Diagnosis of peripheral arterial disease of the lower limb

Abstract

Duplexltrasound is safe, inexpensive, and accurate enough to guide management in most cases

In this week's BMJ, a systematic review by Collins and colleagues compares the diagnostic accuracy of duplex ultrasound, magnetic resonance angiography, and computed tomography angiography for assessing peripheral arterial disease of the lower limb.¹ The review also evaluates the impact of these assessment methods on patient outcomes. It found that contrast enhanced magnetic resonance angiography seemed to be more specific than computed tomography angiography (better at ruling out stenosis of 50% or more in a lower limb vessel) and more sensitive than duplex ultrasound (better at ruling in stenosis of 50% or more). Magnetic resonance angiography was also generally preferred by patients over contrast angiography. So what do these results mean for practising clinicians?

In developed countries up to a fifth of the population over the age of 60 has lower limb peripheral arterial disease, as defined by absent pulses or a reduced ankle brachial pressure index. About a quarter of these people have symptoms—most commonly intermittent claudication. This consists of pain in the leg (usually in the calf) on walking, as a result of atherosclerotic stenosis or occlusion, usually of the superficial femoral artery in the thigh.²

Only a small minority of patients with intermittent claudication undergo imaging with a view to open surgical (bypass, endarterectomy) or endovascular (angioplasty, stenting) intervention. Most claudicants are treated medically in primary³ or secondary care⁴—if they are treated at all.⁵ In contrast, most patients with severe limb ischaemia (rest pain, tissue loss) undergo imaging with a view to interventional treatment, usually by means of bypass surgery or angioplasty.^{6 7}

Imaging studies are of little use in peripheral arterial disease unless intervention is being considered and the imaging results are likely to influence the choice and nature of that intervention. In an era of “high tech” medicine we sometimes forget that the purpose of imaging is not just to obtain pleasing pictures but to answer specific clinical questions that have been thoughtfully framed after undertaking a careful history, thorough examination, and non-invasive assessments.⁸ Not surprisingly, Collins and colleagues found that the availability of appropriate clinical data increased the accuracy and quality of imaging interpretation.

The imaging modality should be carefully chosen, in an evidence based manner, so as to maximise the quality and relevance of information obtained, minimise the risk and inconvenience to the patient, and make the best use of limited resources. But, as Collins and colleagues report, making such a choice can be difficult in people with peripheral arterial disease. They could find few comparative studies and many had serious methodological limitations. Most studies had several potential sources of bias resulting from the nature of the patient population being investigated, the delay between index and reference tests, and the inability to blind observers. Only one study compared patient outcomes. The rest compared diagnostic “accuracy,” which can be hard to define in a clinically meaningful way, especially when data are presented by arterial segment rather than by limb or by patient. Relative sensitivities and specificities, often with wide ranges, for various degrees of arterial stenosis, most commonly 50%—a level of disease with limited biological or clinical relevance—are hard to factor into everyday clinical decision making. In reality, as pointed out by Collins and colleagues, the choice of imaging may be more influenced by patient preference and tolerance as well as the availability of the test.

When a patient with peripheral arterial disease needs diagnostic imaging, it seems sensible to start with the simplest and safest modality, which is undoubtedly duplex ultrasound.^{1 2} Only if this proves insufficient should more sophisticated, potentially risky, and costly tests normally be considered. In practice, this is now unusual given the quality of the machines used and the skill of vascular technologists.

Intra-arterial digital subtraction angiography is the reference standard, but magnetic resonance angiography and computed tomographic angiography can provide more information and can be more accurate than ultrasound.¹ However, in many cases the extra information and accuracy has little effect on patient management and outcome. The only study in the review by Collins and colleagues that compared patient outcomes found no significant difference between duplex ultrasound and intra-arterial digital subtraction angiography.

In summary, the available data,¹ supported by everyday clinical experience, suggest that duplex ultrasound is the only imaging test needed in most patients. If ultrasound is not sufficient, then most clinicians would probably choose magnetic resonance angiography rather than computed tomographic angiography because it is more versatile, more accurate, is not as affected by arterial calcification,^{1 8} and does not involve exposing patients to ionising radiation.^{9 10}

Diagnostic imaging continues to evolve and improve at an astonishing rate. There is a growing consensus that invasive techniques should not be used to visualise the arterial system unless a therapeutic intervention is intended. Thus, diagnostic intra-arterial digital subtraction angiography is likely to become a thing of the past, with open and endovascular treatments for peripheral arterial disease being planned almost exclusively on the basis of duplex ultrasound and, where necessary, magnetic resonance angiography.^{11 12}