Controversies in venous ultrasound

C o n t r o v e r s i e s in V e n o u s U l t r a s o u n d John J. Cronan Ultrasound is the principal method used for diagnosing deep venous thrombosis in the United States, and its accuracy and limitations are well known. As venous ultrasound examination has matured, several controversial issues, primarily clinical, have arisen concerning the application of this diagnostic method. This article addresses some of the more noteworthy and vexing issues, including, but not limited to (a) the need to examine both legs in patients with unilateral symptoms; (b) the role of venous ultrasound in patients with bilateral leg swelling; (c) the necessary extent of the venous ultrasound examination; (d) the importance of calf vein thrombosis; (e) the significance of negative leg veins in a patient with possible pulmonary embolus; and (f) deep venous thrombosis in patients with occult malignancy. Technical aspects of the venous ultrasound examination, and diagnostic accuracy are not described. Copyright © 1997 by W.B. Saunders Company

HE PAST DECADE has defined venous ultrasound as the initial diagnostic tool for assessment of patients for deep vein thrombosis (DVT). l However, many clinical issues remain unsettled regarding the role of this technique. This article reviews several controversial issues and helps to define an acceptable practice of venous ultrasound used to diagnose acute venous thrombosis.

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UNILATERAL SYMPTOMS: SHOULD THE ASYMPTOMATIC LEG BE EXAMINED?

In the era of venography, when the radiologist evaluated a patient for acute deep vein thrombosis, only the leg in question was studied. The asymptomatic leg was not studied because of the risk of reactions to intravenous contrast material and the invasiveness of the procedure. Historically, noninvasivc vascular laboratories that used plethysmography routinely evaluated both the symptomatic and asymptomatic legs. This practice provided a frame of reference that helped diagnose venous thrombosis in the symptomatic leg. After the introduction of venous ultrasound, radiologists continued to evaluate only the symptomatic leg; however, many vascular laboratories continued the practice of evaluating both the symptomatic and asymptomatic legs during the period of transition from plethysmography to venous ultrasound. Contemporary reports in the literature regarding the need to evaluate the asymptomatic leg are conflicting. Of interest, until 1995 a billing code existed (Current Procedural Terminology [CPT]) only for bilateral lower extremity venous ultrasound. In 1995, a code for unilateral, or "limited," ultrasound examination of the lower extremities was provided (CPT 93971). Also, in 1995 the Intersocietal Commission for Accreditation of Vascular Laboratories (ICAVL) acknowledged the need for limited unilateral studies with the publication of

revised guidelines. 2 Until that modification was made, ICAVL indicated that an examination of a symptomatic leg also required a study of the asymptomatic leg. Controversy exists regarding the importance and frequency of finding clot in the asymptomatic leg. Historically, the literature has indicated that the asymptomatic leg does not harbor clot. 3-s More recently, there have been articles suggesting that clot can be found in the asymptomatic leg, but this finding occurred in fewer than 1% of patients with a negative evaluation of the symptomatic leg. 6 Certainly, finding clot in the asymptomatic leg of a patient with clot in the symptomatic leg does not alter treatment. The likelihood of finding clot solely in the asymptomatic leg is between 0% and 1%. This finding suggests that such a low frequency of clot does not justify a routine evaluation of the asymptomatic leg in a patient presenting with unilateral extremity symptoms. BILATERAL LEG SYMPTOMS: ROLE OF BILATERAL VENOUS EXAMINATIONS

In the era of venography, a patient presenting with bilateral leg swelling or bilateral leg pain would have been handled with detailed clinical evaluation and probably with a contrast venogram of only one extremity. Sheiman et al 7 have suggested that most patients with bilateral leg symptoms have cardiac disease or peripheral vascular disease as the dominant cause of leg swelling. A

From the Department of Diagnostic Imaging, Rhode Island Hospital, Providence, RI. Address reprint requests to John J. Cronan, MD, Department of Diagnostic Imaging, Rhode Island Hospital, 393 Eddy St, Providence R102903. Copyright © 1997 by W.B. Saunders Company 0887-2171/97/1801-000555. 00/0

Seminars in Ultrasound, CT, andMRI, Vol 18, No 1 (February), 1997: pp 33-38

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recent article suggested that a significant percentage of patients with bilateral swelling harbor clot, but close reading of this report indicates that many of these patients had significant risk factors for clot formation6; nonetheless, it is important to consider risk factors. If a patient has significant underlying risk for DVT, such as malignancy, then the bilateral examination should be done. If there are no risk factors for DVT, the first assumption should be that the patient has cardiac disease or chronic peripheral vascular disease as the etiology of the bilateral leg swelling. It has been estimated that 96% of patients with DVT have one or more recognized risk factors, s The probability of finding clot in the leg is directly related to the presence of risk factors for DVT. In the absence of any DVT risk factors, it is unusual to find extremity clot in patients with bilateral swelling.

