In type IV atherosclerotic lesions a dense accumulation of extracellular lipid occupies an extensive but well-defined region of the intima. This type of extracellular lipid accumulation is known as the lipid core. A fibrous tissue increase is not a feature, and complications such as defects of the lesion surface and thrombosis are not present. The type IV lesion is also known as atheroma. Type IV is the first lesion considered advanced in this classification because of the severe intimal disorganization caused by the lipid core. The characteristic core appears to develop from an increase and the consequent confluence of the small isolated pools of extracellular lipid that characterize type III lesions. The increase in lipid is believed to result from continued insudation from the plasma. Type IV lesions, when they first appear in younger people, are found in the same locations as adaptive intimal thickenings of the eccentric type. Thus, atheroma is, at least initially, an eccentric lesion. [from
HPO]