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Pathology of Arteriosclerotic, Fibrodysplastic, and Developmental Renal Artery Occlusive Disease
Renal artery occlusive disease is the most common cause of surgically correctable hypertension in adults and in some instances contributes to deterioration in kidney function. It is a less common cause of secondary hypertension in pediatric patients. Pathologic processes causing renovascular hypertension include arterial fibrodysplasia, developmental hypoplasia, and arteriosclerosis. These entities are distinctly different, and discussion of them must be individualized.
Renal Artery Fibrodysplasia
Fibrodysplastic stenoses affecting the renal arteries occur in less than 0.5% of the general population. This uncommon disease is second only to atherosclerosis as the most common cause of surgically correctable hypertension. Dysplastic renal artery disease is usually categorized by the principal portion of the vessel most involved, being classified as intimal fibroplasia, medial fibroplasia, or perimedial dysplasia. These renal artery lesions warrant individual consideration because they represent distinct pathologic entities.
Intimal Fibroplasia
Intimal fibroplasia accounts for approximately 5% of renal artery dysplastic stenoses and occurs with equal incidence in both sexes. It is more likely to affect infants, adolescents, and young adults than older patients.
Primary intimal fibroplasia most often involves the main renal artery as a smooth focal stenosis ( Figure l A). Segmental vessel intimal fibroplasia is uncommon but usually occurs as a weblike lesion. Irregularly arranged intimal mesenchymal cells surrounded by a loose matrix of connective tissue that produce circumferential narrowings of the vessel lumen typify primary intimal fibroplasia ( Figure l B). The internal elastic lamina may be disrupted, but it is always present in these lesions. The cause of primary intimal fibroplasia is unknown. It is possible that this disease represents a proliferative remnant of fetal arterial musculoelastic cushions, similar to the intimal cushions that occur at cerebral artery bifurcations in adults. Lipid-containing foam cells and inflammatory cells are not present in this disease. Medial and adventitial tissues are usually normal in these vessels.
Secondary intimal fibroplasia often has the same morphologic appearance as a primary intimal lesion. Certain secondary lesions accompany developmental ostial narrowings or advanced medial dysplasia, probably as a consequence of flow disturbances along the surface of these stenoses. Blunt vascular trauma or intraluminal injuries can contribute to other secondary intimal lesions. Long tubular intimal stenoses can occur as a consequence of recanalization of a previously thrombosed artery. In this regard, certain cases of intimal fibroplasia have been suggested to represent a resolved arteritis, such as might occur with gestational rubella.
Progression of intimal fibroplasia appears a likely consequence of abnormal surface blood flow, even if the original initiating etiologic factors have resolved. Intimal stenoses in the renal circulation appear to progress more rapidly than do medial fibroplastic stenoses.
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