Unique Presentation of AKI with Polyuria as a Manifestation of Renal Amyloid

Introduction: The presentation of renal amyloidosis varies based on the type of amyloid and the location and quantity of deposits. Histologically, AA amyloidosis is typically localized to the glomeruli. The most common clinical manifestations, hence, include progressive renal dysfunction and proteinuria. This case highlights a rare manifestation of AA amyloidosis presenting as acute kidney injury (AKI) and polyuria. Case Description: Nephrology was consulted for AKI (creatinine 2.2 from baseline 1.1) and polyuria (6 liters) in a 51-year-old male admitted for recurrence of coagulase-negative staphylococcus bacteremia, now with tricuspid-valve vegetation, currently treated with daptomycin. Urine studies indicated increased free water excretion (urine osmolality 207) alongside glucosuria and nephrotic proteinuria. Notably, serum glucose was 70 mg/dL, and albumin was 0.6 mg/dL. A 24-hour urine study revealed 42 grams of protein, 32.6 mEq/L of sodium, and 23.2 mEq/L of potassium. Serologic workup and complement levels were normal. Kidney biopsy (Fig. 1) demonstrated marked amyloid deposition. Immunofluorescence for immunoglobulins and light chains was negative. Electron-microscopy (Fig. 2) revealed amyloid deposits in the glomerulus with foot process effacement, mesangium, and interstitium involving tubular basement membranes, peritubular capillaries, and blood vessels. The diagnosis of AA amyloidosis related to persistent bacteremia was confirmed by mass spectrometry. Discussion: This case illustrates the full spectrum of renal involvement and manifestations of amyloidosis, as well as the interplay of its various components. The nephrotic-range proteinuria and renal dysfunction indicate glomerular involvement, which usually leads to significant peripheral edema. However, this was not observed in our patient, possibly due to the ongoing polyuria stemming from tubular dysfunction. The proximal tubular involvement explains glucosuria, while the distal tubular and collecting duct involvement accounts for water diuresis. More than half of individuals diagnosed with renal amyloidosis eventually require renal replacement therapy, which reflects the unfortunate prognosis of our patient.