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EXTENT OF THE ULTRASOUND EXAMINATION: HOW MUCH OF THE LEG SHOULD BE STUDIED?

The standards of the American College of Radiology (1993) and ICVAL state that the venous system of a symptomatic extremity should be evaluated with ultrasound, as continuously as possible, from the level of the inguinal ligament to the popliteal fossa (Fig 1).2 Recently, it has been shown that symptomatic patients usually have a lengthy continuous clot that often involves multiple venous segments. 9,1° This type of clot is different from clot that develops in asymptomatic high-risk patient; in these patients clot often forms focally on valve cusps in the calf and, more frequently, involves short discontinuous segments. 11This observation was based on a retrospective review of venograms and has been confirmed in the author's ultrasound laboratory: in approximately 99% of symptomatic cases, aboveknee clot can be detected through evaluation of the femoral and popliteal veins, employing the twopoint compression technique. 12 This two-point method requires only the evaluation of the common femoral and popliteal venous areas (Fig 2). Compared with continuous compression ultrasound (groin to popliteal fossa), the overall decrease in the examination time slightly exceeds 50%. The potential of the two-point technique has been confirmed by others who have shown that approximately 95.4% of femoropopliteal clots are detected using a

Fig 1, Traditional compression ultrasound examination interrogates the deep venous system from the unguinal ligament to the popliteal trifurcation. (Reprinted with permission from Radiology, Pezullo et al, 1996, vol 198, pp 67-70. Radiological Society of North America. TM)

2-point method. 13Obviously, there is some compromise between simplicity and accuracy with the 2-point technique. This limited compression technique is not the accepted standard, but it has utility in emergency departments or in the evaluation of patients with extremely restricted mobility.

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cases, the need for follow-up studies, to detect possible propagation of clot from the calf to the popliteal vein, is somewhat less well defined; however, the evidence suggests that if the patient remains symptomatic, a repeat study should be performed 3 to 5 days after the initial examination. 15 As described below, the ability to examine the calf veins directly is a consideration in the need for re-examination. CALF VEIN THROMBOSIS

Fig 2. Limited two-point compression study evaluates the venous system focally in the common femoral and popliteal regions. (Reprinted with permission from Radiology, Pezullo et al, 1996, vol 198, pp 67-70. Radiological Society of North America. 121

SIGNIFICANCE OF A NEGATIVE ULTRASOUND EXAMINATION

Evidence exists that a negative compression ultrasound study of a symptomatic lower extremity, using complete evaluation of both the femoral and popliteal veins, provides sufficient documentation to withhold anticoagulation therapy. 14 In these

The acceptance of ultrasound as a diagnostic technique in the evaluation of the symptomatic patient was based on clinical series in which only the femoral and popliteal veins were examined, and in which direct evaluation of calf veins was not attempted. It was in this examination format that the compression ultrasound technique was acknowledged to be clinically useful. Overall, clot is isolated to the calf veins in approximately 50% of patients with acute DVT; however, the incidence of isolated calf clot is much higher in asymptomatic patients and was reported to be as high as 88% in one series. 11 The presence of calf clot is not likely to lead to clinically significant pulmonary embolization (PE), as noted by Moser and LeMoine.16 In contrast, patients with above-knee clot have pulmonary emboli in more than 50% of cases, even though they may not have clinical signs or symptoms of PE; in such cases the only evidence of PE is via a ventilation/perfusion (V/Q) scan or pulmonary arteriography. If clot is isolated to the calf veins, it is recognized that upward propagation (popliteal vein or above) occurs in approximately 20% of casesJ 7 This propagation of clot can be detected if serial ultrasound studies are performed at 3- to 5-day intervals. I8 However, based on increased technical success in evaluating the calf veins, there is an impetus to evaluate the calf veins directly. Clinical series have suggested that ultrasound evaluation of the calf veins in both symptomatic and asymptomatic patients is as accurate as evaluating above-knee veins in cases in which the veins can be seen adequately, i9 Another issue that may strongly endorse the importance of evaluating the calf veins is chronic venous insufficiency. The great majority of venous valves are located in the calf. Clot in the calf veins may destroy the valves and cause chronic venous

