The main findings of our study were that stent compression and ISR are common complications after iliac vein stenting, with estimated prevalences of 71.2% and 46.8% based on MDCTV, respectively. Ultrasound of gray-scale imaging has very good agreement with MDCTV in diagnosing stent compression. CEUS outperformed DUS in terms of diagnostic accuracy for detecting ISR.
Since there is no dedicated iliac venous stent commercially available in China, arterial-designed nitinol stents have been used as a substitute for the treatment of iliac vein obstruction. However, the arterial design includes some properties that may not suitable for the venous system [25]. A stent with sufficient radial resistive force, crush resistance and outward radial force is needed to resist the sustained compressive forces in venous compressive diseases to maintain blood flow in fibrous venous occlusions. These forces in the arterial-designed stents are most often insufficient.
Raju et al. [17] reported a rate of 25% for stent compression in 103 limbs with residual or recurrent symptoms, indicating that stent compression is a unique feature of iliac vein stenosis. An approximately oval shape can be seen at these levels of high compression, but a residual stenosis of more than 50% would be unacceptable [25]. A study [18] found that the proportion of significant stent compression was 33%; 56% of patients in the significant stent compression group developed stent occlusion, while only 9% developed stent occlusion in the insignificant group, indicating that significant compression of the nitinol stents obviously affects stent patency. Additionally, the diameter reduction caused by stent compression may result in venous hypertension and residual symptoms of lower extremity, although patency may be retained [19]. For these reasons, some researchers have pointed out that laser-cut nitinol stents might not be suitable for treating severe IVCS [21]. Our study showed that the overall incidence of laser-cut nitinol stent compression was 71.2% (99/139), with a 15.8% (22/139) incidence of significant compression based on MDCTV. The ultrasound modality of gray-scale imaging had very good agreement with MDCTV in diagnosing stent compression, which reflects that gray-scale imaging is a good alternative to MDCTV in detecting stent compression.
Another common anomalous feature of iliac venous stents is ISR. Neglén et al. [26] reported that ISR developed to some degree in 77% of limbs at 42 months, and severe (greater than 50%) stenosis was observed in 15% of limbs. A study by Raju et al. [17] showed that ISR was present in all 103 limbs with residual or recurrent symptoms. Our study also detected a high prevalence of 46.8% based on MDCTV. If a significant ISR (greater than 50%) is identified on follow-up imaging, percutaneous transluminal angioplasty (PTA) is recommended to maintain patency, especially in symptomatic patients [27].
Presently, there is no standardized noninvasive screening imaging for iliac vein stent follow-up. DUS has been the most widely used noninvasive method [28]. However, established ultrasound surveillance criteria are rarely mentioned, and independent assessments of the image quality are not provided in most studies. DUS may be insufficient because of ultrasonic attenuation by the deep location of the stents, and the orientation of the flow is almost perpendicular to the transducer. Experience indicates that DUS may be unrevealing in some cases of stent malfunction. A study [29] reported that adequate imaging of the stents could not be achieved with DUS in 23.7% of the cases. In the present study, DUS had only moderate agreement with MDCTV in detecting ISR, and the sensitivity and specificity were relatively low, only 63.1% and 87.8%, respectively. This indicates that some cases of ISR would be missed or be misdiagnosed by DUS.
With the use of contrast agents, CEUS overcomes angular dependence and the shortcoming of DUS by increasing the signal-to-noise ratio [24]. Blood flow is easy to depict with contrast material, and the visualization of the ISR is more intuitive by contrast agent filling defects. In this study, the sensitivity, specificity, and positive and negative predictive value of CEUS were very high (all over 90%), which are significantly superior to those of DUS. Thus, CEUS is an alternative modality to improve the diagnostic accuracy of DUS for the detection of ISR.
CEUS has showed real clinical value because of its advantages, such as its minimal invasiveness, rapid nature, cost-effectiveness, and good tolerability; CEUS also permits multiple follow-up surveillances without the risk of side effects. On the other hand, CEUS is characterized by some limitations that also apply to the unenhanced conventional technique. For example, attenuation because of the deep position of the stent, and obesity and bowel gas can affect the imaging [24]. However, in our experience, as long as adequate intestinal preparation is accomplished, CEUS is easy to perform, and good image quality can be obtained. Furthermore, CEUS is operator-dependent, standardized training and specific skills are required to obtain quality images.
Our study has some limitations. First, as the reasons mentioned earlier, we could not use IVUS as the gold standard, which may affect the real diagnostic accuracy of non-invasive technique of CEUS. In clinical practice, only a few patients with indications of re-intervention were willing to receive further treatment, leading the small number of cases that could be confirmed by IVUS.
Second, the absence of hemodynamic parameters like peak velocity or velocity index or velocity ratio may be a weakness of the study. However, visualization of the blood flow within the stent is necessary to assess stent patency, the abnormalities of hemodynamic parameters detected by pulsed-wave Doppler are useful but are of limited value for the quantification of lesion severity [30].
The last possible drawback was that the specific degree of ISR and the overall degree of stenosis when stent compression presents with ISR was not assessed. Measuring diameter stenosis by DUS and MDCTV is problematic because of the irregular shape of neoplasms and the elliptical shape of the post-stented lumen. The optimal assessment would be to use IVUS to directly delineate the area encompassed by the stent perimeter and then calculate the percentage stenosis with the lumen area within the ISR [17].