Brain Abscess

complete reference · ring-enhancing + central DWI · aspirate + prolonged antibiotics · no LP · Full Card

Symptoms / Associated Sx

• Headache is the most common symptom; the classic triad of headache, fever, and focal neurologic deficit is present in only a minority — and fever is absent in roughly half, so its absence does not exclude the diagnosis. Course is typically subacute over days to weeks. Focal deficits, seizures, and signs of raised intracranial pressure (vomiting, papilledema, declining consciousness) appear as the lesion and surrounding edema grow. Presentation depends on location and size; a frontal/temporal abscess may produce subtle cognitive/behavioral change before clear focal signs.

Neg

• A focal ring-enhancing lesion is present rather than diffuse meningeal inflammation alone — distinguishes abscess from meningitis (and is why LP is avoided — there is a mass)

• Fever is not reliably present — so its absence does NOT argue against abscess (~50% are afebrile; anchor on the imaging and source, not the temperature)

• Central diffusion restriction on MRI is present — argues for abscess over a necrotic tumor (tumors and metastases typically do not restrict centrally)

• Onset is subacute, not maximal-at-onset — argues against acute stroke (though a septic embolus can do both)

Social History (SHx)

• Contiguous infectious sources — otitis media/mastoiditis, sinusitis, dental infection/recent dental work; recent neurosurgery or penetrating head trauma.

• Hematogenous sources — infective endocarditis, pulmonary infection/abscess, right-to-left shunt or cyanotic congenital heart disease (bypasses pulmonary filter), bacteremia, injection drug use.

• Immune status — HIV/AIDS (toxoplasmosis, Nocardia, fungi, TB), transplant, chronic steroids, neutropenia.

Main Etiology

• Contiguous spread (ear, sinus, dental) — viridans/anaerobic streptococci, oral anaerobes, often polymicrobial. Hematogenous (endocarditis, lung, distant infection) — frequently multiple abscesses at the gray-white junction. Direct inoculation (trauma/neurosurgery) — Staphylococcus aureus, gram-negatives. Immunocompromised hosts — Toxoplasma gondii (HIV, ring-enhancing, multiple), Nocardia, fungi (Aspergillus, Candida, Cryptococcus), Mycobacterium tuberculosis. Mixed/polymicrobial infection is common.

RF

• Modifiable: untreated otitis/sinusitis/dental infection, poor dental hygiene, uncontrolled distant infection.

• Non-modifiable: cyanotic congenital heart disease, right-to-left shunt (including pulmonary AVM/HHT), immunocompromise, prior neurosurgery/penetrating trauma.

Data

• MRI brain with contrast (or contrast CT if MRI unavailable) (a ring-enhancing lesion with a smooth wall, surrounding vasogenic edema, and — characteristically — central restricted diffusion on DWI/ADC, which helps differentiate pyogenic abscess from necrotic tumor)

• Blood cultures ×2, CBC, ESR/CRP (may be normal; inflammatory markers support but don't exclude); HIV test; identify the source (echocardiogram for endocarditis, dental/sinus/ear evaluation, chest imaging).

• Stereotactic aspiration or surgical specimen for Gram stain, culture (aerobic/anaerobic), and special stains (the definitive microbiologic diagnosis — and therapeutic)

• Avoid lumbar puncture (mass effect → risk of cerebral herniation; CSF is usually non-diagnostic in abscess)

DDx

Primary or metastatic brain tumor (ring-enhancing but typically no central DWI restriction) · cerebral toxoplasmosis (HIV, multiple ring lesions, basal ganglia) · subacute infarct with luxury perfusion · tumefactive demyelination (incomplete/open ring) · radiation necrosis · neurocysticercosis (cystic, scolex, endemic exposure) · resolving hematoma

Home Meds

• Hold anticoagulation/antiplatelets if neurosurgical aspiration is planned; reconcile and resume per surgical guidance.

• Review immunosuppression with the primary/ID teams in immunocompromised hosts.

Plan

Consults

• Neurosurgery — stereotactic aspiration or excision (diagnostic and therapeutic).

• Infectious disease — empiric and tailored antimicrobials, duration.

• Neurology — seizure management and monitoring.

• Source service — ENT (ear/sinus), dental/OMFS, cardiology (endocarditis); ICU for raised ICP/declining consciousness.

Source control — do not LP

• Lumbar puncture is contraindicated given mass effect/herniation risk and low diagnostic yield.

