Mycotic visceral aneurysm complicating infectious endocarditis: Imaging diagnosis and follow-up.

One of the rarest complications of endocarditis, infected (mycotic) aneurysms result from haematogenous dissemination of septic emboli and occur more frequently in patients with cardiac valvular abnormalities or prosthetic valves, intravenous drug abuse, diabetes and immunosuppression conditions such as HIV infection. Although often clinically unsuspected, mycotic aneurysms are potentially life-threatening because of disseminated sepsis and propensity to rupture. Contrast-enhanced multidetector CT provides prompt detection, characterization and vascular mapping of these lesions, allowing correct planning of surgical or interventional therapies and reproducible follow-up. Because of their characteristically unpredictable behaviour, mycotic aneurysms may undergo spontaneous thrombosis, size reduction, rapid enlargement or rupture, therefore strict imaging surveillance with CT and/or color Doppler ultrasound is necessary.

CASE DESCRIPTION

A 48-years-old male with recent prosthetic replacement of the mitral and aortic cardiac valves three months earlier, was hospitalized with clinical and laboratory evidence of sepsis. Transthoracic echocardiography detected a vegetation consistent with infectious endocarditis of the mitral prosthesis, but the fever persisted despite intensive antibiotic therapy.

Total-body, contrast-enhanced, computed tomography (CT) requested to investigate possible sites of infectious dissemination detected a small metastatic brain abscess and maxillary sinusopathy, excluding lung septic emboli. Incidentally, a saccular aneurysm with a 3-cm maximum diameter and minimal peripheral thrombosis was detected, originating from the distal branches of the superior mesenteric artery [Figures 1a and b].

Figure 1

Axial (a) and sagittal (b) Maximum Intensity Projection (MIP) algorithm CT-angiographic images from initial total-body CT, show a roundish 3-cm aneurysm, originating from the distal branches of the superior mesenteric artery (arrowheads). Repeat CT shows a persistent mesenteric aneurysm, with internal hyperdensity, on an unenhanced image (c) and absent contrast perfusion during a vascular enhanced study (d). Color-Doppler ultrasound (e) confirms spontaneous aneurysmal thrombosis, with internal echogenicity and absent flow signals. The lesion completely disappeared on the six-month CT follow-up (f)

After blood cultures disclosed Aspergillus flavus and Nocardia spp systemic infection, sepsis was successfully treated with intravenous cotrimoxazole plus antimycotic drugs. Two weeks later, a repeat CT-angiography showed a persistent mesenteric aneurysm, with unchanged size, increased basal attenuation, and absent contrast medium perfusion [Figures 1c and d] — findings consistent with its spontaneous thrombotic occlusion. A Color-Doppler ultrasound confirmed a completely thrombosed aneurysm with internal echogenicity and absent flow signals [Figure 1e].

The patient recovered following surgical valvular trigone reconstruction with a pericardial patch. Six months after hospital discharge, a follow-up CT showed complete disappearance of the visceral aneurysm [Figure 1f].

DISCUSSION

Infected (mycotic) aneurysms represent one of the rarest extra-cardiac complications of septic endocarditis, compared to the more frequent involvement of the central nervous system, lungs, spleen, kidneys, and musculoskeletal system. High-risk categories include patients with cardiac valvular abnormalities or prosthetic valves, history of intravenous drug use, diabetes, and immunosuppressive conditions, such as the human immunodeficiency virus infection.[12]

The pathogenesis of mycotic aneurysms involves a hematogenous spread of septic emboli (most, usually due to Staphylococcus aureus) into the vasa vasorum of the aorta, peripheral, cerebral or visceral arteries. The resulting infectious arteritis causes damage and the subsequent contained rupture of the vessel wall, leading to the formation of a saccular luminal outpouching representing a pseudoaneurysm. Mycotic lesions represent only 10% of all visceral aneurysms, most commonly caused by atherosclerosis. The superior mesenteric artery is the most frequently involved visceral vessel, with mycotic aneurysms usually located in the distal arterial bed.[13]

Often, clinically unsuspected in patients with sepsis, mycotic aneurysms may sometimes cause abdominal pain or be appreciated as pulsatile masses. Despite advanced antibiotic therapies, these lesions are associated with significant morbidity and a 50% mortality rate, resulting from both disseminated sepsis and a propensity to rupture, therefore, prompt detection is imperative.[134]

Currently, multidetector CT represents the preferred modality to image patients with sepsis. Allowing a fast panoramic exploration of the systemic arterial bed and production of a detailed multiplanar, and three-dimensional, CT-angiographic reconstructions, the volumetric contrast-enhanced CT allows confident identification, characterization, and vascular anatomy mapping of arterial lesions.[1-35] The usual imaging appearance of a mycotic aneurysm includes a saccular vascular dilatation, sometimes associated with edematous changes in the surrounding tissue planes. The arterial walls may appear indistinct and irregular, whereas, mural calcifications are exceptional compared to uninfected atherosclerotic dilatations.[1-4]

Sometimes incidentally detected mycotic aneurysms may require open surgery, endovascular stent placement or angiographic embolization, medical treatment or a combination of these. Small, non-ruptured aneurysms are managed with intravenous antibiotics for four to six weeks, with imaging surveillance, best performed with repeated CT studies. Alternatively, a Color-Doppler ultrasound allows a non-invasive follow-up, although with limited reproducibility. As demonstrated by this case, a characteristic feature of mycotic aneurysms is their rapid development and unpredictable behavior: These lesions may undergo partial or complete thrombosis as a response to medical treatment, decrease in size or otherwise enlarge. CT-angiography detection of signs of impending or frank rupture, such as, contrast extravasation, consistent with active bleeding, indicate the need for urgent interventional or surgical treatment.[23]