Colour Doppler ultrasound of the post-phlebitic limb: sounding a cautionary note

Colour Doppler ultrasound is accurate in the diagnosis of acute lower limb venous thrombosis. Some patients, however, will re-present with recurrent lower limb swelling following the acute event, and ultrasound may also become the primary imaging modality in these patients. We, therefore, undertook a prospective study of 20 patients with a diagnosis of deep venous thrombosis proven initially on both venography and ultrasound. Sixteen patients had thrombosis in the femoro-popliteal segment and four patients had isolated calf vein thrombosis. Patients with above knee thrombosis were scanned at 1, 3 and 6 months and those with isolated calf lesions at intervals of 1 week, 1 and 3 months. Results showed that in cases of above knee thrombosis, only 50% of patients showed complete recanalization by 6 months. The remaining 50%, by this time, showed only partial recanalization of thrombus, the features of which cannot be distinguished from acute non-occlusive thrombus. With respect to the four cases of isolated calf vein thrombosis, one case showed evidence of popliteal extension of thrombus, which by 3 months had fully recanalized. The remaining three cases recanalized completely without further thrombus extension. These results show that interpretation of the ultrasonic features of the post-phlebitic limb may be difficult in patients who re-present with lower limb swelling. Based on this study, we would advise routine scanning of all patients with above knee venous thromboses at 6 months to provide a baseline scan, permitting more accurate ultrasonic assessment of any future thrombotic events. This study also highlights the question of whether isolated calf vein thrombosis can safely be regarded as a benign entity.