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.