# Brain Abscess > Brain abscess for NEET-PG Medicine covering sources of infection, organisms, ring-enhancing lesion differentials, empirical antibiotics and when to drain. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/brain-abscess - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-08 - Last updated: 2026-10-08 - Reference basis: Harrison's Principles of Internal Medicine; Bailey and Love's Short Practice of Surgery; Greenberg's Handbook of Neurosurgery - How to cite: "Brain Abscess", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/brain-abscess ## Direct answer Brain abscess is a focal, pus-filled infection of the brain parenchyma, most often spreading from an ear or sinus infection, seeding from a right-to-left cardiac shunt or lung focus, or following neurosurgery. Its classic triad is headache, fever and focal deficit, with a ring-enhancing lesion on contrast MRI or CT. Management combines early intravenous antibiotics, usually a third-generation cephalosporin with metronidazole, and stereotactic aspiration or excision when the lesion is large or not responding. ## What you must remember - **Sources:** otitis media, mastoiditis and sinusitis (frontal sinus for frontal lobe, mastoid for temporal and cerebellar), cyanotic heart disease, bronchiectasis and endocarditis, plus post-operative or post-traumatic infection. - **Organisms:** viridans and milleri group streptococci are commonest, with anaerobes; Staphylococcus aureus follows surgery, and fungi or Nocardia affect the immunocompromised. - **Differentials:** glioblastoma, metastasis, tuberculoma, neurocysticercosis, toxoplasmosis and CNS lymphoma can all ring-enhance; restricted diffusion in the centre favours pus. - **Antibiotics:** intravenous third-generation cephalosporin with metronidazole, adding vancomycin when MRSA risk is high after neurosurgery. - **Surgery:** drainage is indicated for lesions larger than about 2.5 cm, significant mass effect, multiloculation, or deterioration on antibiotics. - **HIV:** multiple basal ganglia ring lesions suggest toxoplasmosis, treated with pyrimethamine, sulfadiazine and folinic acid. - **Duration:** antibiotics are commonly continued for six to eight weeks, guided by follow-up imaging. ## A worked approach to a ring-enhancing lesion in fever A 34-year-old man with chronic suppurative otitis media presents with ten days of headache, fever and left arm weakness. Contrast CT shows a ring-enhancing lesion in the right temporal lobe with oedema and mild midline shift. 1. **Avoid lumbar puncture:** A lumbar puncture is avoided, since the raised pressure could herniate the brain once CSF is withdrawn. 2. **Image with diffusion:** Contrast MRI with diffusion imaging follows, to characterise the centre of the lesion. 3. **Search for the source:** The source is sought with an ENT examination, blood cultures, an echocardiogram to exclude a right-to-left shunt, and chest imaging for a lung focus. 4. **Start antibiotics at once:** Intravenous ceftriaxone and metronidazole are started immediately, with vancomycin added only after recent surgery. 5. **Use steroids sparingly:** Steroids reduce oedema but can blunt capsule formation. 6. **Drain and address the source:** Stereotactic aspiration is planned, with pus sent for Gram stain and aerobic and anaerobic culture. Mastoidectomy addresses the ear source, since antibiotics alone will not clear a persistent focus. 7. **Follow the imaging:** Follow-up MRI guides when antibiotics can stop, and repeat imaging confirms that the cavity is shrinking and the oedema is settling. ## Where the ring-enhancing trap catches students Ring enhancement with fever tempts students to diagnose pus, and examiners expect a differential built on context. - **Trap:** calling every ring-enhancing lesion with fever a pyogenic abscess — **Correct:** a glioblastoma has a thick, irregular enhancing wall around a necrotic centre that does not restrict diffusion, whereas a pyogenic abscess has a thin, smooth capsule and a centre that restricts diffusion. Susceptibility-weighted imaging may show the dual rim sign in an abscess. - **Trap:** starting steroids before biopsy in HIV with ring lesions — **Correct:** steroids can destroy the diagnosis of lymphoma, which is periventricular and steroid-sensitive, while toxoplasma lesions are multiple, favour the basal ganglia and often shrink within two weeks of treatment. - **Trap:** looking for an ear or sinus source in every abscess — **Correct:** an abscess in cyanotic heart disease with no ear or sinus history has the heart, not the sinuses, as its source. ## Frequently asked questions ### Which MRI sequence best separates a brain abscess from a necrotic tumour? Diffusion-weighted imaging is the most useful sequence. A brain abscess shows restricted diffusion in its centre from thick pus, with low apparent diffusion coefficient values, while the necrotic centre of a glioblastoma usually does not restrict. Susceptibility-weighted imaging can add the dual rim sign. ### What is the first-line empirical antibiotic for a brain abscess? A brain abscess is commonly treated first with an intravenous third-generation cephalosporin such as ceftriaxone or cefotaxime, combined with metronidazole for anaerobic cover. Vancomycin is added when recent neurosurgery, a skull fracture or high MRSA risk is present. The regimen is narrowed once the culture of pus is available. ### When is surgical drainage needed in a brain abscess? Surgical drainage is needed when a brain abscess is larger than about 2.5 cm, has significant mass effect or multiple loculations, or worsens despite appropriate antibiotics. Stereotactic aspiration suits deep or eloquent sites, while excision suits accessible lesions with a well-formed capsule. ### Why is lumbar puncture avoided in suspected brain abscess? Lumbar puncture is avoided in suspected brain abscess because a space-occupying lesion with raised intracranial pressure can herniate when CSF is withdrawn. CSF analysis is also rarely diagnostic. Diagnosis rests on contrast imaging, blood cultures and microbiology of pus obtained at surgery.