[Clinical pathology of the glomerulus--from phenomenon to entity. Focal sclerosis]

Three main types of global glomerular sclerosis (glomerular obsolescence) can be differentiated by light microscopy, immunohistochemistry and electron microscopy: the compressive type is characterized by an obliteration of the urinary space by type I collagen, the matrix type by a massive accumulation of mesangial matrix (sclerosis), and the insudative type is accompanied by marked subendothelial deposition of hyalin. Thus, different mechanisms may lead to glomerular obsolescence (scarring) even if mixed forms are frequently encountered. Segmental glomerular sclerosis represents a non-specific lesion observed in a variety of conditions. An increase in mesangial matrix and subendothelial deposition of hyalin material are the predominant histological features. The presence of segmental glomerular sclerosis, however, is of particular significance when occurring in the setting of an idiopathic nephrotic syndrome. The majority of patients (90%) are steroid-resistant and prone to develop renal failure. A rapidly progressive course is frequently observed when segmental lesions are superimposed to diffuse mesangial proliferation. The relationship between minimal change nephrotic syndrome and focal segmental glomerular sclerosis is still controversial. It is uncertain whether they represent two distinct, unrelated entities or whether they belong to the spectrum of a disease with focal segmental glomerular sclerosis being the more ominous end of the spectrum. The pathogenesis of segmental glomerulosclerosis is discussed.