Infected abdominal aortic aneurysm graft complicated by lumbar discitis

We present the case of a 68-year-old male, who underwent open abdominal aortic graft in August 2016 owing to a ruptured large infrarenal abdominal aneurysm. He subsequently presented 6 months later with back pain, general weakness, reduced mobility and cachexia. He underwent CT, MRI and fluorodeoxyglucose (PDG)-PET spinal imaging, all modalities showing signs of aortic graft infection complicated by L4/5 discitis. The patient was treated conservatively with intravenous antibiotics and spinal brace support, as his general condition did not allow for surgery. Although he showed initial clinical improvement allowing plans for supported discharge, his improvement was not sustained and he died 4 months after admission.


ClInICal preSentatIon
A 68-year-old male presented as an emergency in August 2016 with a diagnosis of a ruptured infrarenal abdominal aortic aneurysm. He underwent emergency open vascular surgery with the placement of an aortic bifurcated graft with intraoperative right femoral embolectomy. One day post-operatively, he required a left femoral embolectomy. In February 2017-6 months after his surgery-he presented through the emergency department with worsening low back pain, progressive fatigue, decreased appetite and oral intake. He was severely cachexic. His notable comorbidities included chronic renal failure and had a past history of excess alcohol intake.
On admission, he was apyrexial (36.7 C), normotensive (BP 110/70 mmHg) with a normal pulse rate (68 per min). He was severely cachectic weighing 37 kg with a body mass index of 12.4. On palpation of the lumbar spine, there was tenderness without radiation and there were no focal neurological signs in the lower limbs. His breathing sounds were clear and his abdomen was soft with a healthy midline scar of his previous surgery.
Patient was initially managed with intravenous fluids along with optimization of his nutrition including nasogastric feeding with regular follow up by dieticians. A treatment with intravenous pamidronate was initiated to correct hypercalcaemia. A liver screen including a viral hepatitis screen was negative and a magnetic resonance cholangiopancreatography showed a dilated common bile duct and intrahepatic ducts. A prostate specific antigen was done to rule out possible prostate malignancy and bone   metastasis. Myeloma screen was negative. Flexible sigmoidoscopy showed a benign looking polyp with no ulcers or any evidence of malignancy.
An abdominal ultrasound showed normal kidneys and spleen structures and a thrombus in the aortic aneurysm. Later, CT chest, abdomen and pelvis showed soft tissue posterior to the aneurysm as well as periaortic inflammatory process, no malignancy was found ( Figure 1). A diagnostic liver biopsy was considered to rule out possible liver pathologies that potentially explain the elevated liver enzymes but the patient declined owing to possible complications.
After 6 weeks of admission, patient developed hospital-acquired pneumonia and was started on IV tazocin. His bloods showed elevated white blood cells (WBC) of 14.00 10*9 l −1 , CRP of 199 mg l −1 . Although his CRP was initially falling down in the first few days, patient's back pain was progressive and his WBC and CRP climbed up markedly (20.7 10*9 dl −1 , 154 mg l −1 respectively). Of note, patient did not develop high temperature-not once-during his hospital stay. A blood culture was done, one bottle showed positive growth of coagulase negative Staphylococcus. However, repeated blood culture after 48 h was negative.
An MRI (lumbar spine) showed a small abscess collection over the right paravertebral region (Figure 2). Radiological and clinical diagnosis was one of an infected aortic graft and based on microbiology team advice, he was commenced on IV teicoplanin with the addition of rifampicin. Vascular team advised against surgical intervention, whereas neurosurgical team advised continuing intravenous antibiotic treatment and mobilization using Boston brace as able. Patient's general condition was improving gradually; his mobility-on his chair-improved markedly and his infection markers declined on treatment, which was promising to allow for a plan to discharge the patient on long-term oral antibiotics when physiotherapy are happy.
However, follow-up CT (abdomen/pelvis) was done after 10 weeks of antibiotic treatment and showed no improvement of previous findings. Moreover, an FDG-PET scan of lumbar spine showed an increased uptake in L4/5 disc with continuity to the right psoas muscle (Figure 3). Soon patient's condition worsened with a progressive back pain rendering him bedridden. He developed low body temperature of 34.8°C and his WBC and CRP reached (17.1 10*9 l −1 , 73 mg l −1 respectively). A repeated blood culture was negative. Owing to the progressive decline in the patient's general status, despite a long antibiotic course of 15 weeks, he was commenced on an end of life care and died in early July 2017.

ImagIng FInDIngS
Initial CT chest, abdomen and pelvis showed bony erosions of L4/5 with soft tissue posterior to the aneurysm between the psoas muscle and the spine (Figure 1), a follow-up CT scan showed no change in findings.
MRI lumbar spine showed fluid signal in the disc space L4\5 with loss of height, destruction of the endplate and extending collection in the form a small abscess over the right paravertebral region within the right psoas muscle along with a communication between the aneurysmal sac and L4\5 disc (Figures 2, 4

and 5).
FDG-PET scan showed intense FDG uptake within the L4/5 disc with a continuity with increased activity extending both superiorly and inferiorly along the anterior and medial aspect of the right psoas muscle along with increased uptake within the aneurysmal sac (Figure 3).
outCome Sadly, he presented severely cachectic with acute chronic renal failure, and despite the concerted efforts to manage his condition conservatively, he declined and died.

DISCuSSIon
Open surgical aortic grafting is one of the modalities used for the treatment of abdominal aortic aneurysms. One of the significant complications following surgery is the infection of the graft and/or the aneurysmal wall. Although, graft infection following aortic aneurysm repair is uncommon 1 and continuous spread of infection to the spine is rare, 2,3 it needs prompt and urgent management as it is threatening and may lead to graft/arterial interface disruption, haemorrhage, or sepsis. 4 For a proper diagnosis, it is mandatory to do clinical examination, blood tests (CRP, WBC counts) and radiological investigations. As cultures of blood or even samples collected from the infected field are sometimes negative (up to 33% of cases), 4 in those circumstances, diagnosis of graft infections are ultimately based on clinical and radiological evidence. Clinical manifestation of aortic graft infection with discitis is variable and lack specificity. 5 Some in vitro reports have suggested that antimicrobial coated vascular grafts may protect against infection by organisms which can perioperatively colonize the vascular graft, but may not cause clinical findings of infection until a long time after, 6 which could be the reason of the late presentation of our patient. Low-grade infection, which occurs in one-third of total patients (chronic sepsis) 4 is caused by low-virulent bacteria and occurs generally late after the original graft implantation. Usually, these patients suffer from non-specific symptoms such as weakness, weight loss, and malaise; potentially leading to delayed diagnosis. Our patient had non-specific symptoms related to ongoing low-grade infection. Owing to the fact that his blood cultures were negative on two separate occasions and patient refused sampling from the infection field, it was mandatory to ascertain the diagnosis using different radiological investigations.
CT scan is the most commonly used modality for the diagnosis of aortic graft/aneurysmal wall infections. Potential signs of graft infection on CT scan are perigraft fluid, perigraft soft tissue attenuation, ectopic gas and pseudoaneurysm. Yet, perigraft fluid and soft tissue attenuation are not specific for graft infection, as they might arise from chronic postoperative inflammatory process.