CONTENTS
- Brain abscess
- Related intracranial infections
- Podcast
- Questions & discussion
- Pitfalls
evolution of an abscess
- An abscess begins as a localized area of cerebritis. Over about two weeks, this subsequently evolves into an abscess (a purulent collection surrounded by a well-vascularized capsule).
anatomic distribution
- Contiguous sources of infection usually cause a solitary abscess.
- Frontal lobe abscess may result from frontal sinusitis or a dental source.
- Temporal lobe abscess may result from otitis, mastoiditis, or sphenoid sinusitis.
- Hematogenous infection may cause multiple abscesses, often within or at the grey-white junction. Typical locations are within the distribution of the middle cerebral arteries (MCA) or the watershed zones between vascular territories.(30273242)
sources of infection
- Direct inoculation:
- Penetrating trauma.
- Neurosurgery.
- Contiguous spread of bacteria:
- Otitis media, mastoiditis.
- Paranasal sinusitis.
- Meningitis spreads into the brain parenchyma in 0.5% of cases.(31447060)
- Hematogenous spread of bacteria:
- Dental infection.
- Pulmonary infection (bronchiectasis, lung abscess, empyema).
- Endocarditis, IV drug use.
- Gastroenterological source (e.g., diverticulitis, malignancy, perforation).
- Right-to-left shunt (e.g., intracardiac shunt or particularly pulmonary arteriovenous malformation) – increases the likelihood of brain abscess due to various sources (e.g., skin infection).
abscess in severe immunocompromise
- Immunocompromise increases the risk of abscess due to opportunistic pathogens (e.g., Aspergillus, Mucor, Nocardia, Candida, Cryptococcus, Listeria).
- In transplant patients, the rate of brain abscess due to opportunistic pathogens may be considerably higher than the risk from typical bacterial pathogens. (34623105)
epidemiology
- Epidemiology is variable, because different patients may have different underlying risk factors for brain abscess (e.g., the epidemiology of endocarditis-induced abscess is distinct from the epidemiology of trauma-induced abscess).
- Overall, men are 2-3 times more likely to develop brain abscess.(28501669)
- The median age is 30-40 years old.(28501669)
The overall presentation is similar to that of a brain tumor, but the tempo of progression is faster and there may be systemic inflammation. Nonetheless, this is a subacute process, with patients often presenting with symptoms of 1-2 weeks' duration.
neurological manifestations
- Headache is the most common presenting symptom.
- Present in nearly all patients.
- Tends to be worse at night, due to increased intracranial pressure in the supine position.(32845766)
- (Abrupt worsening of headache with the development of meningismus may indicate rupture of the abscess into the ventricular or subarachnoid space.)
- Focal neurological signs/symptoms occur in about half of patients.(34623096) Depending on abscess location, these may include:
- New seizure (25% of cases, may be the presenting symptom).(32845766)
- Visual disturbance.
- Dysarthria.
- Hemiparesis.
- Personality changes (frontal or temporal abscesses).
- Clinical features of elevated intracranial pressure:
- Nausea/vomiting.
- Altered consciousness.
systemic inflammation
- Fever occurs in about half of patients.(Louis 2021) It is often absent or low-grade.
primary focus of infection
- The underlying source of the infection may also cause symptoms (e.g., sinusitis, dental infection, mastoiditis).
- An obvious primary source of infection may draw attention away from the brain abscess (since fever, headache, or other symptoms may be attributed to it – thereby leading to premature diagnostic closure).
alternative presentation: ventricular rupture
- Rupture of the abscess into the ventricular space may cause acute deterioration. This may be marked by worsening headache, neuroworsening, and abrupt onset of meningeal signs.(31447060)
- Management may require prompt surgical drainage with or without an external ventricular drain for management of obstructive hydrocephalus.
laboratory studies
- Leukocytosis is present only in about half of patients.
- C-reactive protein may be elevated in ~90% of patients, but this is nonspecific.(30273242)
- Blood cultures may be positive in ~25% of patients, so these should be performed before antibiotic administration.(31447060) Two blood cultures should be performed initially, and then another two cultures should be obtained following surgery (given the possibility that surgical manipulation could provoke bacteremia).(31447060)
- HIV screen should be obtained (if positive, toxoplasmosis becomes a consideration).
- Coagulation studies (may be required prior to operative interventions).
evaluation for source
- Once a brain abscess is diagnosed, additional investigation should look for an underlying focus of infection.
- Contiguous sources of infectious spread will usually be visible on neuroimaging.
