Incidence

Thromboembolic disease is one of the most significant potential complications after spine surgery, with rates of acute deep venous thrombosis (DVT) ranging from 0.3% to 31%, with an overall DVT incidence of 2.2%.¹ These rates vary substantially depending on a number of factors. Overall, as expected, the lowest rates occur in younger patients undergoing simple elective procedures, whereas the highest reported rates are those in patients with preexisting risk factors that predispose them to DVT.

Risk Factors

Virchow’s triad of venous stasis, endothelial injury, and hypercoagulability is the classic description of the combination of factors that may predispose a person to DVT. In addition, general clinical risk factors include advanced age, trauma, previous DVT, stroke, malignancy, smoking, and exogenous estrogen replacement. While numerous systems have been developed to attempt stratification of DVT risk in surgical patients, many are cumbersome and therefore not of practical utility for most surgeons. However, one of the simpler systems involves assignment of a patient into one of four categories on the basis of complexity of procedure (major vs. minor), age, and additional risk factors such as prior DVT, hypercoagulability, and malignancy.² In general, spine surgery patients are at a higher risk than general surgery patients secondary to the postoperative immobility that may occur due to pain or neurologic deficits.^3,4 Conversely, the risk is significantly lower than that for patients undergoing lower-extremity surgery such as total hip or knee replacement, which can be associated with DVT rates as high as 50%.² While the overall risk of DVT in spine surgery patients can be described as moderate, the degree of immobility, and thus the risk of DVT, can be directly correlated with the type of spine procedure being performed. For example, patients undergoing a single-level anterior cervical discectomy and fusion are often treated in outpatient surgical centers and discharged home the same day, thereby minimizing the amount of postoperative bedrest and risk of DVT. Patients undergoing more extensive and complex surgery, particularly those with traumatic spinal cord injury (SCI), are at the highest risk. With regard to acute SCI, the incidence of DVT has been reported as ranging from 10% to 100% without prophylaxis and from 0% to 7% with prophylaxis. Interestingly, no correlation has been observed between level of injury, American Spinal Injury Association (ASIA) grade, or spasticity and the incidence of DVT.¹

Unfortunately, there is a limited amount of evidence regarding the specific risks of DVT in spine surgery patients. One study that used routine venography in patients undergoing spine surgery who did not receive any prophylaxis reported an incidence of 15.5%.⁴ It is important to note that none of these patients had any clinical evidence of DVT, underscoring the low sensitivity of physical signs in the diagnosis of DVT. Furthermore, the same authors found that lumbar surgery carries a much higher risk of DVT (21%) as compared to cervical surgery (6%).⁴ As was previously noted, the risk of DVT may also increase with the complexity of the procedure because more complex operations have longer operative times and often increased postoperative immobility. The use of ventral and lateral approaches further elevates the risk by requiring manipulation of vessels, particularly major veins, and thus increasing the chance for endothelial disruption. Dearborn et al. reported an incidence of 6% in patients undergoing combined ventral/dorsal approaches compared to 0.5% in patients in whom only a dorsal approach was employed.⁵

Prevention

Recommendations for DVT prophylaxis for patients undergoing spine procedures are varied and inconsistent. This inconsistency stems largely from the lack of rigorous supporting evidence for many of the prophylactic measures that are employed. Available prophylactic modalities include the use of gradient compression stockings (GCSs) or intermittent pneumatic compression devices (ICDs), administration of low-dose unfractionated heparin (LDUH) or low-molecular-weight heparin (LMWH), and the placement of a caval filter. The first step in determining appropriate DVT prophylaxis for an individual patient is assessing the risk of DVT using the criteria described previously. This risk depends heavily on the procedure being performed and the patient’s age and comorbid conditions. In 2004, the American College of Chest Physicians published guidelines for prevention of venous thromboembolism.² These recommendations are summarized in Box 199-1.

While these guidelines may serve as a foundation on which clinical decisions may be based, the decision on when and how to appropriately administer DVT prophylaxis for spine surgery patients remains a subject of significant controversy. The risks of each prophylactic intervention must be compared to the risks of DVT in each individual patient. Most spine surgeons routinely prescribe some form of DVT prophylaxis in almost all patients. As was previously mentioned, the evidence to support these policies is far from unequivocal. At baseline, most spine patients will be fitted with ICDs or GCSs as a primary method of DVT prophylaxis, as the effectiveness of mechanical prophylaxis has been demonstrated and the risks associated with their use is extremely low. One caveat of ICD use is that their effectiveness depends on perioperative and postoperative employment, as the highest risk for DVT development occurs at induction of anesthesia.

While the risk of DVT may be further decreased with the use of pharmacologic anticoagulation, the controversy surrounding its use is significant. Several small studies have demonstrated the effectiveness of LDUH and LMWH in spine surgery patients. In a double-blind randomized controlled trial, Agnelli and Becattini demonstrated a reduction in DVT incidence from 30% in patients treated with ICD alone to 17% in patients treated with ICD plus LMWH.⁶ The paucity of sufficient evidence to definitively support the use of medical anticoagulation combined with concerns for increased intraoperative blood loss and postoperative epidural hematoma formation has led to a lack of standardized guidelines for DVT prophylaxis in spine surgery patients.¹

We adjust the prophylactic regimen according to the neurologic condition, ambulatory status, age, procedure performed, and medical comorbidities of each patient. Preoperatively, hospitalized patients on bedrest or patients with neurologic deficits that affect the lower extremities are treated with ICDs. Intraoperatively, all patients receive continuous treatments with ICDs. It is also customary during prolonged ventral approaches to provide periodic release of retraction to decrease tension of the great vessels. Postoperatively, ICDs alone are continued if the patient will be ambulatory within the first 24 hours. If there are significant neurologic deficits or pain control issues that limit mobility within the first 24 hours, LDUH (given as 5000 IU subcutaneously every 8 hours) is started on the morning of postoperative day 1. If ICDs cannot be tolerated owing to injury of the lower extremities, LDUH is administered perioperatively. For long-term prophylaxis in an outpatient or rehabilitation setting, LMWH is used, the higher cost being traded for improvement of patient compliance and reduction in the need for laboratory monitoring.

