Chapter 131: Thrombosis and thromboembolism

INTRODUCTION

A thrombus is defined as a clot formed in the circulation from the constituents of the blood, whereas emboli are fragments of thrombus that break off and block vessels downstream. Almost half of adult deaths in Western countries are due to thrombosis of coronary or cerebral arteries, or to pulmonary embolism.¹ A thrombus is the result of a cascade of events involving platelets, RBCs, coagulation factors and the vessel wall.

Conditions predisposing to thrombosis:

• thrombophilia

• thrombocytosis (platelets)

• polycythaemia rubra vera

Thrombophilia²

Thrombophilia refers to a disorder of haemostasis in the form of a primary coagulopathy leading to a tendency to thrombosis. This should be considered in patients with major unprecipitated venous thromboembolism with or without a strong family history of venous thrombosis. Several causes can be tested, both inherited and acquired:

• inherited:

  • – factor V Leiden gene mutation (activated protein C resistance)

  • – prothrombin gene mutation

  • – protein C deficiency

  • – protein S deficiency

  • – antithrombin deficiency

• acquired:

  • – antiphospholipid antibodies (anticardiolipin or anti-ß₂ GPI)

  • – elevated homocysteine level

  • – lupus anticoagulant

The above factors can all be measured in the laboratory with specific genetic tests, coagulation or antibody-based tests. Other acquired causes (regarded as risk factors) include malignancy, oral contraceptives, HRT, immobilisation, obesity, smoking, pregnancy and major surgery. Most DVTs do not require investigation for an underlying cause, but consideration should be given to screening patients who have had an unprovoked DVT. Refer to a haematologist if thrombophilia is proven or suspected. Of particular interest is factor V Leiden, which occurs in about 4% of Caucasians; the relative risk of venous thrombosis is increased three to seven times in heterozygotes and 80 times in homozygotes.²

Indications for investigation

• Recurrent or unusual thrombosis

• Arterial thrombosis <30 years

• Skin necrosis, especially on warfarin

• Recurrent fetal loss

• Familial thromboembolism

Venous thromboembolism

A feature of venous thrombosis is that it develops in normal vessels, with key factors being stasis and increased coagulability, including thrombophilia. The classic example is deep venous thrombosis of the leg veins. Another poorly recognised thrombosis is axillosubclavian venous thrombosis, which is associated with pulmonary embolism in 30% of cases.³

Other sites of deep vein thrombosis are the pelvic/ovarian veins, which should be suspected in a person at risk with pelvic pain and swollen upper thighs, mesenteric venous thrombosis and cerebral sinus thrombosis, which are usually due to thrombophilia.