Aortic-iliac thrombosis in horses.

Vascular obstruction of the hindlimbs was diagnosed clinically in 15 horses, and was characterized at necropsy in eight of those horses. The condition was identified as an incidental post mortem finding in two additional horses. The principal clinical signs in affected horses were progressive exercise intolerance and hind-leg lameness. At post mortem the oldest lesions were located at the aortic quadrifurcation and in the distal portions of the femoral and internal iliac arteries, and consisted of partially or completely occlusive masses of well-organized and well-vascularized fibrous tissue, occasionally containing hemorrhagic or degenerate areas. Proximal to these organized masses, large unorganized thrombi were often present. In the region of the occlusive masses, the tunica intima was obliterated, except for the internal elastic lamina which usually remained intact. The tunica media was largely unaffected, except for ischemic necrosis of the media in greatly distended arteries or under thick plaques. The pathogenesis of the lesions is unresolved. The lesions may result from organization of strongyle-related thromboemboli or the inciting cause may be progressive enlargement and organization of spontaneously developing fibrous intimal plaques. Hypercoagulability of the blood may have contributed to thrombosis in one mare with the nephrotic syndrome. Routine examination of the aortic quadrifurcation and its major branches is recommended in order that subclinical changes may be detected and the natural history of the lesion elucidated.