Acute kidney injury with interstitial hemorrhage in renal heavy chain amyloidosis: a clinicopathologic case report.

Heavy chain (AH) amyloidosis is a rare subtype of immunoglobulin-derived amyloidosis, comprising less than 1% of all systemic cases. While kidney involvement is frequently observed in systemic amyloidosis, presentations dominated by acute kidney injury (AKI) with glomerular bleeding and interstitial hemorrhage are uncommon. We report the case of a Japanese man in his 50s who developed AKI requiring dialysis. He had a history of proteinuria noted during previous health checkups but no known underlying diseases. Renal biopsy revealed amorphous mesangial and capillary wall deposits with weak periodic acid-Schiff and negative periodic acid-methenamine silver staining, and positivity for Congo red and direct fast scarlet. Electron microscopy showed non-branching fibrils, and laser microdissection followed by liquid chromatography-tandem mass spectrometry confirmed a diagnosis of AH amyloidosis. Of note, the biopsy also revealed numerous intratubular red blood cells and patchy interstitial hemorrhage in the absence of crescents or necrotizing lesions. The patient was initially treated with corticosteroids under the presumption of rapidly progressive glomerulonephritis. After histologic diagnosis, clone-directed daratumumab-based chemotherapy was introduced. Renal function gradually recovered, and dialysis was discontinued within 3 weeks of admission. This case highlights a rare but important phenotype of AH amyloidosis characterized by disruption of the glomerular capillaries and interstitial hemorrhage leading to reversible AKI. Early biopsy and appropriate therapeutic intervention may allow renal function to recover, even in severe presentations.