Chapter 7: Idiopathic membranous nephropathy

BACKGROUND The diagnosis of MN is made on kidney biopsy. Diagnostic features include capillary wall thickening, normal cellularity, IgG and C3 along capillary walls on immunofluorescence, and subepithelial deposits on electron microscopy. MN is often seen in association with an underlying disorder (secondary MN). Secondary MN is more common in children (75%) than adults (25%) (Table 12). The diagnosis of IMN is made by exclusion of secondary causes, using history, physical exam, and apppropriate laboratory tests (e.g., serology, imaging) and by careful examination of the kidney biopsy by light, immunofluorescence, and electron microscopy. In IMN, deposition of the IgG4 subclass of IgG is dominant, whereas other IgG subclasses dominate in secondary forms of MN. Distinguishing secondary MN from IMN is very important, since the therapy in the former must be directed at the underlying cause and some of the treatments for IMN are potentially toxic both to the patient and the kidney.