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Describe otogenic brain abscess.

ENT · 3rd Professional

Nature and Pathology

An otogenic brain abscess is a focal collection of pus within the brain substance arising from middle ear or mastoid infection.
It carries the highest mortality of all otitic complications, and remains a significant cause of death in young people in India.
The two sites are the temporal lobe and the cerebellum.

Feature	Temporal lobe abscess	Cerebellar abscess
Route	Through the tegmen into the middle fossa	Through trautmann triangle or the sinus plate into the posterior fossa
Frequency	About twice as common	Less common; often with lateral sinus thrombosis
Speech	Nominal aphasia if the dominant lobe is involved — the inability to name common objects. An early and easily elicited sign	Normal
Visual field	Contralateral upper quadrantic homonymous defect (a "pie in the sky") from involvement of MEYER loop	Normal
Motor	Contralateral hemiparesis and facial weakness of upper motor neurone type	Ipsilateral limb ataxia and weakness
Coordination	Normal	Ipsilateral past-pointing, intention tremor, dysdiadochokinesia, dysmetria and hypotonia
Gait	Normal or hemiparetic	Ataxic, with falling toward the side of the lesion
Nystagmus	Absent	Coarse nystagmus, worse and slower on looking toward the lesion
Seizures	Common, and may be the presenting event	Uncommon
Pupil	Ipsilateral third nerve palsy with a dilated pupil in uncal herniation — a terminal sign	Tonsillar herniation with neck stiffness and respiratory arrest

Investigation and Management

Contrast-enhanced CT of the brain is the standard investigation, showing a hypodense centre with ring enhancement and surrounding oedema, with mass effect and midline shift
MRI is superior, particularly for early cerebritis and posterior fossa lesions.
High-resolution CT of the temporal bone defines the ear disease and the bony defect
Lumbar puncture IS absolutely contraindicated — it risks coning and death, and it will not establish the diagnosis
Blood cultures, inflammatory markers, and culture of the aspirated pus including anaerobic culture
Management is a neurosurgical and otological emergency, and it is joint:
High-dose intravenous antibiotics crossing the blood-brain barrier — a third-generation cephalosporin with metronidazole, with vancomycin where staphylococcus is likely — continued for 6 to 8 weeks
Control of raised intracranial pressure with mannitol and dexamethasone, and anticonvulsants
Neurosurgical drainage — repeated aspiration through a burr hole, which is preferred; or excision of the capsule for a multiloculated, thick-walled or recurrent abscess
Mastoid surgery to eradicate the ear disease, which is essential: a brain abscess whose source is left will recur. The timing is decided jointly — the abscess is addressed first, and the ear as soon as the patient is stable, sometimes at the same sitting
Prognosis — mortality has fallen greatly with imaging and antibiotics but remains significant, and depends chiefly on the conscious level at presentation. Rupture into the ventricle is fatal
Survivors may have persistent epilepsy and neurological deficit, and need long-term follow-up
The latent stage is why the diagnosis is missed; the patient seems well for days or weeks between cerebritis and manifest disease
Absence of fever does not exclude a brain abscess, and the temperature is frequently normal in the manifest stage
Ask the patient to name a pen and a watch; nominal aphasia is an early sign of dominant temporal lobe abscess and costs nothing
Temporal lobe abscess is twice as common as cerebellar, and follows erosion of the tegmen
Cerebellar abscess often accompanies sinus thrombosis, both arising through the sinus plate and Trautmann triangle
Falling toward the side of the lesion is characteristic of a cerebellar abscess, along with ipsilateral incoordination
Metronidazole is essential, since otogenic abscesses are frequently anaerobic or mixed
Never do a lumbar puncture; it risks coning and will not establish the diagnosis
Diffusion-weighted MRI separates abscess from tumour, an abscess restricting diffusion where a cystic tumour does not
MRI is superior in the posterior fossa, where bone artefact degrades the CT and a cerebellar abscess may be missed

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