Thromboembolism (Proceedings)

Nephrotic syndrome

The etiology of thrombosis in nephrotic syndrome is multifactorial. The association of increased risk has been found in both human and veterinary patients. Factors contributing include decreased anticoagulant activity, increased platelet aggregability , reduced fibrinolysis and increased amounts of clotting factors. Incidence varies in studies from 10-40% of nephrotics, although severity of disease may not be predictive of risk. Other factors to consider include hyperviscosity , hypovolemia and dyslipidemia. The changes noted may be the result of coagulation activation (thrombin generation and tissue factor production) in the damaged glomeruli.

Platelet activity

Evidence includes thrombocytosis, in vitro aggregation studies showing enhanced reactivity to a variety of agonists (ADP, epinephrine, collagen, arachnidonic acid), spontaneous aggregation and elevated β-thromboglobulin levels.

Studies using in vitro models of blood flow have also cast doubt on the significance of the aggregation studies. Although the platelets are more likely to aggregate in suspension, they do not aggregate under flow conditions. Instead fibrin was formed that blocked endothelial cell/platelet interaction. Addition of excess fibrinogen to normal platelets mimicked the findings in nephrotic patients.

Anticoagulant proteins

Reduced levels of AT III have been found and are related somewhat to degree of proteinuria. This often has been linked to increased risk of thrombosis. Other studies have shown increased levels of Protein C, α2-macroglobulin, and heparin cofactor which may partially counterbalance the low ATIII. Total plasma antithrombin activity is in fact enhanced. Tissue factor pathway inhibitor levels are normal.

Fibrinolysis

Generally it has been shown that fibrinolysis is decreased. Clot lysis times are increased. Elevated levels of PAI, lipoprotein (a) and õ 2-macroglobulin would interfere with fibrinolysis. In addition high levels of fibrinogen and low albumin interfere with plasminogen binding to fibrin.

Elevated clotting factors

Levels of fibrinogen, vWf, factor V, factor VIII and factor XIII are increased in nephrotic syndrome. This is felt to be a result of increased hepatic synthesis or vascular injury.

Pharmacologic manipulations

Aspirin has been shown to be beneficial in slowing the progress of membranous glomerulonephritis in people (950mg/day). Heparin may be very beneficial as it has been able to normalize fibrinopeptide A levels in nephrotics. Fibrinopeptide A is a sensitive indicator of thrombin activity on fibrinogen. Additionally heparin is known to inhibit mesangial cell growth in vitro. The effects of EFA on glomerulonephritis has been investigated in humans and has proven to be beneficial. In those instances where emboli have occurred warfarin may be indicated.

Feline cardiomyopathy

Aortic thromboembolism is a serious complication of feline cardiomyopathy and often predates evidence of heart disease. A retrospective study identified 58% of cats having hypertrophic cardiomyopathy, 27% having intermediate-form, 6.3% having restrictive and 3.2 % having dilated cardiomyopathy. Left atrial enlargement was common with 56.9% having a LA:Ao of 2.0 or greater. Only 5.2% had a normal ratio. Radiographically 88.8% had cardiomegaly and 66.3% had evidence of congestive heart failure. Survival rate was determined as follows: died during hospitalization 28%, euthanized 35%, survived 37%. Warfarin was started in 18 of 22 survivors that were followed. Average survival time of those that died was 9.7 months. Four cases are still alive with an average survival time of 24.3 months. Reembolization occurred in 50% of cases.

Etiology of thrombi

Perturbations of blood flow in the dilated left atrium are felt to be an important factor in pathogenesis of emboli in cardiomyopathy. In addition endomyocardial lesions may be sites for thrombus formation. Platelet aggregation studies have shown a potential hyperaggregable state.

Therapy of established thrombi

Predominantly rest and recreation will cure the cat with an aortic embolus. Improving cardiovascular function is beneficial. The direct effects of cardiac drugs on hemostasis have been investigated but it is still uncertain that this is a significant factor in improving survival. Heparin can be given and may slow down further clot enlargement. The value of thrombolytic therapy in cats is questionable, though TPA is very effective (although it kills 50% of cats treated). The use of phenothiazines may improve collateral circulation or more likely have modulating affects on serotonin to inhibit platelet activity. Nitroglycerin would also inhibit platelet reactivity by being metabolized to NO which decreases calcium influx into the platelet that leads to platelet aggregation by binding to fibrinogen.

Prophylaxis of thrombi

Presently aspirin is commonly used in cats with CM. The benefits of this have not been documented. Recently warfarin has been recommended. Usual dose is 0.5mg/cat/day. Therapy is monitored using PT and INR (international normalization ratio). Goal is an INR 2.0 to 3.0 of baseline. INR = (patient PT/control PT)ISI. The ISI number is supplied with the batch of thromboplastin used. Bleeding complications can occur. The authors initiated heparin therapy concurrently and then tapered this as the warfarin took effect (3-5 days). Survival is purported to be improved.

Autoimmune hemolytic anemia

The pathogenesis of emboli in IMHA remains unexplained. Although the distribution of clots in some cases is consistent with DlC in other cases the large PTE would not be consistent. The effect of RBC membranes and Hb may be procoagulatory.

The hepatic damage occurring in IMHA may potentiate the thromboembolic risk. The liver is vital for the clearance of activated coagulation complexes. Hepatic dysfunction is common in IMHA and may result from hypoxia or blood clots. Elevations of bilirubin have been associated both with thromboembolic risk and survival. Whether this is cause and effect or merely a reflection of the severity of the disease remains unknown.

Cushing's disease and corticosteroid administration

The frequency of thromboembolism associated with endogenous hyperadrenocorticism or exogenous administration of corticosteroids has not been published although the clinical impression does exist that there is an association. In humans the changes occurring in hemostasis after renal transplantation and administration of immunosuppressive therapy with corticosteroids and cyclosporine closely parallels the changes seen in Cushing's disease. It is known that corticosteroids induce increased synthesis of a wide variety of proteins.

Abnormalities of hemostasis

Shortened APTT, increased factor VIII, increased vWf, tissue plasminogen activator and tissue plasminogen activator inhibitor. The increased PAI level would override the increase in TPA and cause a hypofibrinolytic state. The increase in VIII and vWf together with a shortened APTT would indicate a hypercoagulable state. This may also be indicative of vascular disease as it closely parallels MAP and it is known that vWf, PA and PAI are endothelial disease markers. In dogs increases in factor V and X were found. ATIII and plasminogen levels were elevated. It is difficult to interpret the relevance of these findings.