• Neurosurgical aspiration (usually stereotactic) or excision for lesions larger than ~2.5 cm, those causing significant mass effect, when the diagnosis is uncertain, or that fail to respond to antibiotics — it provides the causative organism and decompresses. Small (<2.5 cm) lesions in stable patients may be treated with antibiotics alone with close imaging follow-up.

Empiric antimicrobials

• After cultures, start empiric IV therapy guided by the likely source: ceftriaxone (Rocephin) 2 g IV q12h + metronidazole (Flagyl) 500 mg IV q8h covers the streptococci and anaerobes of contiguous/odontogenic/sinus sources. Add vancomycin (Vancocin) 15–20 mg/kg IV q8–12h for post-neurosurgical/post-traumatic abscess or suspected MRSA. Substitute meropenem for cefepime/ceftriaxone where gram-negative/Pseudomonas coverage is needed.

• Immunocompromised — add host-specific coverage: toxoplasmosis (pyrimethamine + sulfadiazine + leucovorin) in HIV with multiple ring lesions; Nocardia (TMP-SMX/high-dose); antifungals (voriconazole/amphotericin); anti-tuberculous therapy as indicated.

• Prolonged course — typically 6–8 weeks of IV therapy (longer in immunocompromised/incompletely drained), guided by serial imaging and ID.

Adjuncts

• Corticosteroids (dexamethasone) only for significant perilesional edema/mass effect — they reduce edema but may impair antibiotic penetration and abscess capsule formation, so use the lowest effective dose for the shortest time and taper. Treat seizures with levetiracetam (Keppra); many clinicians give seizure prophylaxis given the cortical/supratentorial lesion.

Always

• PT / OT / SLP eval and treat — rehabilitate focal deficits; cognitive assessment.

• Trend: serial neuro exams, serial MRI to document shrinkage (the key marker of response), inflammatory markers, and treat/eradicate the primary source.

• Escalation triggers: declining consciousness or signs of raised ICP/herniation → ICU, emergent neurosurgery · intraventricular rupture (sudden deterioration, meningismus — high mortality) → emergency · status epilepticus → ICU · enlarging lesion despite therapy → repeat aspiration.

• Discharge checklist: complete the prolonged IV antibiotic course via OPAT with ID follow-up and interval imaging · definitive treatment of the source (sinus/ear/dental surgery, endocarditis therapy, shunt evaluation) · AED if seizures occurred with neurology follow-up · rehabilitation for residual deficits · HIV/immunodeficiency management if newly identified · return precautions (worsening headache, new deficit, seizure, fever).

Red Flags — ICU / Neurosurgery

• Declining consciousness / raised ICP / herniation → emergent neurosurgical decompression, ICU.
• Intraventricular rupture of the abscess (abrupt deterioration, severe meningismus) → catastrophic, very high mortality — emergency.
• Status epilepticus → ICU.
• Posterior fossa abscess with brainstem compression/hydrocephalus → urgent surgery/EVD.
• Do NOT LP a suspected abscess — herniation risk.

Senior IM Resident Pearls

• No fever doesn't rule it out. Only about half of brain-abscess patients are febrile, and the full triad is uncommon — a subacute headache with a focal deficit and a ring-enhancing lesion is the abscess until proven otherwise.
• Don't LP a ring-enhancing lesion. The mass effect makes lumbar puncture dangerous (herniation) and the CSF rarely helps — go to imaging and neurosurgery instead.
• Central diffusion restriction is your friend. On MRI, an abscess restricts centrally on DWI; a necrotic tumor usually doesn't — this single sequence often separates the two.
• Aspiration is both diagnosis and treatment. Get the organism by stereotactic aspiration before committing to weeks of empiric antibiotics — and it decompresses the lesion.
• Find the source. Ear, sinus, teeth, heart (endocarditis), lung, or a right-to-left shunt — the source dictates the organisms and must be eradicated or the abscess recurs.
• Steroids only for mass effect. Dexamethasone reduces edema but can impair antibiotic penetration and walling-off — reserve it for significant edema/herniation risk and taper quickly.
• Think Toxoplasma in HIV with multiple ring-enhancing lesions — empiric anti-toxo therapy with follow-up imaging is often diagnostic.
• Common mistake: treating a ring-enhancing lesion as a tumor (or vice versa) without the DWI clue and tissue — and reflexively doing an LP that precipitates herniation.