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 lA). 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 lB). 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.

Medial Fibroplasia

Medial fibroplasia accounts for nearly 85% of dysplastic renal artery stenotic lesions. Nearly all patients exhibiting typical medial fibroplasia are women. This disease is recognized most often during the 4th decade of life. Medial fibroplasia might represent a systemic arteriopathy, yet clinically overt arterial involvement is usually limited to the renal, extracranial internal carotid, and external iliac vessels.

Renal artery medial fibroplasia can occur as a solitary focal stenosis but is more common as a series of stenoses with intervening mural aneurysmal dilations. The thin-walled mural aneurysms have diameters exceeding that of the normal artery, and the stenoses appear as distinct webs projecting into the vessel lumen. Medial fibroplasia is most apt to affect the distal main renal artery (Figure 2A) and is bilateral in 55% of patients. In the presence of bilateral disease, the right-sided lesions usually appear more advanced. Unilateral disease affects the right side in 35% of patients and the left side in the remaining 10% of patients. These medial lesions extend into first-order segmental branches in approximately 25% of patients.

Renal artery medial fibroplasia is less progressive than intimal fibroplasia or perimedial dysplasia. At the very most, progression occurs in a third of patients with medial dysplastic lesions. Progression is more likely to be observed in premenopausal than menopausal women.

Two histologic forms of renal artery medial fibroplasia exist: a peripheral form that is manifested by disease limited to the outer media and a diffuse form that is manifested by disease throughout the entire media. The peripheral form of medial fibroplasia is more likely to be encountered in younger patients. It is possible that diffuse disease represents progression of the peripheral form.

The peripheral form of medial fibroplasia is characterized by the replacement of the smooth muscle of the outer media by compact fibrous connective tissue. Minimal accumulations of collagen and ground substances can affect the inner media of these vessels. Intimal tissues, the internal elastic lamina, and adventitial tissues are rarely affected. The diffuse form of medial fibroplasia is characterized by severe disorganization and loss of normal smooth muscle architecture, with excessive accumulations of fibrous tissue alternating with areas of marked medial thinning throughout the media (Figure 2B). Internal elastic lamina fragmentation and secondary intimal fibrosis may be present. Adventitial tissues usually appear normal in this form of renal artery disease.

True renal artery macroaneurysms affect a little more than 10% of medial fibrodysplastic renal arteries. Fragmentation of elastic tissue of first- and second-order branches is thought to lead to formation of aneurysms in these cases. Dissections affect less than 1% of renal arteries that exhibit medial fibrodysplasia. These dissections can extend into middle and outer medial structures as limited lesions, or they can progress as large intramural hematomas that encroach on the lumen.