A 51-year-old previously healthy man who had no apparent oral cavity lesions presented with a headache and speech disturbance that had been present for a week. He had no history of trauma, surgery, or medical treatment for any serious illnesses, including immunological disorders. At the time of his visit, he had no fever and no signs of meningeal irritation. Blood sampling at the time of admission showed a white blood cell count of 9470/muL, a C-reactive protein level of 1.54 mg/dL, and an erythrocyte sedimentation rate of 85 mm, indicating mildly elevated parameters, but there were no decreases in complement or antibody levels. Head computed tomography (CT) on admission showed a mass lesion with brain edema in the left frontal lobe [Figure 1a]. He had no obvious symptoms or objective signs of infectious diseases, such as sinusitis, otitis media, odontogenic infections, or meningitis. Gadolinium-enhanced magnetic resonance imaging (MRI) revealed a mass in the frontal lobe with ring-enhancement while diffusion-weighted imaging (DWI) showed significant diffusion restriction, leading to the diagnosis of a brain abscess [Figure 1b-d].
On the day after admission, we drained the abscess through a drainage tube from the lesion cavity and aspirated a light red mucus. Although the patient's blood cultures were negative, Prevotella intermedia was detected in the surgical cultures. Since P. intermedia is an oral endogenous bacterium, a dentist examined the patient, but no noticeable lesions were found in the oral cavity. Contrast-enhanced chest, abdomen, and pelvis CT scans, coupled with transthoracic echocardiography to scrutinize the infection source and possible arterio-venous (AV) shunts, were unremarkable. However, a transesophageal echocardiogram (TEE) bubble study (based on bubbles detected in the left atrium) revealed several reproducible bubbles within a three-heartbeat span in the left ventricular system, suggesting the presence of a Grade 1, mild PFO [Figure 2]. We deduced that the brain abscess developed due to a paradoxical bacterial embolism through the PFO since the causative agent was an oral commensal and there was no further evidence suggesting another etiology for the brain abscess other than through the PFO.
Antimicrobial therapy was initially guided by the literature, with third-generation cephem, vancomycin, and intravenous dexamethasone for cerebral edema. Since anaerobic bacteria were suspected as the causative organism of the abscess, the antibiotic was changed to cefmetazole. After detecting P. intermedia, along with the results of sensitivity testing to antibiotics, piperacillin-tazobactam and metronidazole were used [Figure 3]. To confirm the effect of these drugs, an MRI was performed 2 weeks after surgery and the size of the brain abscess was greatly reduced on DWI. These antimicrobial drugs were continued and the abscess clearly shrank 2 months after admission. Three months after admission, the patient was discharged home without any sequelae. After discharge from the hospital, the antibiotics were changed to oral metronidazole and amoxicillin. One month after discharge from the hospital, the oral metronidazole was discontinued due to the appearance of peripheral neuropathy, which was thought to be a side effect of the drug. One and a half months after discharge, the antibiotic was terminated (since the erythrocyte sedimentation rate was normalized) and a head MRI showed a further reduction in abscess size [Figure 3].