Brain abscesses are a potentially life threatening condition requiring rapid treatment, and prompt radiological identification. Fortunately MRI is usually able to convincingly make the diagnosis, distinguishing abscesses from other ring enhancing lesions.
Demographics reflect at-risk groups (see below) with all age groups being affected.
Clinical presentation is non-specific with many cases having no convincing inflammatory / septic symptoms. Symptoms of raised intracranial pressure, seizures and focal neurological deficits are most common forms of presentation. Eventually many abscesses rupture into ventricular system, which results in a sudden and dramatic worsening of the clinical presentation and often heralds a poor outcome.
Pathology and microbiology
Cerebral abscesses result from pathogens growing within the brain parenchyma, initially as a cerebritis and then eventually demarcating into a cerebral abscess. Historically direct extension from sinus or scalp infections was the most common source. More recently haematological spread has become most common. Direct introduction by trauma or surgery accounts for only a small minority of cases 3.
Risk factors for haematogenous spread include 3:
- right to left shunt
- intravenous drug usage (IVDU)
- lung infection
- dental abscess
- systemic sepsis
Streptococcus sp : 35 - 50% 3
- especially S. pneumoniae 4
- sterile : 25%
- mixed: variable, 10-90% of cases depending on source 3
- Staphylococcus aureus and epidermidis : following neurosurgery
- Gram negative species more common in infants
The immunocompromised patient is susceptible to a host of other organisms including 3:
- Toxoplasma gondii
- Nocardia asteroides
- Candida albicans
- Listeria monocytogenes
- Mycobacterium sp
- Aspergillus fumigatus
Both CT and MRI demonstrate similar features, although MRI has the ability to better distinguish cerebral abscess from other ring enhancing lesions.
In patients with suspected intraparenchymal sepsis, pre and post contrast scans should be obtained, unless the plan is to proceed to MRI regardless of the CT findings. Typical appearances include:
- ring of iso / hyperdense tissue, typically of uniform thickness
- central low attenuation (fluid / pus)
- surrounding low density (vasogenic oedema)
- ventriculitis may be present, seen as enhancement of the ependyma
- obstructive hydrocephalus will commonly be seen when intraventricular spread has occurred
MRI is more sensitive and especially with the addition MRS and DWI far more specific for the diagnosis of cerebral abscesses.
- central low intensity (hyperintense to CSF)
- peripheral low intensity (vasogenic oedema)
- ring enhancement
- ventriculitis may be present, in which case hydrocephalus will commonly also be seen
T2 / FLAIR
- central high intensity (hypointense to CSF, does not attenuate on FLAIR)
- peripheral high intensity (vasogenic oedema)
- the abscess capsule may be visible as a intermediate to slightly low signal thin rim 1.
DWI / ADC
- high DWI signal is usually present centrally
- often this represents true restricted diffusion (low signal on ADC)
- peripheral or patchy restricted diffusion may also be seen; this finding is however not as constant as one may think, with up to half of rim enhancing lesions demonstrating some restriction not proving to be abscesses 2.
- in many instances high DWI are associated with high ADC signal, consistent with T2 shine through of the central necrotic portion
- MR perfusion : rCBV is reduced in the surrounding oedema c.f. to both normal white matter and tumour oedema seen in high grade gliomas 2.
- MR spectroscopy : elevation of a succinate peak is relatively specific but not present in all abscesses ; high lactate, acetate, alanine, valine, leucine, and isoleucine levels peak may be present ; Cho / Crn and NAA peaks are reduced.
Treatment and prognosis
The mainstay of treatment for cerebral abscesses is neurosurgical intervention and drainage of the collection. This can be performed either by stereotactic aspiration ro craniotomy 7. Broad spectrum intravenous antibiotics are also needed and can later be changed to agents tailored to the specific organisms.
In cases where the abscess cavity does not completely obliterate, followup with MRI including DWI is useful and lack of restricted diffusion is reassuring. Demonstration of ongoing restricted diffusion in a cavity suggests persistent infection 8.
The differential of an established abscess is essentially that of a ring enhancing lesion, and therefore includes:
metastasis or high grade glioma (e.g. GBM)
- abscesses tend to have smoother inner wall 2
- satellite lesions favour infection 2
- abscesses may have low intensity capsule 1-2
- rCBV elevated in high grade gliomas, reduced in abscesses 2
- sub acute infarction / haemorrhage / contusion
- radiation necrosis
When a lesion demonstrates both ring enhancement and central restricted diffusion the differential is very much narrowed, and although cerebral abscess is by far the most likely diagnosis, the following should also be included on the differential 6:
- necrotic adenocarcinoma
- 1. Haimes AB, Zimmerman RD, Morgello S et-al. MR imaging of brain abscesses. AJR Am J Roentgenol. 1989;152 (5): 1073-85. AJR Am J Roentgenol (abstract) - Pubmed citation
- 2. Holmes TM, Petrella JR, Provenzale JM. Distinction between cerebral abscesses and high-grade neoplasms by dynamic susceptibility contrast perfusion MRI. AJR Am J Roentgenol. 2004;183 (5): 1247-52. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Greenberg MS. Handbook of neurosurgery. George Thieme Verlag. (2006) ISBN:313110886X. Read it at Google Books - Find it at Amazon
- 4. Popp A, Popp AJ, Deshaies EM. A Guide to the Primary Care of Neurological Disorders. Thieme Medical Pub. (2007) ISBN:1588905160. Read it at Google Books - Find it at Amazon
- 5. Ping H. Lai et.al, Brain Abscess and Necrotic Brain Tumor: Discrimination with Proton MR Spectroscopy and Diffusion-Weighted Imaging, American Journal of Neuroradiology 23:1369-1377, September 2002
- 6. Hartmann M, Jansen O, Heiland S et-al. Restricted diffusion within ring enhancement is not pathognomonic for brain abscess. AJNR Am J Neuroradiol. 2001;22 (9): 1738-42. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 7. Johnson RT, Griffin JW, McArthur JC. Current Therapy in Neurologic Disease. Mosby. (2006) ISBN:0323034322. Read it at Google Books - Find it at Amazon
- 8. Cartes-zumelzu FW, Stavrou I, Castillo M et-al. Diffusion-weighted imaging in the assessment of brain abscesses therapy. AJNR Am J Neuroradiol. 2004;25 (8): 1310-7. AJNR Am J Neuroradiol (citation) - Pubmed citation
Synonyms & Alternative Spellings
|Synonyms or Alternative Spelling||Include in Listings?|