According to research[4], the incidence of IVCFT in 598 cases of filter implantation was approximately 18.6%, of which 4 cases were associated with thrombosis above the filter. A prior study[5] reported 1 case of IVC filter thrombosis on a retrieval hook of a filter tilted and attached to the IVC wall, in which the thrombus detached without endovascular therapy. However, no report has been found on IVCFT associated with a free-floating thrombus on retrieval hook endovascular therapy.
Causes of free-floating thrombi on retrieval hooks
The causes of IVCFT mainly include the following: (1) free-floating thrombi are captured, (2) the filter stimulates the IVC wall resulting in IVCFT, and (3) iliac vein thrombi extend into the filter and lead to IVCFT[6]. In addition, factors including various shapes of filters produced by different manufacturers, filter implantation time, blood hypercoagulability (cancer, obesity, pregnancy, antiphospholipid syndrome, etc.), and patient age can cause IVCFT[7]. In our case, no tilt was observed in the IVC filter and standard anticoagulant therapy was initiated. The filling defects in the filter are considered to be detached thrombi captured by the filter. When there is a small amount of thrombus in the filter, if the IVCF is close to the renal vein, the fluid dynamics of the renal venous vein will be affected. High wall shear stress (WSS) occurs along the caval wall downstream from the thrombus in infrarenal filter positions. The lower the WSS, the slower the blood flow velocity in the section, leading to an increased possibility of thrombosis. When the IVCF is placed just below or near the level of the renal vein, the lowest WSS is close to the low flow area downstream of the bilateral renal veins[8]. This is the recovery hook location (Fig. 3). Therefore, the causes of a free-floating thrombus on a retrieval hook could be as follows: a vortex of venous return forms near the retrieval hook after the detached IVC thrombi are captured by the filter, leading to platelet adhesion, so stagnation and even coagulation can occur among platelets. Blood cells are wrapped in fibrin strands and eventually form mixed thrombi (Fig. 2).
Diagnosis of free-floating thrombi on retrieval hooks
Most IVCFT patients have no clinical symptoms[9]. Generally, IVCFT is diagnosed when the patient is scheduled to undergo filter retrieval surgery and DSA detects filling defects in the filter. This examination is the gold standard for IVCFT diagnosis. Zhang et al.[10] conducted multi-angle IVC angiography on 1 patient with suspected IVCFT in which the filling defects were eventually assessed as artifacts in the filter; hence, artifacts in filters should not be overlooked. RDSA conducts data acquisition and imaging on selected areas using a fast rotating C-arm, acquires dynamic subtraction images from different continuous angles, and generates CT-like images through 3D reconstruction to provide more accurate and detailed image information[11]. IVC angiography using RDSA can avoid the interference of intestinal gas and centrum while improving the accuracy of IVCFT diagnosis. Moreover, RDSA can initiate multi-angle and dynamic observation on IVC thrombi. The round filling defect floating above the retrieval hook in our case had a candlelight shape, which was unattainable via regular IVC enhanced CT scan.
Endovascular treatment of free-floating thrombi on retrieval hooks
IVCFT therapies involve anticoagulants, CDT, and PMT. The American College of Chest Physicians (ACCP) in Antithrombotic Therapy for VTE Disease: CHEST Guidelines[12] recommend that when there are no anticoagulant medication contraindications, anticoagulant therapy should be initiated after filter placement. New oral anticoagulants with low risks of bleeding and no need for INR measurement such as rivaroxaban are also recommended. A prior study[7] indicated that once IVCFT is observed, the patient should undergo immediate anticoagulant therapy as soon as anticoagulant medication contraindications are ruled out, and the possibility of fatal PE and serious complications of the lower extremity should be assessed. In cases of low possibilities, anticoagulant therapy should continue and angiography reexamination should be performed in 2 to 3 weeks; in cases of high possibilities, further treatment is required.
CDT ensures the effect of thrombolysis while reducing the dosage and thrombolysis time, significantly lowering the risk of bleeding and benefitting patients the most[12]. CDT is superior to peripheral venous thrombolysis (PVT)[13]. Aiming at IVCFT, Tian et al.[14] noted that the therapeutic effect of local CDT far exceeded that of PVT and the filter removal rate in a local CDT group was higher than that in a PVT group.
In our case, the therapeutic effect of oral rivaroxaban combined with standard anticoagulant therapy and CDT for 5 days was not satisfactory, which could have been caused by the following: (1) in CDT therapy, the catheter’s side hole of the is usually placed inside the thrombi; however, it is relatively difficult to insert the wire through retrieval hook thrombi, which is especially true for free-floating thrombi; hence, thrombolytic drugs have only limited access to the thrombi; (2) thrombolytic drugs in the IVC filter may be removed by venous return before the thrombi is fully accessed; (3) the thrombi formation time is rather long, while the therapeutic effect of CDT depends on the age and thrombi load. The thrombolytic effect of chronic thrombosis is not ideal; therefore, the combination of AT or percutaneous mechanical thrombectomy (PMT) with CDT could be more effective for removing thrombi and reducing the dosage of thrombolytic drugs and the thrombolysis time, which is superior to the exclusive use of either therapy[15,16]. Free-floating thrombi on retrieval hooks have a detachment risk; hence, new retrievable filters should be placed above original filters to prevent secondary PE caused by retrieval hook thrombi detachment during AT. There are also indications[17] that when the IVCFT load is relatively heavy and the thrombi are larger than 50% of the IVC diameter, retrieval filters should be placed before AT. For AT therapy against IVCFT, aspiration catheters should be placed as close as possible to the thrombi to reduce the amount of bleeding, since the blood flow around the thrombi is relatively heavy; hence, adjustable curved sheaths or MPA catheters with a diameter over 8F are suggested. RDSA excels at locating the positions of thrombi and guiding CDT catheters. Retrieval hook thrombi, free-floating thrombi in particular, should be crushed by the catheter and captured by the filter above after detachment and then removed by AT. Filters should be retrieved as soon as possible after IVCFT removal to eliminate further risks.
When IVCFT develops into chronic thrombosis, CDT and PMT may no longer effectively remove the thrombi, so balloon dilatation and stent implantation should be considered to reconstruct the IVC. Branco et al.[18] initiated balloon dilatation on 4 patients and stent implantation on 2 out of 9 IVCFT patients, and the occlusions were successfully removed. Cui et al.[19] implanted IVC stents in 40 IVCFT patients. The stents and filters were in good shape with no serious bleeding or PE, and the restenosis rate was 7.7% 12 months after surgery. Marmagkiolis et al.[20] reported successful IVC reconstruction with PMT, balloon dilatation, and stent implantation on 1 IVCFT patient. Self-expandable braided stents are recommended (such as Wallstent) [21]. The stents can be placed either parallel or through the filter in accordance with the filter shape; that is, the filter can be regarded as the reference for stent anchorage. In brief, it is both feasible and safe to implant stents in patients with chronic IVCFT to remove occlusions, whereas the long-term patency rate of stents still needs to be further improved[20-23].