with Biochemical C1 Inhibitor Deficiency: A Case Report of Suspected Overlap with Hereditary Angioedema.

RATIONALE: Chronic spontaneous urticaria (CSU) and hereditary angioedema caused by C1 inhibitor (C1-INH) deficiency are mechanistically distinct disorders. When persistent wheals coexist with recurrent throat or abdominal symptoms, however, assigning individual episodes to mast-cell- or bradykinin-mediated pathways can be difficult. PATIENT CONCERNS: A 36-year-old Chinese woman developed recurrent wheals and pruritus in 2020. Beginning in 2024, she also reported episodic throat tightness and intermittent abdominal pain despite high-dose H1 antihistamines, systemic corticosteroids, and omalizumab. DIAGNOSES: Repeated complement testing in 2025 showed low C4, reduced C1-INH concentration, and impaired C1-INH function. Her mother had similar biochemical abnormalities. These findings supported biochemical C1-INH deficiency in a patient with active CSU, but the mechanism of individual throat and abdominal episodes remained uncertain. Whole-exome sequencing, including exome-based copy-number analysis, identified no reportable pathogenic or likely pathogenic variant in the prioritized angioedema-related genes; however, not all currently recognized HAE-associated genes were included, and gene-level coverage metrics were unavailable. INTERVENTIONS: Documented treatment included H1 antihistamines, systemic corticosteroids, cyclosporine, and omalizumab. Subcutaneous icatibant and lanadelumab were also administered, but the retrospective records did not permit reliable assessment of icatibant response, and lanadelumab exposure was too brief to assess prophylactic efficacy. OUTCOMES: Following methylprednisolone, cyclosporine, and reintroduction of omalizumab, symptoms remitted briefly but recurred the day after discharge. At follow-up in February 2026, the patient reported fewer episodes, a lower wheal burden, less pruritus, and milder throat tightness and chest pain while receiving omalizumab plus an H1 antihistamine. LESSONS: In patients with CSU, recurrent throat tightness or abdominal pain should prompt complement testing and episode-level assessment. Objective airway findings, abdominal evaluation, attack timing, complement results, and response to on-demand therapy should be documented before symptoms are attributed to a specific mechanism.