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Appearance: The nubbin sign is a focal outpouching of contrast usually located along the anterior portion of the ascending aorta. It appears as a small, projecting nodule with a smooth rounded contour.
Explanation: A nubbin is defined by the Merriam-Webster dictionary as a small, usually projecting part or bit, which is stunted, undeveloped, or imperfect. The nubbin sign refers to a completely occluded saphenous vein graft (SVG) which when visualized on computed tomography angiographic (CTA) imaging, appears as a focal outpouching of contrast (Fig. 1A), creating the appearance of a high-density nubbin.1 (All references cited in this article can be found at http://links.lww.com/JTI/A44.) The proximal portion of the graft, near its origin and attachment to the ascending aorta, is often the site of occlusion and is depicted optimally on three dimensional reconstructed images (Fig. 1B).
Discussion: SVG’s were first used as a coronary artery bypassing channel in 1964.2 In this procedure, veins are harvested from the legs and attached proximally to the ascending aorta and distally to the coronary artery beyond the point of obstruction. They have been shown to offer several advantages over internal mammary artery (IMA) grafting including availability, accessibility, ease of harvest, and resistance to spasm.1 However, several post operative complications have been documented, most notably vein graft occlusion. Recently, multidetector computed tomography (MDCT) has been used to evaluate graft patency, supplementing conventional angiography as an important tool in assessing graft status.3
The nubbin sign was first described by Jeudy et al. as an important CTA sign for proximal occlusion of an SVG.4 Complications related to SVG bypass grafts can occur early or late in the postoperative period. Early graft occlusion during the first postoperative month is usually due to thrombosis caused by endothelial injury during surgery with ensuing platelet dysfunction.5,6 Patient hypercoagulability, the inherently weaker antithrombotic properties of a venous graft and mechanical stretching of the vessel wall all contribute to endothelial damage and thrombus formation.7 Late graft occlusion after the first postoperative month is principally due to vessel arterialization caused by systemic blood pressures, with resultant atherosclerotic narrowing and eventual thrombosis.5,6 One study showed occlusion rates of 12% and 25% at early postoperative and at 5 year follow up, respectively.5
Recognizing the appearances of graft complications is an essential part of the radiologist’s interpretation of post bypass CT imaging. However, other entities can be confused with graft occlusion, including oversewn aortic graft side branches, pledgets from prior cannulation sites and pseudoaneurysms.8 These can often be differentiated from bypass graft occlusions based on history, prior surgical procedures, associated high density on non-contrast CT images and a flatter, irregular appearance of aortic graft post-surgical material. Occasionally, a thrombosed graft can be seen as an intermediate density thread distal to the nubbin.
Graft thrombosis resulting in symptoms of ischemia often occurs in degenerated venous grafts with advanced atherosclerotic disease. Prevention of graft occlusion and management strategies include smoking cessation, lipid-lowering drug therapy and anti-thrombotic agents such as aspirin and warfarin.6 However, prompt recognition and correct interpretation of this important finding is imperative as it can lead to early intervention with potentially life-saving benefits.