- CT scanning of the chest, abdomen, and pelvis may be reasonable.
- Echocardiography with bubble study may be considered to evaluate for endocarditis or the presence of a right-to-left shunt.
- ⚠️ Lumbar puncture is generally contraindicated (as this may risk causing herniation or rupture of the abscess into the subarachnoid space).
cerebritis
- Cerebritis refers to an early stage of pyogenic inflammation that may eventually develop into an abscess. Unlike a mature abscess, cerebritis lacks a purulent core or a fibrous capsule. Cerebritis may involve vasogenic edema and petechial hemorrhages.
- CT scanning may reveal a low-density lesion with or without partial ring enhancement.
- MRI may reveal an ill-defined T2 hyperintensity. There is no contrast enhancement initially, but some rim enhancement may develop as cerebritis is starting to transition into an abscess.(33741796)
brain abscess on CT scan
- Abscess appears as a hypodense lesion with ring enhancement, often with surrounding vasogenic edema.
- The fibrous capsule may create a hyperdense rim on noncontrast CT scan.(26046515)
- Abscesses due to hematogenous spread are typically found at the gray-white matter junction.
- Associated pathology:
- If the abscess occurred secondary to sinusitis, otitis, or mastoiditis, then these underlying pathologies may also be visible.
- Ear or mastoid infection may cause CVT (cerebral venous thrombosis), which may occur in combination with an abscess. Consider the addition of a CT venogram when obtaining CT imaging of the brain.
brain abscess on MRI
- MRI is the gold standard for the imaging of brain abscesses. Compared to CT scan, MRI is especially superior for early lesions and lesions in the posterior fossa. MRI with diffusion-weighted imaging has sensitivity and specificity of >95% for brain abscesses.(Wijdicks, 2019)
- Diffusion restriction is typically pronounced within the necrotic core of the abscess. This may be helpful to differentiate abscess from most brain tumors, toxoplasmosis, or neurocysticercosis. However, this finding is not completely reliable:
- (1) Rim enhancement with central diffusion restriction can occur in some high-grade neoplasms, in radiation necrosis, and rarely in demyelination.
- (2) DWI is susceptible to artefact due to motion and blood products, so absence of diffusion restriction doesn't rule out an abscess.(33741796)
- Fibrous capsule with rim enhancement:
- Contrasted MRI often shows an enhancing rim which is smooth and thin. Alternatively, if rim enhancement is thick, irregular, or nodular, that may suggest a malignancy or fungal abscess.(31378868)
- The fibrous capsule may be hyperintense on T1 and hypointense on T2 (due to paramagnetic oxygen free radicals inside macrophages).(34623096; 33741796)
- The capsule is often thicker where the abscess faces grey matter and thinner where the abscess faces white matter. This may create a weakness in the abscess capsule, with a tendency to rupture into the ventricles (leading to catastrophic ventriculitis).(33741796) This pattern may help differentiate abscess from tumors or demyelination.(Louis 2021)
- Demyelination typically causes greater rim enhancement on the white matter side (sometimes causing a characteristic open-ring pattern). 📖
timing of antibiotics
- If the patient is stable and surgical drainage is imminent, then antibiotics may be delayed to maximize the yield of intraoperative cultures. Otherwise, patients should be started on empiric antibiotics (after obtaining blood cultures).(32845766)
microbiology of brain abscess
- ~50% are due to Streptococcus spp. (e.g., S. anginosus, S. intermedius, S. viridans, S. pneumoniae).(32845766)
- ~25% are due to anaerobes.
- Styphlococcus spp. (S. aureus, S. epidermidis) are often seen in postsurgical abscess.
- Enterobacteriaceae regularly are isolated (e.g., Klebsiella pneumoniae, E. coli, Proteus spp.).
- Among immunosuppressed patients, additional organisms should be considered (especially Nocardia spp.).
sources of infection & empiric antibiotic regimens (30273242)
- Dental or sinus infection:
- Common pathogens: Streptococcus spp. (especially S. milleri group), Staphylococcus aureus, Haemophilus spp., anaerobes (e.g., Fusobacterium, Actinomyces spp.).
- Empiric antibiotic regimen: ceftriaxone + metronidazole. Vancomycin may be added if high risk for MRSA (e.g., chronic sinusitis, recent sinus surgery, known colonization with MRSA).
- Otogenic infection:
- Common pathogens: Streptococcus spp., Enterobacteriaceae, Haemophilus, Pseudomonas aeruginosa, anaerobes (e.g., Bacteroides spp.)
