A Dark Horse Diagnosis

Because of increasing reports of antibiotic resistance in GNRs, even in community-acquired infections, it is appropriate to initially treat with a broad-spectrum antibiotic such as a fourth-generation cephalosporin or carbapenem while awaiting identification and susceptibility results to guide definitive therapy. In addition to antimicrobial therapy, treatment of psoas abscess usually requires drainage. Vertebral osteomyelitis from a hematogenous source can often be treated with antibiotics alone, as long as there are no associated complications such as epidural abscess and spine instability. Imaging should be reviewed for pathology of the surrounding structures, and surgical consultation should be obtained.

Neurosurgery, Interventional Radiology, and Infectious Disease services were consulted. Antibiotic coverage was expanded to vancomycin, cefepime, and metronidazole due to the possibility of polymicrobial infection. No surgical intervention was recommended since the abscesses were too small to drain.

The next day, the GNR was identified as Serratia marcescens.

S. marcescens is a widely distributed organism in the environment, but not a common component of endogenous human flora. Serratia is generally considered as an opportunistic nosocomial pathogen. Community-acquired infection with this organism is unusual and implies exogenous acquisition. A careful re-evaluation of exposures, including injection drug use or other parenteral exposures is important to identify the likely source of infection, as these have been previously linked to outbreaks of environmental organisms. Based on the presumed pathogenesis of infection and the initial microbiology suggesting monomicrobial Serratia infection, antibiotics should be narrowed based on the susceptibility results. There is concern that antibiotics might not adequately penetrate the abscesses and result in a lack of clinical improvement and/or lead to the emergence of antibiotic resistance during therapy. This is an important concern with Serratia, which typically harbors an AmpC beta-lactamase that can mediate resistance to broad-spectrum cephalosporins. If medical therapy alone without drainage is planned, short-interval re-imaging is warranted.

Blood cultures from days two and three of hospitalization also grew S. marcescens. No other organisms grew. Based on culture sensitivity data, antibiotics were narrowed to ceftriaxone.

This surprising culture result prompted the medical team to obtain screening laboratory tests for immunocompromising conditions and to revisit the patient’s history. His type 2 diabetes mellitus was well controlled with a hemoglobin A1c of 6.5%. HIV testing was negative. Further questioning of the patient revealed that he had fallen from a truck onto rocks four months prior, injuring his back and hip, but without puncture of the skin or loss of consciousness; he denied recent falls or other injuries but reported significant chronic knee pain. He had not been hospitalized recently. He had never taken corticosteroids or immunomodulatory medications. He continued to deny injection drug use. He did, however, clarify that his work with racehorses, which was originally understood to be a prior hobby, was ongoing, including recent work of cleaning the stables.

The following morning, he experienced confusion, rigors, and hypoxia, which prompted transfer to the intensive care unit (ICU).

Acute worsening during treatment is worrisome, and could be a potential complication of his infection or treatment – or even a separate process altogether. Knee pain in the setting of bacteremia raises the possibility of septic or crystal-induced arthritis and warrants imaging. Confusion and hypoxia might represent secondary sites of seeding from bacteremia (CNS infection and pneumonia, respectively) or manifestations of endocarditis, the latter being unusual for Serratia. An echocardiogram should be obtained. Other neurologic causes, including seizure, should also be considered. Further evaluation by chest imaging and repeat neurologic examination and imaging should be performed. Emergence of resistance during therapy is a theoretical concern with Serratia as an AmpC beta-lactamase-containing organism. While awaiting additional microbiology data, an empiric change to an AmpC beta-lactamase stable antibiotic such as a carbapenem should be made, especially since he has clinically deteriorated on therapy with a β-lactamase susceptible antibiotic, raising concerns of the emergence of resistance on initial therapy.

Antibiotics were changed to meropenem, vancomycin, and metronidazole given the clinical worsening and concerns that this represented infection unresponsive to prior antibiotics. The acute episode resolved spontaneously after one hour. His neurologic examination remained nonfocal. Chest radiography, urinalysis, urine culture, and right upper quadrant ultrasound were unremarkable. Transesophageal echocardiogram revealed no heart valve vegetations. MRI and bone scan of the lower extremities did not show any evidence of septic arthritis or other infection. He remained stable and was transferred out of the ICU the following day. Antibiotic coverage was switched to cefepime. On discussion with his significant other, this event was found to be similar to the intermittent confusion that occurred in the days prior to admission.

The acute onset and other features of these intermittent periods of deterioration are compatible with infection; intermittent seeding of the blood with microbes or their products (eg, lipopolysaccharides) from an abscess or vascular infection could explain these episodes. Some of the previous hypotheses to explain the episodes, such as a secondary infectious process, have not been supported by diagnostic testing or the clinical course. He needs close clinical monitoring and interval assessment of the known sites of infection.

Ten days after osteomyelitis and discitis were diagnosed, the patient developed worsening low back pain, prompting repeat spine MRI. This was significant for bilateral psoas abscess enlargement and extension of osteomyelitis and discitis (Figure 3). He was re-evaluated by Neurosurgery and Interventional Radiology and underwent psoas abscess drainage; abscess cultures grew S. marcescens.