Complex Aorto-Spinal Fistula With Multifocal Abscesses in an Immunosuppressed Elderly Man Following Endovascular Aortic Repair

Introduction

Endovascular aneurysm repair (EVAR) is a well-established treatment for abdominal aortic aneurysms (AAAs), offering improved safety compared to open repair in elderly patient [1]. However, EVAR is associated with complications, including endoleak (with an incidence rate of approximately 30%) and graft infection (with an incidence rate of 0.6% at a mean 25 months postoperatively), as well as contrast nephropathy, local wound complications, and ischemic-related complications [2–4]. The prognosis of endoleak can vary based on which type is present (there are 5 types), but over 50% resolve spontaneously and less than 10% require surgical repair [3]. Infection is a more serious complication, in which surgical explantation is associated with a 27% mortality at 30 days and 46% mortality overall [2]. Diagnosis of infection can include clinical symptoms, a variety of radiographic imaging modalities, routine laboratory testing, and interventional radiologic testing [5]. Staphylococcus aureus has traditionally been the most commonly identified organism, along with other gram-positive microbes, but multidrug-resistant organisms are an increasing concern, having risen by 67% globally in the past 2 decades [6]. EVAR infections with methicillin-resistant species, Pseudomonas aeruginosa, and polymicrobial cultures have become increasingly common [7]. The risk of infection increases when an endoleak is present [7].

Abscess formation after EVAR is rare, with few cases reported [8–10]. Image-guided aspiration has been performed successfully, although in 1 case there was recurrence, necessitating open drainage [9]. Surgical procedures to explant the graft or wash out the infection, in conjunction with broad-spectrum antibiotics with transition to oral outpatient antibiotics, have been successful, with no recurrences. Linn et al described a psoas abscess and aorto-enteric fistula in an immunocompromised patient with Mycobacterium avium complex [11]. Surgical washout, fistula repair, and maintenance antibiotic therapy were effective in controlling the infection. Similarly, White et al reported an aorto-enteric fistula with polymicrobial infection successfully treated with graft explant, repair, and prolonged antibiotic therapy [12].

Here, we report multiple abscesses involving the psoas muscles, epidural space, and disc space, with communication to the aortic sac, in an immunocompromised patient with polymicrobial infection after EVAR complicated by endoleak. To the best of our knowledge, no prior EVAR infection has been reported in the literature that caused multifocal abscesses with communication with the disc space, posing a unique challenge given the multidisciplinary approach required to treat this condition. This case report presents a potential approach clinicians could use to treat a rare but serious complication in EVAR.

Case Report

INITIAL PRESENTATION:

A 74-year-old male nonsmoker with a history of hypertension, hyperlipidemia, chronic kidney disease, and asymptomatic aortic stenosis presented with 6 months of progressive low back and bilateral hip pain. He had previously undergone EVAR for an asymptomatic infrarenal AAA measuring 8.6 cm anteroposteriorly by 9.6 cm transversely. The procedure was complicated by type 1b endoleak of the right common iliac artery, requiring further intervention with stenting 4 weeks later.

Shortly after stenting, the patient developed progressive back and bilateral hip pain, intermittent fevers, and elevated inflammatory markers. Given these symptoms, along with inconclusive repeat outside imaging and a family history of rheumatologic disease, he was presumptively diagnosed by an outside rheumatologist with polymyalgia rheumatica. Initially, 35 mg of prednisone daily provided some relief; however, attempts to taper the dose exacerbated his symptoms. This led to an increase in prednisone to 60 mg daily as well as adding 200 mg of sarilumab every 2 weeks. His symptoms persisted, resulting in multiple emergency department visits and a hospital admission for pain management. Subsequent imaging was inconclusive for signs of infection or postoperative complications.

Six months following endoleak repair, he presented to an outside emergency department, where lumbar magnetic resonance imaging (MRI) demonstrated discitis and osteomyelitis at L4–L5 with an anterior epidural abscess (1.0×1.9×11.6 cm) and bilateral psoas abscesses. He underwent decompressive laminectomy and abscess evacuation. Intravenous vancomycin (1750 mg twice daily) and aztreonam (1 g every 8 hours) were initiated, as he had allergies listed to penicillin and cephalosporins. Cultures grew Cutibacterium acnes and methicillin-sensitive Staphylococcus aureus, and aztreonam was discontinued in favor of rifampin (300 mg daily). Blood cultures were also positive for C. acnes. Follow-up imaging showed a new fluid collection in the surgical bed at L5-S1, but no further surgeries were performed.