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insufficiency. Logic thus strongly suggests direct evaluation of the calf veins to search for clot. Detection of clot permits treatment to be initiated that might limit the destruction of venous valves. COMPRESSION ULTRASOUND CAUSING PULMONARY EMBOLUS

Compression ultrasound has the potential to break off clot in the femoral vein and cause PE. In the past this consideration has been theoretical; however, several recent reports have noted PE subsequent to diagnostic compression ultrasound. 2°,21 The temporal relationship of the ultrasound examination to the actual occurrence of PE is somewhat uncertain in these reports. This finding is particularly noteworthy, given the fact that patients with above-knee DVT have clinically suspected PE in more than 50% of cases. 22 It should be noted, however, that the risks associated with compression ultrasound are quite small if one takes care to avoid excessive venous compression and manipulation of the vein beyond that necessary for diagnosis. DVT IN OCCULT MALIGNANCY

Trousseau's sign concerns hypercoaguability associated with cancer and is based on the finding of spontaneous thrombosis in patients with underlying malignancy. 23 When patients present with DVT and have no known risk factors, there is an underlying concern that they may indeed have an occult malignancy. Several published series have looked at this issue and observed a 23% to 24% incidence of cancer developing in patients who lack any apparent cause for venous thrombosis. 24-27 If a patient presents with recurrent episodes of DVT and has no known risk factors, the risk of an underlying malignancy is increased further. DVT associated with malignancy tends to be much more extensive and aggressive than DVT in the nonmalignant setting. The clinical examination shows an extremity that is very swollen and painful. Controversy exists regarding whether a diagnostic workup should be initiated after documentation of DVT in a patient with no risk factors. 28 There is evidence to indicate that a search for occult malignancy is not valuable; alternatively, there is a strong opinion that an aggressive workup is warranted in these situations. Patients with DVT related to Trousseau's syndrome manifest the cancer clinically within 1 to 2 years. Malignancies associated with

venous thrombosis may originate in the breast, the gastrointestinal/gastrourinary tract, the lung, or the brain. ULTRASOUND FOR ASSESSMENT OF PULMONARY EMBOLUS

Clinicians have become reluctant to perform pulmonary angiography to confirm or exclude the presence of PE. The workup of PE thus often stops after an indeterminate V/Q scan. 29 Nearly 75% of patients, after a V/Q scan, do not fit either a normal or high-probability category for PE, and thus the diagnosis of PE remains uncertain. Because most PEs are thought to originate from the lower extremities, a teleologic assessment with noninvasive venous imaging has been used to clarify an indeterminate lung scan or confirm a clinical impression of PE. 3° By establishing the presence of clot in the lower extremity, adequate therapy can be initiated, because the treatment for DVT and PE is essentially the same, ie, anticoagulation therapy. 31,32 It has been observed for several decades, however, that even when studied with bilateral venography, nearly one third of patients who have documented PE do not show any clot in the lower extremities. 33 Hence, a negative, noninvasive, venous examination certainly cannot exclude PE. This concept is important to convey to clinicians, because physicians should continue to consider the diagnosis of PE after a negative noninvasive study if there is a strong clinical suspicion for PE. Recently, spiral CT has been proposed in the workup of PE in conjunction with compression ultrasound. 34 The suggestion has been made that if a patient presents with symptoms of deep venous thrombosis and PE, ultrasound of the lower extremities should be obtained first. If the ultrasound examination is positive for clot, treatment of venous thromboembolic disease should be initiated. A negative venous ultrasound study would permit anticoagulation therapy to be withheld, pending additional diagnostic tests. Alternatively, if the patient presents with symptoms of PE, a spiral CT might be done initially; if that test were negative, ultrasound could then be done to determine whether there was clot in the lower extremities. Either a spiral CT scan showing pulmonary artery clot or a compression ultrasound study showing lower extremity DVT would permit treatment to be started. Negative studies would permit anticoagulation therapy to be

