Article: Deep vein thrombosis

This article is about Deep-vein thrombosis. For other uses of 'DVT, see DVT (disambiguation).

Deep-vein thrombosis, also known as deep-venous thrombosis or DVT, is the formation of a blood clot ("thrombus") in a deep vein. It commonly affects the leg veins, such as the femoral vein or the popliteal vein or the deep veins of the pelvis. Occasionally the veins of the arm are affected (known as Paget-Schrötter disease). Thrombophlebitis is the more general class of pathologies of this kind.

Signs and symptoms

There may be no symptoms referrable to the location of the DVT, but the classical symptoms of DVT include pain, swelling and redness of the leg and dilatation of the surface veins. In up to 25% of all hospitalised patients, there may be some form of DVT, which often remains clinically inapparent (unless pulmonary embolism develops).

There are several techniques during physical examination to increase the detection of DVT. These include measuring the circumference of the affected and the contralateral limb at a fixed point (to objectivate edema), and palpating the venous tract, which is often tender. Physical examination is unreliable for excluding the diagnosis of deep vein thrombosis.

A careful history has to be taken considering risk factors (see below), including the use of the oral contraceptive pill, recent long-haul flying, a history of miscarriage (which is a feature of several disorders that can also cause thrombosis). A family history can reveal a hereditary factor in the development of DVT.

It is vital that the possibility of pulmonary embolism is included in the history, as this may warrant further investigation (see pulmonary embolism).

Diagnosis

In a low-probability situation, current practice is to commence investigations by testing for D-dimer levels. This cross linked fibrin degradation product is an indication that thrombosis is occurring, and that the blood clot is being dissolved by plasmin. A low D dimer level should prompt other possible diagnoses (such as a ruptured Baker's cyst, if this has not been considered as part of the history).

Other blood tests usually performed at this point are:

• complete blood count
• Primary coagulation studies: PT, APTT, Fibrinogen
• liver enzymes
• renal function and electrolytes

Compression ultrasound scanning of the leg veins, combined with duplex measurements (to determine blood flow), can reveal a blood clot and the extent of it (i.e. whether it is below or above the knee) is often used to complement the above tests.

The gold standard is intravenous venography, which involves injecting a peripheral vein of the affected limb with a contrast agent and taking X-rays, to reveal whether the venous supply has been obstructed. Because of its invasiveness, this test is rarely performed.

Impedance plethysmography can also be used as a non-invasive alternative.

Therapy

Anticoagulation is the usual treatment for DVT. Thrombolysis is generally reserved for extensive clot, e.g. an iliofemoral thrombosis.

In general, patients are initiated on a brief course (i.e., less than a week) of heparin treatment, while they start on a 3- to 6-month course of warfarin (or related vitamin K inhibitors). Low molecular weight heparin (LMWH) is the type of heparin generally used, though unfractionated heparin is given in patients who have a contraindication to LMWH (e.g., renal failure or imminent need for invasive procedure). In patients who have had recurrent DVTs (two or more), anticoagulation is generally "life-long."

In patients who cannot have anticoagulant treatment (e.g., cerebral hemorrhage) or those who have recurrent PEs while on anticoagulation, an inferior vena cava filter (also referred to as a Greenfield filter) may prevent pulmonary embolisation of the leg clot. However these filters are themselves potential foci of thrombosis, IVC filters are viewed as a temporizing measure for preventing life threatening pulmonary embolism.

Prophylaxis (Prevention)

In patients who have undergone surgery, low molecular weight heparins (LMWH) are routinely administered to prevent thrombosis. LMWH can only currently be administered subcutaneously by injection. Prophylaxis for pregnant women who have a history of thrombosis may be limited to LMWH injections or may not be necessary if their risk factors are mainly temporary.

Early and regular ambulation (walking) is a treatment that predates anticoagulants and is still recognized and used today. Walking activates the body's muscle pumps, increasing venous velocity and preventing stasis. Intermittent pneumatic compression (IPC) machines have proven protective in bed- or chair-ridden patients at very high risk or with contraindications to heparins. IPC machines use air bladders that are wrapped around the thigh and/or calf. The bladders arternately inflate and deflate, squeezing the muscles and increasing blood velocity by as much as 500%. IPC machines have been proven effective on knee and hip surgery patients (a population with a risk as high as 80% with no prophylactic treatment) of developing DVT and PE.

Pathogenesis (factors leading to the disease)

Main article: Thrombosis

Many factors are involved in the formation of a thrombus (clot). Virchow's triad is a group of 3 factors that are known to affect clot formation: rate of flow, the consistency (thickness) of the blood, and qualities of the vessel wall. Among the many risk factors, obesity, immobilisation, male sex, use of oral contraceptives, tobacco usage and air travel ("economy class syndrome", a combination of immobility and relative dehydration) are some of the better-known causes^[1]. Thrombophilia (tendency to develop thrombosis) often expresses itself with recurrent thromboses.

It is recognised that thrombi usually develop first in the calf veins, "growing" in the direction of flow of the vein. DVTs are distinguished as being above or below the popliteal vein. Very extensive DVTs can extend into the iliac veins or the inferior vena cava. The risk of pulmonary embolism is higher in the presence of more extensive clots.

Epidemiology

DVTs occur in about 1 per 1000 persons per year. About 1-5% will die from the complications (i.e. pulmonary embolism).

DVT is much less common in the pediatric population. About 1 in 100,000 people under the age of 18 experience deep vein thrombosis, possibly due to a child's high rate of heartbeats per minute, relatively active lifestyle when compared with adults, and fewer comorbodities (e.g. malignancy).