Phospholipid-bound prothrombin is often the target of lupus anticoagulants and has been rather well characterized. There is, also, considerable interest in the possibility that APS-associated autoantibodies may inhibit the protein C pathway, a clinically important physiologic anticoagulant mechanism. Virtually all known cases of vascular occlusive disease in the APS, regardless of the type, size, and location of the blood vessel involved, are thrombotic in nature and not vasculitic. In approximately one third of patients, it is accompanied by pulmonary embolism and, occasionally, thromboembolic pulmonary hypertension. Infrequently, venous occlusions may involve the superior or inferior vena cava.
Upper limb veins such as the subclavian are less commonly obstructed than lower limb veins.
Venous occlusions are also liable to occur after intravenous procedures and catheter placement or passage. This association has been well-documented. It may present without identifiable focal neurological deficit, and it may indeed closely resemble other dementias of the Alzheimer type. It is usually ascribed to diffuse cerebritis in symptomatic patients in the absence of infection, stroke, hemorrhage, steroid psychosis, or aseptic meningitis. A number of these patients have been noted to have aPL antibodies which were present at levels higher than those of patients with less severe neurological dysfunction.