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withheld. Clearly, there is a move to reassess the diagnostic methods of evaluating for PE. This new approach, using spiral CT and compression ultrasound, indicates that clinicians are searching for an alternative to pulmonary angiography and V/Q scanning. CONFUSING TERMINOLOGY

Use of proper anatomic terminology in the venous system is important, because many primary care physicians misconstrue examination reports written by radiologists. Radiologists commonly identify the vein below the bifurcation of the common femoral vein as the superficial femoral vein. This terminology creates confusion because the superficial femoral vein actually is a component of the deep venous system, and a clot in this location requires treatment. By referring to this venous segment as superficial, a clinician unfamiliar with the terminology might assume that the vein is not important, because it is "superficial," and thus might not initiate treatment) 5

The suggestion has been made that the common and superficial femoral veins be called the femoral vein over their entire length. A permissible alternative would be to refer to that portion of the femoral vein above the inguinal ligament and before the bifurcation as the common femoral vein. This is certainly worth considering, because it would be unfortunate for a casual definition of venous anatomy to prevent a patient from being treated appropriately.

OTHER ISSUES Several issues about venous ultrasound that are clinically important and somewhat controversial have been presented. It is important to realize that this discussion is not exhaustive, and that many other issues could be discussed. However, despite these issues, ultrasound for evaluation of the venous system, particularly in the acute situation, remains the dominant venous diagnostic technique employed in the United States today.

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causing acute pulmonary embolism: A brief report. J Vasc Surg 15:1082-1083, 1992 22. Huisman MV, Btiller HR, Ten Cate JW, et al: Unexpected high prevalence of silent pulmonary embolism in patients with deep venous thrombosis. Chest 95:498-502, 1989 23. Silverstein RL, Nachman RL: Cancer and clotting-Troussean's warning. N Engl J Med 327:1163-1164, 1992 24. Goldberg RJ, Seneff M, Gore JM, et al: Occult malignant neoplasm in patients with deep venous thrombosis. Arch Intern Med 147:251-253, 1987 25. Aderka D, Brown A, Zelikovski A, et al: Idiopathic deep vein thrombosis in an apparently healthy patient as a premonitory sign of occult cancer. Cancer 57:1846-1849, 1986 26. Monreal M, Lafoz E, Casals AN, et al: Occult cancer in patients with deep venous thrombosis. Cancer 67:541-545, 1991 27. Prandoni P, Lensing AWA, Biiller HR, et al: Deep-vein thrombosis and the incidence of subsequent symptomatic cancer. N Engl J Med 327:1128, 1992 28. Prins MH, Lensing AWA, Hirsh J: Idiopathic deep vein thrombosis. Is a search for malignant disease justified? Arch Intem Med 154:1310-1312, 1994 29. Stein PD, Hull RD, Saltzman HA, et al: Strategy for

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diagnosis of patients with suspected acute pulmonary embolism. Chest 103:1553-1559, 1993 30. Killewich LA, Nunnelee JD, Auer AI: Value of lower extremity venous duplex examination in the diagnosis of pulmonary embolism. J Vasc Surg 17:934-939, 1993 31. Smith LL, Iber C, Sirr S: Pulmonary embolism: Confirmation with venous duplex US as adjunct to lung scanning. Radiology 191:143-147, 1994 32. Rosen MP, Sheiman RG, Weintraub J, et al: Compression sonography in patients with indeterminate or low-probability lung scans: Lack of usefulness in the absence of both symptoms of deep vein thrombosis and thromboembolic risk factors. AJR Am J Roentgenol 166:285-289, 1996 33. Hull RD, Hirsh J, Carter CJ, et al: Pulmonary angiography, ventilation lung scanning, and venography for clinically suspected pulmonary embolism with abnormal perfusion lung scan. Ann Intern Med 98:891-899, 1983 34. Goodman LR, Lipchik RJ: Diagnosis of acute pulmonary embolism: Time for a new approach. Radiology 199:25-27, 1996 35. Bundens WP, Bergan JJ, Halasz NA, et al: The superficial femoral vein. A potentially lethal misnomer. JAMA 274:12961298, 1995