Rapidly Progressive GlomerulonephritisDue to Hypocomplementemic Urticarial Vasculitis Syndrome (HUVS) Masquerading as ANCA-Associated Vasculitis: A Rare Presentation

Introduction: Hypocomplementemic urticarial vasculitis syndrome (HUVS) is an immune-complex mediated small-vessel vasculitis with low complement levels. Due to the rarity of HUVS, the existing literature on renal involvement is limited. We present a case of an 87-year-old male who developed rapidly progressive glomerulonephritis (RPGN) due to HUVS. Case Description: An 87-year-old female was referred to the nephrology clinic with a creatinine surge to 3.5 mg/dL from 0.9 mg/dL a year prior. Urinalysis showed proteinuria and hematuria, with subsequent urine albumin-creatinine ratio of 2g/g and an unremarkable renal ultrasound. Immunological studies indicated normal C3, low C4, elevated p-ANCA and elevated MPO antibody. Concerns for RPGN in the context of ANCA-associated vasculitis (AAV) prompted a renal biopsy, revealing focal crescentic glomerulonephritis with mesangial C3-dominant immune complex deposition. A detailed history revealed episodes of burning facial rash lasing recurring over the past two years alongside arthralgias, weight loss, eye redness and floaters. Physical examination revealed periorbital edema and pitting edema to the ankles. Repeat lab assessment again showed elevated anti-MPO, now with both low C3 and C4 levels. Despite lab findings consistent with microscopic polyangiitis (MPA), the history of urticarial rash, periorbital edema, normal CRP, low complement levels, and C3-predominant immune complex deposition in the mesangium raised suspicion for HUVS. Subsequent testing confirmed low C1Q levels, supporting the diagnosis. Induction therapy was started with cyclophosphamide, rituximab, and steroids, followed with avacopan. She has demonstrated ongoing improvement in creatinine, most recently measuring 2.17 mg/dL. Discussion: RPGN is characterized by a rapid decline in kidney function and although its etiologies are broad, 40-50% of cases in adults are deemed to be secondary to AAV. However, in our patient, renal biopsy findings contradicted the typical pauci-immune pattern, instead revealing immune complex deposits in the mesangium. This, in addition to the angioedema and low complements, raised suspicion for HUVS, as confirmed using the Schwartz criteria. This case highlights the need for HUVS as consideration of RPGN differentials and emphasizes the necessity for further research on optimal management strategies for this rare condition.