[Diagnostic pathway of an unusual case of nephrotic syndrome: immunotactoid glomerulopathy]

Immunotactoid glomerulopathy is a clinicopathological entity characterized by extracellular deposition of microtubular substructures, which are negative for the usual staining that identifies amyloid within the mesangium and capillary walls of renal glomeruli. Despite ongoing debate in the nephrological community, it is kept distinct from fibrillary glomerulonephritis on the basis of the size and arrangement of the microtubules and microfibrils. It is clinically characterized by the presence of glomerular proteinuria in the nephrotic range, microscopic hematuria and hypertension, and is often associated with hypocomplementemia, monoclonal gammopathy, and lymphoprolipherative disorders. A 47-year-old woman was referred to our unit for evaluation of proteinuria associated with nephrotic syndrome. Laboratory findings revealed a serum M component and hypocomplementemia. Renal biopsy yielded three fragments for optical microscopy, immunofluorescence, and electron microscopy; Congo red staining was used. Renal histology showed a morphological pattern of membranoproliferative glomerulonephritis. Immunofluorescence showed IgG deposits with monoclonal kappa light chain restriction as well as C3 and C1q deposits. Electron microscopy revealed the presence within the mesangium of microtubules measuring >35 nm that were focally parallel oriented. The final diagnosis was nephrotic syndrome caused by immunotactoid glomerulopathy. The clinical diagnosis of immunotactoid glomerulopathy is based on pathological, clinical and hematological features and requires the exclusion of other diseases that are associated with organized glomerular deposits. We discuss the diagnostic options offered by the clinical and morphological elements of this case; the use of electron microscopy is emphasized, especially when a renal syndrome is associated with paraproteinemia.