Surgical Treatment of Infected Aortic Aneurysms

William J. QQuinones-Baldrich

In 1885 Sir William Osler1 presented a comprehensive analysis of infected aneu-rysms and coined the term mycotic aneurysm specifically to refer to patients who presented with infected aneurysms secondary to endocarditis. Since then, however, the term has been used to refer to infected aneurysms regardless of pathogenesis. The fact that other sources of infection could also cause infection in the arterial wall was suggested by Stangel and Wolfed in 1923,2 describing 30 of 213 patients in whom there was no evidence of bacterial endocarditis and yet they demonstrated infected aneurysms. The potential for bacteremia leading to an infected aneurysm was proposed. In essence arterial wall infection can be caused by bacteremia of any source. It tends to affect more commonly segments of an artery with atherosclerosis or a congenital abnormality. Whether or not an aneurysm forms will depend on the clinical course and the institution of effective antibiotic therapy. When the infection is not treated promptly, continued degeneration of the wall leads to pseudoaneurysm formation and thus the presentation of an infected and/or mycotic aneurysm. Existing aneurysms can also become infected usually in the infrarenal aorta given the higher incidence of aneurysms in that location. Contiguous infection eroding into the aorta can also occur, most often associated with osteomyelitis and/or vertebral or retroperitoneal infection, and present as a mycotic aneurysm. Lastly, posttraumatic infected false aneurysms usually occur secondary to drug abuse or iatrogenic causes, most commonly seen in the femoral, carotid and brachial artery.

In this chapter the term mycotic aneurysm refers to an aneurysm that has formed secondary to infection either from bacterial endocarditis and/or bacteremia of any source. This discussion will specifically exclude bacterial infection with aneurysm formation secondary to primary aortoenteric fistula and/or prosthetic graft infection.

Incidence

Mycotic aneurysms are most common in the femoral artery usually secondary to drug abuse and/or iatrogenic causes. Mycotic aneurysms of the abdominal aorta represent approximately 34% of all reported cases,3 the second most common site. An incidence of 0.85% has been reported in a review of2,585 patients with abdominal aortic aneurysms.4 Interestingly, 50% of these mycotic aneurysms were observed in the perivisceral or thoracoabdominal region.5 In our experience, six of 12 patients with rupture of the perivisceral aorta proved to have a mycotic aneurysm.6 Thus, involvement of the perivisceral aorta with a contained rupture should alert the clinician that a mycotic process may be evolving. Only 31 cases of mycotic perivisceral thoracoabdominal aneurysm had been reported up to 1992.5 In contrast, it is estimated that there are 5.3 thoracoabdominal aneurysms per 100,000 person years and 21.8 abdominal aortic aneurysms per 100,000 person years.8

Bacteriology

The bacteriology of mycotic aortic aneurysms has changed over the years. Although initially the predominant organisms were nonhemolytic Streptococci, Pneu-mococci and Staphylococci, recent reviews since 1965 has suggested that Staphylococcus aureus, Streptococcus and Salmonella are the predominant organisms.3,5 In 1984 Brown3 reported that Staphylococcus aureus and various streptococcal species were found in 37% of infected aneurysms when all types were considered. Gram negative organisms have been reported with increasing frequency. Of particular importance is Salmonella which appears to have a predilection for the arterial wall, particularly when atherosclerotic, and accounts for most cases of microbacterial arteritis. Patients with positive cultures for Salmonella from an infected aneurysm should also have their gallbladder examined as many of them are carriers and thus cholecystectomy should be considered part of the management. Other gram-negative organisms have been reported in much lower incidence including Pseudomonas, Escherichia coli, Proteus, Serratia, Enterobacter, Neisseria, Enterococcus and Bacteroides. The latter is more commonly seen in immunosuppressed patients. Fungal infection has also been reported including Candida species. Importantly, up to 25% of patients will have a negative culture in spite of the presence of all other characteristics suggestive of mycotic process.3 The absence of a positive culture should not deter the clinician from establishing the diagnosis.

Diagnosis

The clinical presentation of patients with mycotic aortic aneurysms is different than those patients presenting with contained rupture of an atherosclerotic aneu-rysm. In our experience,5 we noted that all patients with ruptured atherosclerotic aneurysm presented with symptoms of less than 24 hours duration. In contrast, all patients in the mycotic group had symptoms that ranged between two to six weeks with a mean of 3.5 weeks. History of sepsis was absent in all patients presenting with atherosclerotic aortic rupture whereas two-thirds of patients with mycotic aneu-rysms had history of sepsis. Abdominal, chest and/or back pain was present in all patients regardless of etiology. The age at presentation was similar with a mean age of 73 years in the atherosclerotic group and 74 years in the mycotic group. There were no differences noted in the incidence of cigarette smoking, hypertension or the presence of chronic obstructive pulmonary disease. Coronary artery disease was present in both groups in similar frequency. Previous abdominal aortic aneurysm repair was also present in 50% of the atherosclerotic group and 30% of the mycotic group.

