In a recent decade, great attention has been paid on the correlation between DVT and surgery, especially for the effect of surgery on the DVT occurrence. Just like injury, surgery will also induce hypercoagulation state of patients or make damage to the blood vessels directly. When general anesthesia performed, intraoperative muscle relaxants may result in venous stasis. For spine surgery, prone position is the most common position which may result in compression of the blood vessels in the groin and affecting the venous drainage of the lower limbs. Bone cement usage in some spine surgery may also induce hypercoagulation state of patients.[20, 21] Collectively, patients undergoing spine surgery will take a risk of having DVT after surgery. However, for patients with DVT before spine surgery, little is known about how the surgery affects the DVT prognosis precisely, how the safety of the surgery for these patients, what is the best surgical timing and what are risk factors predicting DVT progression after surgery. This research was therefore carried out aiming to answer above clinical questions.
Based on the results of this study, the overall incidence of DVT progression after spine surgery is 24.5%. This incidence is not low. However, only two patients (0.98%) had symptomatic PE following the surgery and not life-threatening. All patients in this case series received anticoagulant treatment as early as possible after surgery depending on the risk of the epidural hematoma. There was no case of DVT steady worsening or death occurred in this study. It is still safe for the patients with preoperative DVT receiving spine surgery while postoperative anticoagulant treatment and DVT surveillance with ultrasonography can be timely applied.
D-dimer level has been widely advocated as a risk factor for DVT occurrence both preoperatively and postoperatively in the literature.[6, 8, 18, 22] For the patients who already had DVT before surgery, lower D-dimer level normally means old or mild thrombosis which will not easily grow worse. By contrast, the higher D-dimer level normally means fresh or active thrombosis. The subsequent spine surgery may exacerbate DVT more frequently. In this study, higher preoperative D-dimer level was also significantly associated with DVT progression after surgery both in univariate analysis and multivariate analysis. The cut off value of D-dimer for prediction of DVT progression was 1.605 mg/L. It is similar to the D-dimer value for prediction of DVT occurrence in other reports (1.4 mg/L in Ikeda’s research and 1.81 mg/L in Wang’s research). Besides, the D-dimer for prediction had a fair sensitivity of 72.0% but a low specificity of 53.9%. That means D-dimer < 1.605 mg/L for prediction of DVT not progression is more reliable than D-dimer ≥ 1.605 mg/L for prediction of DVT progression.
Based on the multivariate analysis, operating time is another risk factor for DVT progression after spine surgery. This is a comprehensible result because longer surgical time usually correlates with advanced surgical trauma and induces severer systemic stress response, followed by advanced hypercoagulation state of patients and contributing to DVT progression. Several studies also confirmed the longer operating time was associated with DVT occurrence after spine surgery.[7, 13] In our study, comparing with the operating time of less than 175 min, advanced operating time (≥ 175 min) results 9.567 fold increase in the risk of DVT progression. As a predictor, the operating time had a very high specificity of 97.4% but a very low sensitivity of 18.0%. That means operating time is valuable for prediction of DVT progression (≥ 175 min) but not for prediction of DVT no progression.
Intraoperative blood loss and blood transfusion as well as high energy trauma were significantly associated with DVT progression in the univariate analysis but not in the multivariate analysis. These three factors are all responsible for hypercoagulation state of patients and may contribute to DVT progression. The negative results in the multivariate analysis may be attributed to the sample size is still not large enough. Besides, the effect of high energy trauma may not always last long enough to induce DVT progression after surgery.
Operative position (supine/lateral/prone) is another potential risk factor for DVT progression. Because differ to supine or lateral position, prone position may result in compression of the blood vessels in the groin and affecting the venous drainage of the lower limbs. Based on our results, there are 49 out of 188 patients (26.1%) with prone position had DVT progression while only one out of 16 patients (6.3%) with supine/lateral position had DVT progression. However, there is still no significant difference according to the statistical analysis. The small amount of patients with supine/lateral position in this case series may be an important reason.
Age has been identified as an important risk factor for DVT occurrence after spine surgery in several studies.[6, 13] However, there is no significant difference of the average age between DVT progression group and no progression group in this study. It should be noted that normally elder patients had preoperative DVT who were included in this study (66.7 years and 68.2 years for DVT progression group and no progression group, respectively). By contrast, the average age reported in other studies (most participants did not have preoperative DVT) are less than 55 years.
It is surprising that the minimal invasive surgery is not significant associated with lower risk of DVT progression. For all patients with minimal invasive surgery, there is still 15 out of 71 patients (21.1%) had DVT progression. It seems not necessary to choose minimal invasive surgery as far as possible when consideration of minimizing the risk of DVT progression. However, most of the minimal invasive surgery in this study is percutaneous kyphoplasty (PKP). It has been reported that the average hidden blood loss of PKP was 282 ± 162 mL which was underestimated before. The bone cement usage in PKP procedure may also induce hypercoagulation state of patients.[20, 21] Although the bone cement usage was not identified as a risk factor for DVT progression in this study, the bone cement was mostly used in the PKP procedure and rarely used in other open surgeries.
As old DVT is usually stable and dose not easily progresses or contribute to PE. It is supposed that the delayed surgery is beneficial to reduce the risk of DVT progression after surgery. However, the time from DVT detection to surgery was not found to be significantly different between the DVT progression group and no progression group in this study (5.1 days vs. 4.4 days). In view of the fact that our study is retrospective but not prospective, the patients with advanced DVT or severer injury may prone to receive delayed surgery. It may affect the accuracy of the results. Anyhow, the time from DVT detection to spine surgery still can’t be identified as an important factor while consideration of the surgical timing, especially for those patients with neurological impairment who need operation of neural decompression as soon as possible. By contrast, preoperative D-dimer level should be an important basis for making the decision of the surgical timing.
Chemoprophylactic anticoagulation for reducing the incidence of DVT after spine surgery has been proved in some studies.[13-15, 24, 25] Nevertheless, some researches didn’t advocate the effect of chemoprophylaxis and concerned the risk of epidural hematoma.[6, 24] Usage of anticoagulant drug such as low molecular heparin for treating DVT has been widely accepted. However, if it is necessary and how long to use anticoagulant drug preoperatively for controlling DVT progression? Especially for patients with high level of D-dimer, the anticoagulant treatment and delayed surgery are normally beneficial for lowering D-dimer, and may further lowering the risk of DVT progression. Based on our results, there was still no difference of preoperative anticoagulation time between the DVT progression group and no progression group. Maybe the usage of anticoagulant drug in most cases of our study was not long enough to have effect on the controlling of DVT progression (except for the cases without anticoagulant, average 4.2 and 4.0 days in the two groups, respectively). However, longer-time anticoagulant treatment preoperatively may increase intraoperative bleeding or not be permitted in some cases of emergency surgery. At least by now, there is still absent evidence that preoperative chemoprophylactic anticoagulation is beneficial for reducing the risk of DVT progression after surgery.
Ultrasound detection for DVT has been widely advocated in clinical practice and most researches focusing on DVT. However, the false positives and negatives while using ultrasound for DVT detection have also been reported. A meta-analysis concluded ultrasound for whole-leg DVT had a pooled sensitivity of 59% and a pooled specificity of 95%. That means ultrasound for DVT had a relative high rate of false negative but low rate of false positive. For our study, the incidence of preoperative DVT and DVT progression postoperatively may be underestimated due to the false negative. Nevertheless, it still has a high accuracy for the risk factors of DVT progression, because the false positive is low.