Commonly VTE follows neurosurgery, but surgery might occasionally be required in patients with a recent VTE.19 Diagnosis of VTE is based on algorithms incorporating clinical probability, D-dimers, and imaging (such as duplex ultrasound for DVT and spiral CT or ventilationperfusion scans for PE). D-dimers, the smallest degradation product of the fibrin clot, are useful because they have a high negative predictive value. In the postoperative setting, however, when D-dimers are quite often raised, their utility is limited. Duplex ultrasonography has largely replaced venography as the imaging of choice and convenience to diagnose DVT, but sensitivity for pelvic and calf vein DVT is sub-optimal. CT with pulmonary angiography has replaced ventilation-perfusion scanning as the imaging of choice for PE, due to higher sensitivity and specificity and the ability to demonstrate alternative diagnoses if PE is excluded. In future, magnetic resonance imaging (MRI) is likely to be increasingly important for diagnosis of DVT and PE.
After VTE has been diagnosed, conventional therapy comprises once daily LMWH injections and warfarin.20 The LMWH is usually given for at least five days and discontinued when the INR is within the therapeutic range. Below-knee DVT is usually treated for three months and proximal DVT or PE for six months. In the neurosur-gical setting, therapy will depend on risk of bleeding in the individual patient together with the timing and the location of thrombosis. In the immediate postoperative period, systemic anticoagulation is best avoided due to risk of intracerebral bleeding. Remote from this period, patients should be cautiously anticoagulated. If there is still a concern about the risk of bleeding then intravenous UFH maybe used in the first instance, aiming for an APTT ratio at the lower limit of the therapeutic range. The advantage of this approach includes the short half-life of the heparin together with the availability of an antidote in protamine. An alternative approach is the initial use of split doses of LMWH ensuring smoother anticoagulation control, with lesser peaks and higher troughs. If monitoring is required, due to concerns about bleeding risk, anti-factor Xa levels may be measured. LMWH is as efficacious as UFH in the treatment of DVT and submassive PE and is more convenient, with the use of once or twice daily subcutaneous injections without routine monitoring. Adverse effects such as hep-arin induced thrombocytopenia and osteoporosis are reduced. In the bleeding patient, however, it must be remembered that LMWH's anticoagulant effect lasts 18 to 24 hours and protamine will reverse only a fraction of this.
When anticoagulation is contraindicated, vena caval filters are an important alternative (Fig. 5).21 The primary purpose of implanting
a vena caval filter is to prevent a potentially fatal PE. A high complication rate has, however, been seen in brain tumor patients with VTE treated with IVC filter. In a small study of 42 patients, 12% experienced recurrent PE and 57% developed IVC or filter thrombosis, recurrent DVT or post-phlebitic syndrome. When IVC filters are used there is a significant decrease in early PE, but a higher incidence of recurrent DVT in the long-term. Therefore, IVC filters should only be used in selected neurosurgery patients, to enable us to withhold anticoagulation during the period of maximal bleeding risk. At other times, anticoagulation remains the best option. The availability of new generations of retrievable IVC filters provide a valuable tool for the treatment of DVT in neurosurgical setting as the contraindication to anticoagulation is only temporary. These filters can often be retrieved many weeks after insertion, following which the patient can be anticoagulated as usual.
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