Brain abscess is an intraparenchymal accumulation of pus. It begins with localized areas of cerebritis in the parenchyma and evolves into collections of pus enclosed in a well-vascularized capsule. It may occur as a result of spread from a contiguous focus of infection or may be of unknown origin. The reasons why brain abscess occurs include predisposing factors, such as underlying disease, a history of immunosuppressive drugs, disruption of the protective barrier surrounding the brain (e.g., neurosurgical procedure, head trauma), or a systemic source of infection (endocarditis or bacteremia)¹⁾. To manage patients with cerebral abscesses successfully, the infectious process is to be eliminated with an appropriate antimicrobial agent and the mass effect should be reduced using drainage or excisional procedures, thus avoiding or minimizing secondary cerebral injury and neurological deficit. Surgical excision or aspiration combined with prolonged antimicrobial therapy remains the treatment of choice. However, some factors should be considered when choosing the appropriate treatment. This study aimed to collect and analyze the information of patients diagnosed with primary brain abscess at our hospital over the last 10 years to determine the predisposing risk factors, clinical characteristics, and predictors of outcome.

The retrospective study evaluated hospital records and radiology data of patients diagnosed with and treated for brain abscess in our hospital from 2013 to 2022. The study was approved by the Institutional Review Board (IRB) of Konyang University Hospital (No.2023-03-029). Patients who met at least one of the following criteria were included: (1) evidence of infection in brain specimens collected from surgical aspiration or excision and (2) brain magnetic resonance imaging (MRI) and/or computed tomography (CT) findings of brain abscess and reversal of brain lesions with antibiotic therapy. Patients with intracranial empyema, including those with subdural and epidural abscess, were excluded. The following factors were analyzed in all patients: age, sex, predisposing factors, location and volume of the brain abscess, initial neurological status, biological organisms, surgical procedures, duration and type of antibiotic therapy, and neurological outcomes. The neurological status of the patients was assessed on admission via physical examination. Brain abscess volume (mm³) was calculated using the formula 0.5×X×Y×Z, where X, Y, and Z are the largest diameters of the abscess in the X-, Y-, and Z-axes, respectively. If multiple brain abscesses were present, the largest was measured.

We conducted standard laboratory tests, including complete blood counts, c-reactive protein levels, blood and cerebrospinal fluid (CSF) cultures, and serum chemistry. Imaging diagnostic tools include enhanced CT and MRI. Stereotactic navigation-guided aspiration and resection via craniotomy were performed under general anesthesia. Empirical antibiotic treatment was initiated after diagnosis in all cases. All patients were administered intravenous antibiotics for 4–8 weeks. Factors, such as predisposing conditions, result of antibiotic susceptibility test, patient treatment response, as noted clinically and radiologically, and inflammatory laboratory parameters, were the main determinants of antibiotics. Follow-up CT imaging was performed within 24 h of the surgical procedure and weekly as part of therapeutic monitoring. At these time intervals, white and red blood cell counts and C-reactive protein levels were also monitored. Outcomes were assessed in all patients according to modified Rankin Score scores. A score of 3–6 was regarded as a poor outcome, and a score of 0–2 was regarded as a good outcome, indicating that the patient was independent in daily life.

Statistical analyses were performed using IBM SPSS 22.0. Continuous data were expressed as means±standard deviation or median and were analyzed using the independent t-test or Mann–Whitney U test. Categorical data were analyzed using the Chi-squared test. Multivariate logistic regression analysis was performed to identify factors affecting the outcome. Statistical significance was set at p <0.05.

The average age of the study participants was 66 years, and 44% of them were aged more than 65 years. This is inconsistent with previously published reports associated with the age predilection of brain abscess. Some studies showed that individuals older than 40 years are more susceptible to brain abscess^2,3,4), whereas others revealed that brain abscess occurs more often in individuals younger than 40 years^5,6). The average age of participants in a meta-analysis conducted in 2014 was 33.6 years⁷⁾. Therefore, it is considered difficult to determine the most affected age group.

Twenty-one (84%) patients suffering from brain abscess had some predisposing factors. The neurosurgical procedure was the most common predisposing factor. When the blood-brain barrier is damaged, the brain becomes extremely vulnerable to bacterial infections. Each of the five patients (20%) experienced infection in adjacent areas, such as sinusitis, chronic otitis media, and dental or hematogenous infection. Most patients with brain abscess have predisposing conditions, which may provide clues toward causative micro-organisms⁸⁾. The contagious spread of bacteria occurs in half of the cases and can result from penetrating trauma, neurosurgery, or infections^1,8). Comorbidities, including diabetes mellitus, hypertension, tumor, liver cirrhosis, and a history of stroke, were noted in 72% of the patients in our cohort. Diabetes mellitus was the most common comorbidity observed in this study. The relationship between diabetes mellitus and susceptibility to infection has been reported⁹⁾. Impaired glucose control may affect host defense and increase the risk of brain abscess. Our study differs from the previous reports. Other reports explained that a history of stroke was reported to be a common comorbidity, as damaged brain tissues might be vulnerable to invasion by infectious organisms¹⁰⁾. Other comorbidities, including human immunodeficiency virus infection, autoimmune disease, and immunosuppressive therapy, were reported in a previous study¹¹⁾. However, only a few patients had these comorbidities.