DIAGNOSTIC WORK-UP:

The patient arrived to our emergency department via private ambulance directly from the outside hospital in severe pain with limited mobility. He was afebrile, with a normal leukocyte count, but his inflammatory markers were markedly elevated. Differential diagnosis of his presentation included intra-abdominal abscess, psoas abscess, osteomyelitis, and discitis. A computed tomography (CT) scan revealed a posterior epidural collection (4.5×5.1 cm) in the surgical bed, fluid-filled L4–L5 disc space communicating with the AAA sac via a 1.2×1.6-cm opening, and abscesses in the bilateral psoas and posterior paraspinal muscles (Figures 1–3). Intravenous (i.v.) daptomycin was initiated at 8 mg/kg for 6 weeks and an interventional radiologist placed a 10.2F multipurpose drain for the epidural abscess, from which cultures eventually grew C. acnes.

SURGICAL MANAGEMENT:

A vascular surgeon performed EVAR explant through a retroperitoneal approach, aortobiiliac reconstruction with rifampin-soaked Dacron grafts, surgical debridement, left psoas abscess drainage, and placement of vancomycin, gentamicin, and daptomycin antibiotic beads into the L4 disc space. Neurosurgery evaluated the obliterated disc space intraoperatively; however, given the infected surgical field, it was felt that placing neurosurgical hardware would not be beneficial. The patient tolerated the procedure and was transferred to the intensive care unit, receiving 5 units of packed red blood cells due to large-volume blood loss.

POSTOPERATIVE PERIOD:

Postoperatively, a rheumatologist from our facility ruled out polymyalgia rheumatica, indicating an initial misdiagnosis, which led to a permanent discontinuation of sarilumab and a corticosteroid taper. His postoperative course was notable for a thrombus in the right internal iliac vein, for which high-intensity heparin was initiated. Blood cultures remained negative, and he was discharged to inpatient rehabilitation with a 6-week course of intravenous daptomycin with plans to transition to oral doxycycline.

The patient was doing well until 2 months postoperatively, when he presented with back pain and was found to have new type Ia endoleak necessitating further EVAR repair and prophylactic i.v. ceftriaxone. He recovered from this procedure and is currently doing well based on the last visit nearly 8months following his initial presentation to our facility (Table 1).

Discussion

Our case demonstrates an unusual and severe complication of EVAR, emphasizing the high index of suspicion required when evaluating such a patient and the need for comprehensive source control. Infection following EVAR is a rare but potentially devastating complication with a pooled mortality rate of approximately 46% [2]. Surgical explant of the graft with debridement and perioperative and postoperative antibiotics is recommended [13]. Risk factors for infection include reintervention and corticosteroid use, both of which were present in our case. Endoleaks have been associated with a higher rate of graft infections following EVAR [14].

To the best of our knowledge, this is the first report of a patient with previous EVAR who developed a communicating abscess between the AAA sac and a disc space, as well as diffuse abscesses throughout adjacent tissue. It is uncertain when our patient developed this infection, although we suspect it occurred immediately following and as a result of the endoleak repair, as this is when his symptoms began and his inflammatory markers became persistently elevated. Our patient’s back imaging over the first 5 months following his endoleak repair did not reveal definitive signs of infection, but it is unclear if this was due to the early and occult nature of the infection, the quality of outside imaging machines, or some other cause. It is likely, however, that his condition worsened substantially due to the introduction of corticosteroids and the biologic agent. This ultimately led to the hospital admission prior to coming to our facility, at which time infection was confirmed via MRI. Linn et al similarly reported a devastating AAA infection involving an aorto-enteric fistula in a patient living with HIV, highlighting the increased risk of serious complications in immunosuppressed patients [13]. There also has been reports in the literature for aorto-right atrial fistula in a type A dissection, which emphasizes that our case was likely a rare complication of the endoleak repair [15]. Both sarilumab and corticosteroids pose an increased risk of infection, with the latter being linked to deep surgical site infections following vascular surgery [16,17]. Long-term corticosteroid use is also a known risk factor for deep wound infection following posterior spine surgery [18]. Our report highlights the need for caution in prescribing these medications for patients who recently underwent surgery, particularly without a confirmatory diagnosis.