- Empiric antibiotic regimen: cefepime + metronidazole (or, if these are contraindicated, meropenem monotherapy).
- Status post neurosurgery:
- Common pathogens: Staphylococcus aureus (including MRSA), Streptococcus spp., Enterobacteriaceae, Pseudomonas aeruginosa.
- Empiric antibiotic regimens:
- i) Vancomycin + cefepime +/- metronidazole (anaerobic coverage may be considered depending on the nature of the surgical procedure and risk of exposure to anaerobic pathogens).
- ii) Vancomycin + meropenem.
- Status post penetrating trauma:
- Common pathogens: Staphylococcus spp., Streptococcus spp., Enterobacteriaceae, anaerobes (e.g., Clostridium spp.).
- Empiric antibiotic regimen: vancomycin + ceftriaxone + metronidazole.
- Multifocal abscesses suspicious for hematogenous source (e.g., endocarditis or pulmonary infection).
- Common pathogens: Staphylococcus aureus, Streptococcus spp., Enterococcus spp., anaerobes (e.g., Fusobacterium, Actinomyces spp.), Nocardia spp.
- Empiric antibiotic regimen: usually vancomycin + ceftriaxone + metronidazole. If high degree of suspicion for Nocardia, consider the use of meropenem (in place of ceftriaxone + metronidazole) and/or linezolid (in place of vancomycin).
- Pulmonary infection: (34623096)
- Pathogens: Staphylococcus aureus, Streptococcus spp., Enterococcus spp., Nocardia spp., anaerobes, enterobacteriaceae.(Louis 2021)
- Empiric antibiotic regimen: ceftriaxone + vancomycin + metronidazole. May consider coverage for Nocardia depending on context.
- Unknown source:
- Pathogens: Variable/unknown.
- Empiric antibiotic regimen:
- Usually vancomycin + ceftriaxone + metronidazole.
- Immunocompromised patient: Consider vancomycin + meropenem, or vancomycin + cefepime + metronidazole.
- Immunocompromise: other pathogens to consider may include:
- Toxoplasmosis.
- Listeria.
- Nocardia.
- Aspergillus, cryptococcus, cocciodomycosis.
- Tuberculosis.
narrowing antibiotics based on culture data
- ⚠️ Up to 50% of abscesses may be polymicrobial (especially those due to contiguous extension). Therefore, isolating a single organism doesn't exclude the presence of other organisms. Thus, relatively broad coverage (e.g., ceftriaxone + metronidazole) should often be continued even after a specific organism is identified.(32845766) In particular, commonly used techniques for obtaining cultures are often suboptimal for isolation of anaerobes, so anaerobic coverage may be continued regardless of negative culture results for anaerobes.(28501669)
- Brain abscess due to endocarditis is a monomicrobial process, so antibiotics should be narrowed based on culture results.
duration of therapy
- A six-week duration of therapy is often utilized (although this may vary between patients, depending on organism and clinical course).
- Patients will require longitudinal follow-up with specialists in infectious diseases to determine the optimal antibiotic course.
potential indications for drainage (vs. empiric medical therapy without any intervention)
- Need to obtain culture material (e.g., immunosuppressed patient with a broad differential diagnosis that is challenging to cover empirically). Stereotactic aspiration currently allows for sampling of small abscesses (e.g., lesions >1 cm).(31447060)
- Larger size is an indication for therapeutic drainage, including:
- Size >2.5 cm diameter (although this size cutoff is arbitrary and controversial).(32845766)
- Lesion is exerting significant mass effect on surrounding tissues.
- Abscess adjacent to ventricular surface, with risk of rupture into the ventricle.
- Abscess in the posterior fossa (threatening brainstem compression).
- Refractory to medical management.
drainage via stereotactic aspiration vs. surgery (total resection via craniotomy)
- Stereotactic aspiration:
- Most abscesses can be drained via stereotactic-guided aspiration, rather than surgical excision.
- Potential indications for surgical excision may include:
- Multiloculated abscess.
- Abscess is superficial and well circumscribed (more amenable to surgery).
- Stereotactic aspiration was unsuccessful.
- Abscess in the posterior fossa.
- Severely elevated intracranial pressure.
external ventricular drain (EVD)
- Potential indications:(31447060)
- Rupture of the abscess into the ventricular system.
- Large abscess causing hydrocephalus.
serial imaging
- Serial CT scan after drainage may be useful to ensure that the abscess is not re-expanding. Note that radiological improvement often lags behind clinical improvement.
