4.1. ATN (so-called acute tubulonecrosis)

Definition: Morphological lesions of the tubulo-interstitial space accompanied by acute renal failure.

ATN (acute tubular necrosis) is a misnomer. The term is used to describe a wide variety of morphological lesions with predominant (if any) lesions in the tubular interstitial space without overt cellular infiltrates which have one feature in common i.e. acute renal failure. Since necrotic tubular cells (with the exception of tubulonecrotic ATN) are rarely found by light microscopy the term “acute tubular interstitial damage (ATID)” is in most cases more appropriate. The term ATN is however retained because of its uniform use.

The morphology is highly variable. This is due to several factors: etiology of ATN, duration of the insult, single, recurrent or persistent events and finally the time point of investigation at autopsy or by biopsy.

Early stages are characterized by tubular dilatation, loss of brush border etc., edema eventually (single) cell necroses, late stages show interstitial fibrosis (about 3 weeks after onset), interstitial cellular infiltrates (mainly lymphocytes and monocytes/histiocytes) as well as repair mechanisms (mitoses). All these factors make the interpretation in biopsies difficult.

Many drugs cause ATID accompanied by more or less prominent additional characteristics, ATID is in the following subdivided according to the accompanying lesions which may give a hint to the etiololgy e.g. vacuolar changes (hydropic change,  “osmotic nephrosis”), calcification, oxalosis, crystals or overt necrosis. Since drugs may cause mild moderate or severe lesions, they are described under the heading which is most characteristic.