Bacterial Abscess

Before an abscess forms, seeding of the brain parenchyma with bacteria will cause a localized cerebritis.  Treating a brain infection at this stage may be successful with antibiotics alone.  However, once an abscess is formed, optimal therapy requires a combination of antibiotics and surgical intervention.  At presentation, if brain imaging indicates an abscess that would be amenable to neurosurgical intervention, the patient should have CT-guided stereotactic aspiration of the abscess immediately without antibiotic pretreatment.18  In patients that are not good surgical candidates, or in patients where the abscess is located in deep or eloquent parts of the brain, empirical antibiotics are started and surgical aspiration is deferred.19  When antibiotics and aspiration do not control the infection, excision of the abscess can be a definitive treatment.  Excision is used only when less invasive techniques have failed because of the risk for permanent neurological deficit with surgery.  Excision is contraindicated when the abscess is located in deep or eloquent parts of the brain.

Empirical antibiotics should be started after aspiration of the abscess or immediately at the time of diagnosis in patients that are not candidates for aspiration.  Empirical therapy includes a third- or fourth-generation cephalosporin, vancomycin, and metronidazole (Table 3).  These antibiotics offer coverage for most of the potential bacterial pathogens associated with abscesses regardless of the underlying cause.  If the source of the abscess is known, empirical antibiotics can be modified to cover the most common pathogens in the specific subgroup.  Sinusitis-associated abscesses are usually caused by streptococci and anaerobes, but can be caused by Hemophilus species as well.  In sinusitis-associated abscesses, empirical antibiotics consist of metronidazole for anaerobic coverage and either penicillin G for streptococcus coverage or a third- or fourth-generation cephalosporin to cover both streptococci and Hemophilus species.  In otitis-associated abscesses, the most common causative organisms are streptococci, enterobacteriaceae, Pseudomonas aeruginosa, and Bacteroides spp.  Empirical therapy of otitis-associated abscesses includes penicillin G for streptococci, metronidazole for Bacteroides species, and ceftazidime for enterobacteriaceae and Pseudomonas aeruginosa.  A brain abscess from penetrating head trauma is most commonly caused by Staphylococcus aureus, Clostridium species, and enterobacteriaceae.  Empirical therapy of abscesses due to penetrating head trauma includes a third- or fourth-generation cephalosporin and vancomycin.  A brain abscess that occurs as a complication of a neurosurgical procedure is usually caused by staphylococci, enterobacteriaceae, or Pseudomonas species.  Empirical antimicrobial therapy for an abscess complicating a neurosurgical procedure should include vancomycin for staphylococcus coverage and either meropenem or ceftazidime for coverage of enterobacteriaceae and Pseudomonas sp.18

Once the causative microorganism is identified, the antibiotic regimen can be narrowed to target the specific bacteria as shown in Table 4.  All antibiotics are given intravenously and should be continued for 6 to 8 weeks.  A head CT or MRI should be performed at least every two weeks to follow the progress of treatment.  If the abscess enlarges after two weeks of intravenous antibiotics or fails to decrease in size after four weeks of antibiotic treatment, further neurosurgical intervention is required.20  A brain abscess caused by Nocardia asteroides requires 6 to 12 months of trimethoprim-sulfamethoxazole.  A Nocardia abscess usually requires complete excision to obtain a cure.

Corticosteroids can lead to decreased antibiotic penetration into the abscess and slow the formation of the abscess wall, so they should be avoided if possible.  In patients with increased intracranial pressure, mass effect, or significant edema, a short course of corticosteroids can provide benefit.21  Dexamethasone 10 mg every 6 hours for 3 to 7 days is the recommended dosing.  Seizures occur in approximately 50% of patients during the initial hospitalization and in 70 percent of patients after discharge.22  Patients should be treated routinely for 2 years with antiepileptics to prevent seizures.  If patients are seizure-free for 2 years, the antiepileptic medication can be discontinued.18

Intracranial epidural abscess and subdural empyema

Empirical therapy for an epidural abscess or subdural empyema is the same as for an intraparenchymal abscess of unknown etiology a third- or fourth-generation cephalosporin, vancomycin, and metronidazole.  Surgical evacuation is required for a cure and should be performed immediately.  Intravenous antibiotics specific to the isolated organism should be continued for 4 to 6 weeks , followed by 2 to 3 months of oral antibiotics for an epidural abscess.  For a subdural empyema, patients should receive 3 to 4 weeks of intravenous antibiotics followed by oral antibiotics to complete a 6 week course.18

Table 3.  Empirical therapy for bacterial abscess and empyema

Population/etiology

Microorganism(s)

Treatment

Unknown etiology

Streptococci, anaerobes, Hemophilus spp., enterobacteriaceae, P. aeruginosa, Bacteroides spp., S. aureus, Clostridium spp.

3rd or 4th generation cephalosporin plus vancomycin plus metronidazole

Sinusitis-associated

Streptococci, anaerobes, Hemophilus spp.

Metronidazole plus either penicillin G or a 3rd or 4th generation cephalosporin

Otitis-associated

Streptococci, enterobacteriaceae,

P. aeruginosa, Bacteroides spp.

Metronidazole plus penicillin G plus ceftazidime

Penetrating head trauma

S. aureus, Clostridium spp., enterobacteriaceae

3rd or 4th generation cephalosporin plus vancomycin

Post-neurosurgery

Enterobacteriaceae, Pseudomonas spp., staphylococci

Vancomycin plus meropenem or ceftazidime

 

Table 4.  Antibiotics for specific bacteria in brain abscess or empyema (recommendations are in bold).

Pathogen

Antibiotic

Bacteriodes fragilis

Metronidazole 2000 mg/day (500 mg every 6 hours)

Enterobacteriaceae (e.g. Klebsiella, E. coli, Proteus)

Ceftriaxone 4 grams/day (2 grams every 12 hours)

   or

Cefotaxime 12 grams/day (2 grams every 4 hours)

   or

Cefepime 4 grams/day (2 grams every 12 hours)

   or

Meropenem 6 grams/day (2 grams every 8 hours)

Hemophilus influenzae

Ceftriaxone or cefotaxime

Nocardia asteroides

Trimethoprim-sulfamethoxazole 15-20 mg/kg per day of TMP component (5-6.67 mg/kg every 8 hours)

Pseudomonas aeruginosa

Meropenem

   or

cefepime

   or

ceftazidime 6 grams/day (2 grams every 8 hours)

Staphylococci

Methicillin-susceptible

 

 

Methicillin-resistant

 

Nafcillin or oxacillin 12 grams/day (2 grams every 4 hours or 3 grams every 6 hours)

 

Vancomycin 2-3 grams/day IV (500-750 mg every 6 hours)

Streptococcus spp.

 

Penicillin G 20-24 million units/day (3-4 million units every 4 hours)

   or

ceftriaxone or cefotaxime or cefepime