- For example, a scan may be performed a day after drainage, and then repeated a week later.
steroid
- Shouldn't be used routinely.
- May be used for patients with substantial peri-abscess edema causing mass effect and elevated intracranial pressure.
- If used, a low dose should be selected and steroid should be tapered off as soon as possible.
antiepileptic
- Seizures are common, occurring in ~25-50% of patients.
- There is no solid evidence about whether to provide seizure prophylaxis, with varying recommendations in the literature. Seizure prophylaxis with a newer antiepileptic agent is reasonable for patients with abscess in more epileptogenic brain areas (e.g., frontal, temporal, or insular cortex).
- If seizure is suspected, there should be a low threshold to obtain EEG monitoring.
basics (Nelson, 2020)
- More common than epidural empyema.
- Subdural empyema tends to be more severe than epidural abscess:
- i) The subdural space is less constrained than the epidural space, so infection may spread more rapidly.
- ii) The subdural space is closer to brain tissue, so there may be a greater tendency to develop cerebritis.
causes
- (1) Often occurs in otherwise healthy patients as a result of extension from sinusitis, mastoiditis, or otitis. Subdural empyema can also complicate meningitis.
- (2) Following penetrating head trauma or neurosurgery (e.g., status post evacuation of a chronic subdural hematoma).(31964490)
- (3) May occur in context of chronic subdural hygroma or hematoma that becomes superinfected.
presentation
- Deterioration may occur rapidly. Presentation is often more severe than epidural abscess (discussed further below).
- Patients may present in a nonspecific fashion, with the following main findings: (Louis 2021)
- Headache.
- Fever.
- Neurologic abnormalities (emesis, mental status changes, focal neurologic deficits, focal seizures).
- Complications may include:(31964490)
- Intracranial pressure elevation, midline shift, and herniation.
- Seizures are common.
- Cerebritis and subsequent abscess formation.
- Epidural abscess.
- Meningitis.
- Cortical vein thrombosis with venous ischemia.
imaging
- Configuration of subdural empyema is similar to subdural hematoma (figure above):(31964490)
- Cannot cross the midline falx.
- Able to cross suture lines.
- Overall configuration follows the convexity of the brain, or lies along the interhemispheric fissure.
- Imaging characteristics on CT scan:
- Extra-axial, hypodense or isodense collection.
- Enhancement with contrast.
- Imaging characteristics on MRI:
- Bright on T2/FLAIR.
- On T1, hypointense to isointense compared to brain parenchyma, and mildly hyperintense compared to CSF. This may help sort out empyema versus sterile effusions (which have lower intensity on T1) or hematomas (which are hyperintense on T1).
- Restricted diffusion is a key finding (which differentiates this from nonrestricted collections, such as subdural effusion, hygroma, or hematoma).(31964490) However, restricted diffusion is only ~70% sensitive for empyema, so the absence of restricted diffusion doesn't exclude infection.(30761443)
- A thin rim of enhancement may be seen, usually more prominently along the inner table of the skull.(Torbey, 2019)
management
- Urgent surgical intervention is needed in nearly all cases. (Louis 2021)
- Broad-spectrum antibiotic therapy.
- Most centers recommend prophylactic antiepileptic therapy, as seizures are common and may increase morbidity.(31964490)
basics
- Rare – less common than subdural empyema.
causes
- Secondary to otitis or sinusitis.
- Status post trauma.
- Following intracranial, transnasal, or transmastoid surgery.
presentation (Nelson, 2020)
- Usually subacute.
- Nausea/vomiting, headache.
- Fever.
- May have altered mental status, focal deficits, or seizure.
imaging
- Similar configuration compared to epidural hematoma:
- Biconvex, lens-shaped structures.
- Cannot cross suture lines, but can cross the midline by dissecting across dural sinuses.
- Imaging characteristics:(31964490)
- Hyperintense on T2 sequences.
- Restricted diffusion suggests the presence of purulence.
- Dural thickening and enhancement on CT and MRI.
- Underlying pathology leading to the epidural empyema may sometimes be visible (e.g., sinusitis or otitis).
management
- Surgical drainage and debridement (may include removal of infected bone).
- Antibiotic regimens are similar to those for brain abscess (above). For patients who are stable, antibiotics may be held prior to surgical drainage.
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- Don't forget to include subdural empyema or epidural empyema within the differential of fluid collections involving these spaces (it's not always a hematoma).
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References
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