Discussion
Infective endocarditis (IE) is a focal infection of the endocardium that has a high potential for systemic dissemination. In the United States, IE is rare and occurs at a rate of 11.4 per 100,000 adults [7, 8]. Common signs of endocarditis include fever, new or increasing cardiac murmur, and embolic phenomenon, such as Osler nodes or Janeway lesions [9]. Fever, the most commonly associated sign of endocarditis, was notably absent in this patient. Nevertheless, even if a fever was present in this patient it could have been attributed to the more evident diagnosis of osteomyelitis. This underscores the importance of a thorough evaluation, especially in patients with accompanying risk factors. IE risk factors include a past history of intravenous drug abuse, embedded medical hardware, aberration in heart structure, and immunosuppression [6, 10, 11]. While this patient denied intravenous drug use, the use of a dialysis catheter in combination with immunosuppression by virtue of uncontrolled diabetes increased the overall risk of bacterial growth and subsequent spread [12].
The presence of structural heart defects and bacteremia should raise the index of suspicion for IE, but not all defects are outwardly evident. In this case, the patient did not display overt signs of structural heart defects, such as an audible murmur or patient-reported diagnosis. A broad screening effort included the use of a TTE to rule out the heart as a source of persistent infection. While TTE is less specific for right atrial pathology than transesophageal echocardiography (TEE) due to limitations of probe positioning, TTE remains the preferred initial screening modality [11, 13, 14]. Fortunately, the patient’s embryological eustachian valve remnant was detected on TTE as a potential source of seeding infection, thus there was no need to pursue a TEE. This remnant is present in 2-4% of the adult population [15]. The eustachian valve functions in-utero to direct vital oxygenated blood from the inferior vena cava (IVC) to the foramen ovale and then to the left atrium.  After birth and closure of the foramen ovale, the eustachian valve regresses, but this regression is variable and can lead to a remnant of the valve located on the superior aspect of the IVC [15, 16]. The remnant is normally of little clinical significance [16, 17]. However, in the presence of bacteremia, the abnormality can contribute to progressive nidus formation and persistent systemic spread.
In the setting of IE, bacterial dissemination to the spine most commonly occurs in the lumbar vertebrae [1]. Vertebral osteomyelitis occurs at a rate of 22.4 per 100,000 adults and should be considered in the presence of severe back pain, cognitive disturbances, and bacteremia [18]. The link between IE and concomitant vertebral osteomyelitis can partially be explained by vascular anatomy. The posterior intercostal artery branches from the descending aorta and feeds blood to the spinal arteries. Then, the spinal arteries supply blood to the vertebrae. This pathway serves as a migratory route for hematogenous spread of bacteria following cardiac colonization in the setting of IE. Contiguous spread to nearby organs and extension of the infection throughout the axial spine can occur. As the disease progresses, worsening bacteremia can result in a pronounced inflammatory state and attenuation of existing renal failure. Increases in interleukin-6 (IL-6) and interleukin-1β (IL-1β) can trigger iron sequestration and shorten the lifespan of erythrocytes leading to anemia as seen in this patient [19]. Further, exacerbation of renal failure can suppress excretion of neurotoxins, such as ammonia, causing cognitive deficits and encephalopathy [20].
Clinical treatment of coexisting IE and vertebral osteomyelitis relies on pathogen-directed therapy. In this case, the presence of MRSA in serial blood cultures limited effective therapy to narrow-spectrum intravenous antibiotics, such as vancomycin and daptomycin. A minimum of six weeks of therapy is recommended, with eight to twelve weeks warranted in treatment-resistant strains [21]. Nevertheless, dose adjustment in the setting of renal compromise is necessary with regular monitoring to detect complications. In this case, after careful dialogue between the patient and care team, the patient was discharged from the care facility in a stable condition with arrangements to return for scheduled intravenous antibiotic infusions. While this management decision may not be warranted in every clinical situation, it was made to balance the patient’s cooperation, autonomy, and perceived quality of life.