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Original Report |
1 All authors: Department of Radiology, University of Alabama at Birmingham, 619 19th St. S, JTN363, Birmingham, AL 35249-6830.
Received February 16, 2004;
accepted after revision May 17, 2004.
Address correspondence to M. E. Lockhart
(mlockhart{at}uabmc.edu).
Abstract
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CONCLUSION. The augmentation component of the lower extremity sonogram rarely provides additional information in the diagnosis of deep venous thrombus. In our large series, no deep venous thromboses were discovered with augmentation. Factors such as the lack of usefulness and patient discomfort may justify removal of augmentation from the routine study. However, augmentation should still be applied as a diagnostic tool in difficult or uncertain cases.
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The routine sonographic evaluation for DVT includes compression and duplex color and spectral Doppler assessment of flow [2, 4, 8, 9]. The deep veins of the leg are sequentially imaged in transverse orientation along their courses with dynamic evaluation using realtime direct compression of the vein. With pressure applied to the skin over the veins, the vein walls can be coapted to show that no clot is present. Longitudinal images of the vein using color and spectral Doppler sonography are then used to confirm vein patency and to identify filling defects.
Evaluation of respiratory phasicity and augmentation provide indirect venous assessment of adjacent vein segments. Occlusive thrombus cranial to the level of insonation is suggested if lack of normal respiratory phasicity is documented in the vein. Augmentation of flow is performed by squeezing the leg, typically the calf or distal thigh, at a level below the area of insonation. If the subsequent increased venous flow is transmitted to the level of evaluation, it suggests that no occlusive thrombus exists between the level of compression and insonation.
To our knowledge, no studies have evaluated the usefulness of the augmentation component of the sonographic examination for DVT. The primary goal of this study was to determine the usefulness of vein augmentation in the sonographic evaluation of DVT. A secondary goal was to prospectively evaluate and describe the prevalence of DVT in a large cohort of patients and to note the location of thrombi in the lower extremities.
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Patient Population
Our study population included 1,096 female and 884 male patients. The mean
age of the patients was 56.7 years (range, 14.6–98.7 years). They were
60.4% (1,197/1,980) white and 37.0% (732/1,980) African American.
Radiographic Evaluation
Lower extremity duplex sonography was performed according to the American
College of Radiology (ACR) Practice Guidelines in our ACR-accredited hospital
and outpatient laboratories. Sonographic findings were evaluated in the common
femoral vein, femoral vein, and popliteal veins bilaterally. Initial
transverse compression imaging and longitudinal duplex Doppler sonography with
augmentation were routinely performed using a 3–6-MHz linear transducer.
In cases of severe swelling, a curved 2–4-MHz transducer was used to aid
in visualization of deep vessels. Static images or short videotape clips of
venous compression were recorded at the level of the common femoral vein,
femoral vein, and popliteal vein. Then color and spectral Doppler sonography
of the venous flow was performed at each level.
Unless a thrombus was detected on gray-scale or color Doppler images, augmentation was performed. The technologist rapidly squeezed the leg below the level of insonation. A rapid increase in blood flow was a normal response on spectral Doppler sonography. If no rapid increase in flow was elicited by the maneuver, further evaluation was performed to exclude a thrombus below the level of insonation. If a DVT was detected by the initial gray-scale compression technique, then color Doppler flow and spectral Doppler phasicity were always evaluated with duplex sonography. However, in these cases, augmentation was not performed at or below that level in a leg with documented thrombus. Focal evaluation of calf veins was performed if there were symptoms or signs associated with the calf region. Bilateral examinations were performed unless they were specifically contraindicated (such as severely limited access because of bandages over a recent surgical site). The diagnosis of DVT was made if clot was visualized in the vein, the vein was noncompressible because of hypoechoic or echogenic clot, or a filling defect was noted on color Doppler.
