6.10.5 Aorto-iliac thrombosis
Thrombosis of the terminal section of the aorta, internal iliac arteries, external iliac arteries, or combinations of these vessels, has been reported to result in an ischaemic myopathy of the hind limbs. The aetiology of the condition is unclear, although it may be associated with larval Strongylus vulgaris migration.
One or both hind limbs may be affected, depending on the extent of the occlusion of the vessel(s) by thrombus. The typical history is of a hind-limb lameness which is induced or exacerbated by exercise. The condition is usually insidious in onset. Clinical examination may reveal a weak greater metatarsal arterial pulse and slow filling of the saphenous vein. These signs may be more marked following exercise. Tachypnoea and sweating may be observed during exercise, possibly due to pain.
Rectal examination is usually diagnostic. Thrombus may be palpated in any of the arteries at the terminal aorta, although they are most commonly recognised in the terminal aorta itself. The pulse quality may be reduced. A thorough examination of the aorta and both branches of the iliac arteries on the left and right side is required.
A more definitive method of diagnosis is ultrasonography of the affected vessel. This has been diagnostic in some cases where clinical examination did not pinpoint the source of the lameness. A 5 or 7.5 MHz linear array real-time scanner is recommended, although some sector scanners are suitable for examinations per rectum. A mass of variable heterogeneous echogenicity may be seen on either the dorsal or ventral wall of the aorta, or in its branches (Figure 6.5). The degree of occlusion can be estimated.
Treatment consists of box rest, NSAIDs such as phenylbutazone, and larvicidal doses of anthelmintics such as ivermectin. Resolution of the thrombus may occur in some cases, however the outcome is seldom favourable. Ultrasound is a useful method of assessing progress.