The typical patient with an infected aneurysm presents with no antecedent history of arterial injury, is elderly and usually within the atherosclerotic population. The patient may present with fever of unknown origin. The most common clinical presentation is abdominal pain which occurs in most patients. Fever and leukocytosis can be seen in two-thirds of patients, with positive blood cultures and a palpable abdominal mass occurring in about 50% of patients.5

Physical findings may include a tender palpable pulsatile mass. Some patients may present with other signs suggestive of septic emboli such as petechial, occasionally purulent or erythematous lesions in the lower extremities. The presence of a septic foci should be sought such as bacterial endocarditis, pyelonephritis, osteomyelitis, pneumonia or an intraabdominal source of infection.

CT scan remains one of the more important diagnostic studies in the workup of a patient with a suspected mycotic aneurysm. The presence of a contained rupture in the perivisceral and infrarenal aorta with an adjacent normal segment of aorta, particularly in the absence of calcification, is highly suggestive of a mycotic process (Fig. 20.1). On arteriography mycotic aneurysms tend to be saccular whereas atherosclerotic processes tend to be diffuse (Fig. 20.2). Angiography should be included as part of the workup as it will help plan operative repair. The presence of a normal aorta above and below the area of aneurysmal degeneration supports the diagnosis of a mycotic process. This combined with the clinical presentation should give the clinician a high index of suspicion and thus help in planning therapy.

Other imaging modalities include ultrasound which may help establish the presence of an aneurysmal process but will not be helpful in differentiating between a mycotic and an atherosclerotic process. Although radionuclide white blood cell scans can be helpful when positive in the area of the aneurysm, a negative white cell scan does not exclude the diagnosis of a mycotic process.9

Imaging studies are aimed at establishing the location of the process with important distinctions between the thoracic, visceral and infrarenal aorta. In addition they may assist in establishing the etiology of the infection, identifying a source of sepsis. Bone erosion of the vertebral bodies in the presence of a diffuse aortic aneurysm perhaps with a contained chronic rupture is suggestive of an infected preexisting atherosclerotic aneurysm (Fig. 20.3). On the other hand bony changes consistent with osteomyelitis in the presence of a contained chronic saccular rupture with normal adjacent aorta is suggestive of a mycotic process secondary to primary bone infection.

Management

Proper management of a mycotic aortic aneurysm includes adequate preoperative preparation with control of overwhelming sepsis if present using broad-spectrum antibiotics, adequate hydration and stabilization of vital signs. On occasion, the process may lead to a cardiac arrest in which case immediate resuscitation and operation will be necessary. In the majority of cases, however, patients present with a contained rupture and, therefore, there is enough time to adequately prepare the patient for major surgical intervention.

Preoperative imaging should establish the proximal and distal extent of the mycotic process with important differences noted in the thoracic, visceral and infrarenal location. General principles at operation include preparatory extra-anatomic bypass particularly for infrarenal mycotic aneurysms. Other important principles include

Ruptured Mycotic Aneurysm
Fig. 20.1. CT scan of the lower descending thoracic aorta region showing a contained rupture of a mycotic aneurysm. Note the absence of calcification in the aortic wall which supports the diagnosis of mycotic aneurysm.

proximal and distal control above and below the contained rupture, resection and debridement of the aneurysm particularly if in-line replacement is anticipated. Gram stain and cultures should be obtained understanding that a negative culture does not exclude the diagnosis of a mycotic aneurysm. Revascularization should be extra-anatomic if feasible or in situ if necessary.

Mycotic aneurysms limited to the descending thoracic aorta can be approached through a left thoracotomy through the fifth intercostal space. The patient should be positioned however in a right semilateral decubitus with the hips flattened so that if extension into the retroperitoneum is necessary, it can be handled by extension of the incision, exposing the infradiaphragmatic aorta in the retroperitoneum.

Extra-anatomic bypass can be used for revascularization in the management of a descending thoracic mycotic aneurysm. A right axillofemoral bypass would be preferable so that a left thoracotomy can be carried out without the encumbrance of a bypass graft. Care must be taken during positioning of the patient so that the axillofemoral reconstruction on the right side is not compressed during the operation. Bypass must be performed prior to resection of the mycotic process, otherwise the ischemia time to the kidneys, gastrointestinal organs and lower extremities would be intolerable. In addition, blood flow through an extra-anatomic bypass may be somewhat limited and the patient may experience renovascular hypertension in the postoperative period. In the author's opinion, in-line replacement is the preferred method for revascularization following resection of a descending thoracic mycotic

Aneurysm Aorta Scan
Fig. 20.2. Aortogram of patient with CT scan seen in Figure 20.1. Note normal proximal and distal aorta without calcification or aneurysmal changes. These findings on aortography support the diagnosis of a mycotic process.

aneurysm. It is imperative that the area of the aneurysm be thoroughly debrided and that no purulence be present prior to in-line replacement. In the presence of a mycotic process, reimplantation of intercostals in this region is not advisable, as it would likely include infected aortic tissue increasing the incidence of recurrent infection. Thus the patient should be advised as to the possibility of paraplegia following this intervention. Similarly, utilization of adjunctive procedures to lower the incidence of paraplegia such as spinal fluid drainage should be utilized. On occasion we have routed a descending thoracic graft around the infected aortic bed under a pleural flap to avoid direct apposition of the graft with the infected area (Fig. 20.4).

Mycotic aneurysms involving the visceral segment of the abdominal aorta may be approached through an 11th rib retroperitoneal incision with visceral rotation. If

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