Examinations were performed by one of 13 departmental sonographers. All sonographers were certified as Registered Diagnostic Medical Sonographers. Five sonographers were also certified as Registered Vascular Technologists. The sonographers had between 2 and 16 years of experience. A single sonographer routinely performed the examination. Occasionally, a second sonographer was enlisted to help with augmentation in the evaluation of a difficult case. Data were not obtained from studies performed by radiology resident trainees during the late evening and early morning on-call coverage.
The sonographer performing the examination recorded the findings of each venous segment on a data sheet after completion of the study. The sonographer documented whether augmentation of the venous flow was performed, and whether it aided in the evaluation. If the augmentation was deemed useful, the reason was noted on the data sheet. Phasicity in the common femoral veins was recorded as normal or abnormal. The patient age, sex, race, clinical symptoms, and indications were collected, and the results were subsequently recorded in a database for analysis.
Statistical Analysis
Statistical analysis of the prevalence of DVT at initial sonography was
performed on a per-patient basis. This method avoids correlation of data,
because a patient with a DVT in one leg may have higher likelihood of DVT in
the contralateral leg. Results are presented using proportions with 95%
confidence intervals (CIs) based on a binomial population distribution.
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In this series, no DVTs were identified using augmentation of the leg veins. Evaluation of one patient with DVT described the augmentation as useful in the affected leg. This patient had a nonocclusive popliteal thrombus identified by initial compression imaging, but augmentation of flow aided in visualization of a small nonthrombosed femoral vein in the thigh. In two patients with DVT, the augmentation helped improve confidence that no thrombus was present in the contralateral leg. The sonographic studies that judged augmentation as useful were evenly distributed among the various sonographers. Interestingly, the experience level of the sonographer did not affect the frequency of whether augmentation was judged to be useful.
Prevalence and Characteristics of DVT
The prevalence of lower extremity DVT at initial lower extremity sonography
was 11.2% (221/1,980; 95% CI, 9.8–12.6%)
(Table 1). Most DVTs were
nonocclusive (68.1% [322/473 segments]), and most patients with DVT had
involvement of more than one segment (57.5% [127/221]). Approximately 20%
(45/221) of patients with DVT had bilateral thrombosis. Isolated femoral vein
thrombus was identified in only 0.86% (17/1,980) of patients.
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The prevalence of DVT varied depending on the clinical indication. Higher rates of DVT were noted in studies performed for lower extremity symptoms or clinical signs. In studies performed for only lower extremity swelling, thrombus was discovered on 12.3% (94/764) of sonograms. When lower extremity pain and swelling were both listed as the indications, 14.1% (57/404) of cases were positive for lower extremity deep venous thrombus. Furthermore, when there was a combination of lower extremity and pulmonary symptoms or signs, 23.4% (11/47) of cases were positive for DVT. Of cases evaluated for lower extremity pain without swelling or pulmonary symptoms, only 7.2% (23/319) were positive.
Approximately 7.1% (15/210) of studies for respiratory distress or hypoxia without lower extremity pain or swelling revealed lower extremity thrombus. In patients with a combination of respiratory distress and chest pain, DVT was detected in 6.0% (5/84) of patients. In patients with only chest pain as the indication, no patient (0/11) had a positive study. The most common indication given for the examination in the remaining patients with a DVT who did not have lower extremity or pulmonary signs or symptoms was prolonged immobility.
In our series, it was rare to find DVT in the asymptomatic contralateral leg when the study was performed for unilateral leg pain or swelling. Only 0.5% (5/966; 95% CI, 0–0.9%) of patients had isolated DVT in the contralateral leg. This is similar to previously described rates of 0–1.2% in the literature [10–13].
Venous wall thickening was noted in 2.8% (56/1,980) of patients. Most (34/56) cases of wall thickening were limited to the common femoral veins and were without collateral vessels. The rate of venous wall thickening is similar to a previously described rate of 2% in the radiology literature [3].