The pathophysiology of graft infection involves microorganisms colonizing the EVAR stent graft, most commonly due to intraoperative exposure, although seeding of the graft due to a new infection is also possible [19]. Biofilm development impairs host immune clearance and prevents penetration by antibiotics, leading to persistent infection. Abscess formation is common due to local tissue necrosis, which can then erode directly through adjacent structures, as in the prior report of aorto-enteric fistula by White et al [12], forming large fluid collections or sinus tracts. Broadly speaking, biologic immunosuppression contributes to initial and persistent infection by impairing innate and adaptive responses. Corticosteroids downregulate proinflammatory cytokines, preventing immune defenses from effectively clearing invading microorganisms and allowing infection to progress to further tissue necrosis [20]. Sarilumab, an IL-6 receptor antagonist, prevents the effective recruitment and function of neutrophils, similarly impairing the immune response. Of note, Nguyen et al found no increase in post-surgical skin and soft-tissue infections based on whether patients on biologic agents held their medication preoperatively, but the rate of infection nearly doubled with concomitant corticosteroid use [21]. This indicates corticosteroids may be the main driver of immunosuppressive infection, or perhaps there is a synergistic effect between them that needs further elucidation by future studies. The infection was not recognized by imaging until our patient had been on prednisone for approximately 2 months and on sarilumab for 4 weeks.

Spinal infection can occur through 3 main mechanisms: hematogenous spread, direct inoculation (eg, surgery), or contiguous tissue spread [22]. Given our patient’s prolonged symptoms and immunosuppressed nature, all 3 are plausible explanations to explain spread to the intervertebral discs. He may have had indolent bacteremia that did not return positive on our blood cultures due to prolonged antibiotic therapy, or the procedure the month prior to evacuate the abscess and decompress the spinal canal could have directly introduced new infection, but we suspect that contiguous spread to the disc space occurred, as evidenced by the aorto-disc connection. The abdominal aorta is positioned only several millimeters from the L4–L5 disc space, and local tissue necrosis due to the persistent inflammatory state likely allowed for the fistula creation, although his prior surgery may have also helped disturb the integrity of the local tissues [23].

Following the patient’s presentation and imaging at our institution, a multidisciplinary team worked to control his symptoms, plan surgical intervention, and optimize the antibiotic regimen. While there is no standard of practice regarding infectious discitis, 6 weeks of i.v. antibiotic therapy is usually recommended, with transition to oral antibiotics afterwards [24]. Use of a rifampin-soaked Dacron-based endograft, which was necessary to reconstruct the damaged aorta, has been reported previously with great success, although larger studies are needed to further assess efficacy and outcomes [25]. Antibiotic-loaded bead placement is an effective method of treatment for patients with discitis [26]. While some reports indicate spinal reconstruction with instrumentation is relatively safe for infected patients to improve spinal stability, this is controversial, and ultimately our team decided not to increase the risk of biofilm adherence given his florid infection and immunosuppression [22,27]. Our team combined these treatments with abscess drainage, debridement, and postoperative broad-spectrum antibiotic therapy to achieve comprehensive source control. Lifelong surveillance and corticosteroid discontinuation will be necessary to minimize adverse events.

There are some notable limitations to this case report. The patient’s delayed diagnosis and nonspecific symptoms made the exact timing and source of infection unclear, and this was not helped by outside imaging that was inconclusive for infection and delayed treatment. Furthermore, while we understand the patient had allergies to penicillin and cephalosporins, we are not sure why vancomycin was selected as an antibiotic of choice given his cultures, so our team switched him to i.v. daptomycin on presentation to our facility.

Conclusions

We present a rare case of multiple abscesses, including discitis communicating with the AAA sac, in a patient who underwent EVAR complicated by endoleak. The patient’s risk for serious infection was increased by long-term use of chronic corticosteroids and interleukin-6 inhibitors for misdiagnosed polymyalgia. This case highlights the need for a high index of suspicion for deep post-surgical infection in patients who have undergone EVAR, reevaluation when early imaging is inconclusive, avoidance of treating presumptive diagnoses when the clinical criteria are unclear, caution in the postoperative use of corticosteroids and immunosuppressants, and comprehensive source control with a multidisciplinary approach in treating complex infectious surgical cases.