The clinical presentation of brain abscess depends on the site, size, number of lesions, and any secondary cerebral injuries. Similar to previous study¹²⁾, our cases were also frequently presented with classical symptom triad of this disease, such as headache, fever, and hemiplegia in our study, occurring in 52%, 40%, and 28% of the patients, respectively. The symptoms were atypical, and the absence of this classical clinical triad decreased the likelihood of brain abscess being suspected on initial examination. In addition, the advancement of MR technology, including perfusion- and diffusion-weighted images and MR spectroscopy, has made a more accurate diagnosis of cerebral abscess and differentiation from other ring-enhancing lesions possible^13,14). Sixty percent of the patients in this study had an elevated peripheral white blood cell count. Indicators of inflammation, such as white blood cell count and c-reactive protein level, were useful indicators of clinical response to treatment, but not helpful in diagnosing brain abscess or in predicting outcome due to another infection in which the patient is present¹⁵⁾. An increased cell count in the CSF, which indicates leptomeningeal infection, was observed in 70% of the patients who underwent lumbar puncture in this study, and this finding helped us determine the nature of the brain lesion in those patients. However, not all patients with brain abscess show leptomeningeal involvement, and the role of lumbar puncture in diagnosis is limited.

The rate of brain abscess purulent material culture positivity in our study was 48%, which is far lower than the previously reported rate of approximately 70%^7,16). This may be because we initiated antibiotic therapy before obtaining samples for culture, and the standard culture protocol followed at our hospital may result in certain organisms being undetected. In addition, operations in the referral cases from other non-neurosurgical departments were delayed. In our study, gram-positive Staphylococcus was the most common pathogen observed in the material culture of brain abscesses. In previous studies, the most commonly observed bacteria in patients with brain abscesses were those of the Streptococcus species from 1952 to recent days¹⁵⁾. With the development of culture techniques and microbiological analysis, the trend is toward a high incidence of infection with polymicrobial and gram-negative organisms in today’s causative organism^17,18).

The most common brain abscess location in our study was the frontal lobe, followed by the parietal and parietal lobes. Most patients had single lesions. This result is consistent with a previous report^1,3). However, another study found that the temporoparietal region is the most common brain abscess location¹⁹⁾. The location of a brain abscess partly depends on the route of infection transmission²⁰⁾. Brain abscesses related to direct spread from sinus or odontogenic foci tend to be frontal and are caused by aerobic or anaerobic streptococci, Enterobacteriaceae, Staphylococcus aureus, and anaerobes. An abscess secondary to an otic infection is usually temporal or cerebellar, with mixed flora, including anaerobes and streptococci (aerobic and anaerobic), Enterobacteriaceae, and Pseudomonas aeruginosa.

The treatment of brain abscesses is aimed at reducing space-occupying activity, lowering intracranial pressure, and eradicating pathogenic microorganisms; thus, brain abscess is basically a surgical lesion. Surgery allows the immediate reduction of intracranial pressure from a mass effect and the identification of the causative organism. In addition, surgical treatment by aspiration or excision is more likely to lead to shorter treatment and decrease the possibility of serious clinical deterioration With intraventricular rupture. However, recently, there have been a few reports demonstrating successful non-operative treatment of brain abscess with antibiotics alone^12,15). The cerebritis phase refers to abscesses in an early stage of formation, in which there is inward migration of leukocytes and significant secondary edema, but no well-formed fibrous capsule. Therefore, these abscesses are usually observed in the first 0–9 days of development^21,22), and because they are not encapsulated, antibiotic therapy is more likely to provide adequate therapy without surgery^23-25). In our study, 68% of the patients underwent surgery. We observed a significant difference in abscess volume between patients with and without surgery; however, no difference was observed in the number of abscesses. A previous study showed that the number or size of abscesses or midline shifts may affect the decision for surgical therapy¹⁰⁾. Patients with headaches or hemiplegia were significantly more likely to undergo surgery. When these symptoms are visible, families tend to make more surgical decisions to improve their quality of life.

In choosing between aspiration or excision, the choice of procedure remains controversial^26,27). Several reports have advocated excision as the procedure of choice because it is often followed by a lower incidence of recurrence and shorter hospitalization^28-30). Recently, based on their good treatment outcome with craniotomy and lesionectomy, they recommended that indication for early surgical excision of brain abscess are single, accessible, superficial, and larger lesion accompanying aggravated neurological deficit³¹⁾. However, recent studies indicated that the stereotactic management of brain abscess allow both confirmation of the diagnosis and institution of therapy by the aspiration of its contents and identification of the offending organism, which has become primarily widespread with the introduction of CT-guided stereotaxy^6,29,32). In our cohort study, there was no significant difference between the excision and aspiration groups (Table 5). A previous study found no difference in the effects, outcomes, and complications of these two surgical techniques³⁾. Xiao et al.¹⁵⁾ also reported that favorable outcome was not significantly different between the patient treated by excision or aspiration. These varying results could be considered that the aspiration group may be associated with more multiple abscesses and poor general conditions, whereas the excision group may have a more favorable abscess position that can be completely removed by craniotomy and a good general condition.

Previous studies have reported inconsistent findings regarding the factors associated with outcomes. Age, immunosuppression, and hematogenous spread were found to be associated with a poor outcome in brain abscess patients²⁾. Meanwhile, it was revealed that sex was associated with an unfavorable outcome³⁾. Another study found that consciousness at presentation had prognostic value¹⁹⁾. In our study, headache and confusion influenced outcomes. As a classical symptom of brain abscesses, headache indicates possible intracranial hypertension, which may lead to a poor outcome. A confused mentality can lead to a poor prognosis because it suggests high intracranial pressure. This study has several limitations. The sample size is small. In addition, this was a retrospective single-center study. Therefore, our findings may not be generalizable to patients in different regions, and some data could not be collected or analyzed. Multicenter studies with larger sample sizes are warranted.

The most common comorbidity in the patients with brain abscess included in this study for over 10 years was diabetes mellitus, and Gram-positive Staphylococcus was the most common pathogen. Headache and confusion are also associated with poor outcomes. In addition, aspiration and excision had no significant differences in terms of outcomes.