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The ACR Practice Guidelines for lower extremity duplex Doppler sonography include compression, color, and spectral Doppler sonography for phasicity, and venous flow augmentation. Bilateral examination of common femoral veins and femoral veins to the popliteal veins is usually performed. However, several variations in the examination have been recommended in the literature. Some investigators have suggested a limited-compression sonographic study of only the common femoral veins and popliteal veins because of a low reported rate of isolated femoral vein thrombus [15–17]. Other authors have suggested that unilateral studies are adequate in patients with unilateral lower extremity indications, although some studies have shown positive contralateral findings that suggest a bilateral study should routinely be performed [10, 12, 13, 18]. Our findings indicate that isolated contralateral DVT is rare.
In our prospective series of 1,980 patients, no additional DVTs were discovered by augmentation. In theory, the method assists in the detection of occlusive thrombus between the transducer and the level of augmentation of flow if augmentation is abnormal. Abnormal spectral Doppler response to augmentation should prompt additional evaluation of the intervening vein to exclude thrombus. However, in actual practice, all clots in our series were first detected by compression on grayscale imaging.
Secondary uses of augmentation include increasing the venous flow to help visualization of venous segments on color Doppler sonography in patients with very swollen legs. In addition, the increased flow can help to exclude hypoechoic thrombus in the vein. In one case, augmentation helped visualize a small patent femoral vein in a patient with nonocclusive popliteal vein thrombus, thereby excluding extension of thrombus into this vessel.
Several potentially negative consequences may be associated with vein augmentation in a leg with DVT. The risk exists of mobilizing a thrombus by the rush of blood created through the vein. Moreover, many patients are evaluated for DVT because of lower extremity pain and swelling. In these patients, the rapid leg compression used in the augmentation process may be painful. This discomfort may be repeated as many as six times during a single DVT sonographic study. If uncertainty exists, repeated augmentations may cause further discomfort. In addition, in patients with extremely swollen legs, the augmentation maneuver may be difficult, especially for an inexperienced sonographer or radiology resident. To expedite the examination, an additional sonographer may occasionally be requested to aid with the examination in a patient with very swollen legs.
Our study had several weaknesses that must be considered. Ours was a descriptive series of patients. The lack of stratification by difficulty of examination and by patient risk factors other than symptoms and signs may limit generalization of the data from our series. Specifically, the prevalence of DVT may be higher after orthopedic surgery. All examinations were performed using our routine technique as clinically requested by our referring physicians; however, our current experience in a large tertiary institution may not mirror the results of other facilities.
The outcomes and treatment of patients after the sonographic evaluation were not considered. According to the sensitivity of the sonographic examination reported in the literature, 1–2% of DVTs may have been missed on the sonographic examination [19, 20]. Also, the benefit of serial sonographic studies in patients with a high clinical suspicion of DVT to detect propagation of calf vein thrombus was not addressed by our study. We did not perform routine calf vein sonography at the time of this study because calf vein thrombosis was not treated with anticoagulation by our referring clinicians.
In conclusion, the lack of clinical benefit and the discomfort to the patient may justify removal of the augmentation component as part of the routine examination for lower extremity DVT. Augmentation of the leg veins did not detect any DVT in our large series. Patients often report severe pain due to the augmentation if the lower extremity is swollen or inflamed. This augmentation procedure is repeated for each region of the leg, causing discomfort several times during the examination. In straightforward cases, the time spent is less than 1 min. However, in difficult examinations, repeated attempts may be necessary, or a second sonographer may be required to assist. For these reasons, augmentation may function better as a diagnostic adjunct in uncertain cases rather than as a routine component of the lower extremity DVT sonographic examination.
Acknowledgments
We thank Andrew Westfall of the Department of Statistics for his
statistical support, the sonographers from the University of Alabama at
Birmingham and The Kirklin Clinic, and Trish Thurman for her help with the
reference list and manuscript preparation.
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