Screening and Diagnosis

Clinical diagnosis of DVT remains a concern as less than 50% of patients will exhibit clinical signs.^1,7 This raises the question of whether routine screening should be employed as a method of early detection. While there are some authors who advocate routine screening, there does seem to be at least a majority consensus that routine screening for DVT with ultrasound or venography is not clinically indicated after spine surgery. Similarly, a comprehensive review by Furlan and Fehlings concluded that there is insufficient evidence to support routine screening for DVT in patients with acute SCI.⁷

If routine screening is not indicated, the question arises as to the most sensitive and cost-effective method for diagnosis of DVT in patients in whom DVT is suspected. Lower-extremity pain and tenderness, leg edema, and low-grade fevers can be nonspecific indicators of DVT. While DVT is confirmed in only 10% to 25% of patients in whom it is suspected clinically, clinical suspicion remains an important first step in the initiation of more accurate diagnostic testing.⁸

Beyond clinical suspicion, objective confirmatory tests remain mandatory for accurate diagnosis of DVT. Contrast venography continues to be the gold standard for diagnosis of DVT against which other tests are measured. No other modality is as sensitive and specific for both proximal and distal DVT. However, high cost, limited availability, patient discomfort, and contrast reactions have led to the increased use of less invasive diagnostic modalities. By using a pressurized cuff, impedance plethysmography measures the change in electrical impedance of the lower extremity in response to occlusion of the deep venous system. The sensitivity and specificity are high for proximal DVT and lower for distal DVT on single examinations. The accuracy can therefore be increased with serial examinations. By comparison, B-mode ultrasonography is as sensitive as plethysmography for proximal DVT and more sensitive for distal DVT. The addition of Doppler flow analysis in conjunction with ultrasonography has demonstrated sensitivity and specificity of 95% to 100% and has therefore become the diagnostic modality of choice in most clinical settings.^9–11

Treatment

Management of acute DVT is directed toward reducing both the short-term (pulmonary embolism [PE], clot propagation) and long-term (postphlebitic syndrome) complications. General management includes bedrest, elevation of edematous extremities, and administration of appropriate analgesics (non–platelet-active agents). Definitive management of acute proximal DVT requires a decision regarding risk of anticoagulation to the patient. If the risk for systemic anticoagulation is acceptable, treatment of established DVT may be initiated in several ways. Because the risks of using oral anticoagulation agents alone have been well documented, a safe and effective treatment strategy must include an initial course of continuous intravenous unfractionated heparin (IVUH), subcutaneous LMWH, or subcutaneous fondaparinux. The need for an initial course of heparin has been demonstrated in a double-blind, randomized trial with a threefold reduction in recurrent venous thromboembolic events compared with oral anticoagulants alone.¹² This is thought to be the result of the long half-life of factor II (compared to proteins C and S), which results in an initial hypercoagulable state at the onset of oral anticoagulant therapy. Recommendations for treatment of acute DVT, summarized in Box 199-2, are based on the ACCP guidelines for patients with DVT.¹³

The aforementioned regimen remains our preferred means of managing thromboembolism. It does, however, carry risks of morbidity. The medical and surgical literature contains numerous reports of complications related to heparin therapy. These complications include thrombocytopenia and thrombotic disorders, skin necrosis, priapism, spontaneous hemorrhage, gastrointestinal bleeding, and epidural hematoma formation.^14,15 Decortication of portions of the vertebral column and the creation of large potential dead space during exposure predisposes the spine surgery patient to an even higher risk of hemorrhagic complications and hematoma formation.¹⁶ Furthermore, following decompressive surgery, hematomas are often in direct continuity with the thecal sac, placing neural structures at risk of injury, thus necessitating further surgical intervention and its additional risks.

Initial Evaluation

Common clinical manifestations of PE include tachypnea, dyspnea, and pleuritic chest pain. The initial evaluation for clinical suspicion of PE includes chest radiograph, arterial blood gas measurements, and electrocardiogram. The arterial blood gas measurement is useful to demonstrate alterations of oxygen transfer that accompany the ventilation of lungs that have a reduction of pulmonary vascular inflow (ventilation/perfusion mismatch). Arterial blood gases typically reveal respiratory alkalosis, variable reduction in partial arterial oxygen pressure, and widening of the alveolar-arterial oxygen pressure gradient. Chest radiographs and electrocardiograms are more important and are used to rule out other diagnoses, such as pneumonia, pneumothorax, myocardial infarction, or pulmonary edema. Occasionally, the electrocardiogram may reveal right axis deviation or a right bundle branch block that may aid in the diagnosis of PE. Most commonly, chest radiographs reveal nonspecific findings such as pleural effusion, infiltrate, atelectasis, or elevation of the hemidiaphragm or are negative. Measurement of brain natriuretic peptide (BNP) is sensitive but not specific for diagnosis of PE.¹⁸ Similarly, measurements of serum d-dimer (by enzyme-linked immunosorbent assay) have a high sensitivity but low specificity, particularly in postoperative patients, as the d-dimer may be elevated from the procedure.¹⁹ However, the utility of d