A Common Bug, an Uncommon Road to Diagnosis

A 71-year-old man presented with encephalopathy and worsening tremors for 5 months. He also reported a 25-pound unintentional weight loss over the past 6 months. He had a history of paroxysmal atrial fibrillation with an ablation procedure 3 months prior to presentation but no other surgical or medical history. He denied chest pain, palpitations, hemoptysis, history of seizures, chemical exposures, recent travels, or exposure to deserts, caves, or tuberculosis. Physical examination revealed right-sided facial weakness and right arm tremor that worsened with movement. Lumbar punctures showed a reactive pleocytosis without malignant cells. Magnetic resonance imaging (MRI) of the brain revealed bilateral frontoparietal paramedian leptomeningeal fluid-attenuated inversion recovery (FLAIR) hyperintensities and enhancement, concerning for central nervous system infection versus leptomeningeal carcinomatosis versus inflammatory processes (Figure 1A).  An electroencephalogram (EEG) was unremarkable, and blood cultures were negative throughout the workup. Finally, dural biopsy showed abscesses and inflammation with eventual cultures growing Cutibacterium acnes. The patient was started on penicillin and ceftriaxone and underwent a transesophageal echocardiogram, which was concerning for aortic valve endocarditis. Magnetic resonance imaging after a few days of antibiotics showed decreased bilateral frontoparietal leptomeningeal fluid-attenuated inversion recovery hyperintensity and leptomeningeal enhancement (Figure 1B). The patient reported improved tremor after the 6-week antibiotic course and physical therapy.

C. acnes (formerly known as Propionibacterium acnes) is a slow-growing, pleomorphic gram-positive rod that is widely found within skin flora. It lies deep within the pilosebaceous glands, particularly on the scalp and face, and can recolonize within 4 hours of surgical skin decontamination. Though the presence of C. acnes in cultures is frequently dismissed as a contaminant, C. acnes has been shown to be the culprit in serious illnesses, most commonly after endovascular, neurosurgical, and orthopedic procedures.

Most relevant to our case, there have been previously described case reports of brain abscesses secondary to C. acnes. Usually these occur after neurosurgical instrumentation, though seemingly de novo or spontaneous C. acnes abscesses have also been observed. Our patient's concomitant aortic valve vegetation suggests that his brain abscesses and leptomeningitis were likely secondary to embolic spread from endocarditis because C. acnes is a known source of endocarditis. The etiology of the patient's endocarditis is not clear, though prior literature has shown that endocarditis is a rare complication of atrial ablation, which our patient had 3 months prior to presentation.

Systemic infection due to C. acnes is not an easy diagnosis because blood cultures can take up to 6 to 14 days to yield positive growth. Additionally, the Gram stain is an unreliable diagnostic marker in clinically significant C. acnes infections, especially in the presence of heavy inflammation; detection rates have been reported to be as low as 10.5%. Lastly, the clues to the diagnosis may not be immediately apparent in the patient's history because the instigating event may have been a remote procedure.

In this report, we present a rare case of leptomeningitis secondary to C. acnes with concomitant endocarditis. Ultimately, the key to this difficult diagnosis was a dural biopsy.  A diagnosis of C. acnes infection should be considered in patients with an undiagnosed inflammatory illness, especially in the setting of recent procedural or surgical instrumentation.