Neuroradiology

Abscess, cerebral

infectious process in which a phase of inflammation involving vessels and brain tissue, develops an organized pus collection surrounded by a capsule. Clinical presentation is that of a fast-developing space-occupying syndrome (median duration of symptoms before diagnosis = 15 days) usually beginning with headache and focal neurological deficit that may progress to a raised intracranial pressure syndrome with nausea, vomiting and impairment of consciousness with fever. Fever is absent in up to 50% of cases. The frequency of seizures is estimated to be around 25 - 40%.

Owing to its high morbidity and mortality if undiagnosed and untreated, brain abscess continues to be a medical challenge, unchanged despite the introduction of antibiotics and the refinement of surgical techniques. Delay in diagnosis is considered to be the major obstacle to the success of therapeutic intervention.

It is only in the last 10-15 years that improvements in the outcome for patients with brain abscess have been obtained. Advances in noninvasive diagnostic techniques are thought to be responsible for this change.

In developed countries it is estimated that brain abscess accounts for approximately one in 10,000 general hospital admissions. A median age of 30-40 years has been reported and 25% of cases occur in children less than 15 years of age.

Extension of infection from a contiguous focus is estimated to be the cause in about 45% of cases. Otologic infections are the most frequent, accounting for about 30% of cases, being complicated by brain abscess in 1 in 3,500 cases. Sinusitis follows, accounting for about 15% of cases.

Infection from a contiguous focus follows two possible pathways: direct extension through areas of associated osteitis or osteomyelitis; or retrograde thrombophlebitis along diploic or emissary veins into the intracranial compartment. Haematogenous seeding from a distant focus of infection accounts for about 25% of cases. Endocarditis is the most frequent source of haematogenous infection. Chronic cyanotic heart disease is highly associated with brain abscess and is the most frequent cause in children. Other causes of brain abscess are dental and facial infections, trauma with penetrating injuries and surgery. In 15-20% of cases brain abscess develops without an identified cause. Microbiology of the positive abscess cultures (about 60%) yields multiple organisms in about a third of cases. Overall aerobic and anaerobic streptococci are the most frequently encountered organisms accounting for about 40% of cases, followed by Gram-negative bacilli (16%), staphylococcus aureus (13%) and bacteroides (10%). Of note, the most frequent pathogens in abscesses complicating neonatal meningitis are of the enteric Gram-negative group including Salmonella group B, and the vast majority of cases of meningitis complicated by abscesses in neonates are caused by Proteus mirabilis or Cytrobacter diversus.

Imaging

Neuroradiological examinations are an emergency priority, definitely before any lumbar puncture. CT scan is still the easiest neuroradiological examination that can be performed in emergency (Fig.1). The typical appearance of a brain abscess on CT scan is that of a focal, usually round or oval expanding lesion characterized by a hypodense centre with an outlying uniform ring surrounded by a variable hypodense region of brain oedema. Contrast enhancement is usually always seen at the level of the ring. The localization may suggest the aetiology: abscesses resulting from otological infection are usually localized in the temporal lobe sometimes in the posterior fossa; those resulting from sinusitis are usually in the frontal lobe; those resulting from haematogenous seeding are usually multiple and follow a vascular distribution.

Ring-enhancing lesions are not diagnostic of abscess: tumours, granulomas and resolving haematomas may have the same appearance. The differential diagnosis is open in particular when fever and systemic signs of inflammation are lacking. As compared to tumours abscesses usually exhibit thinner, more regular and more homogeneously enhancing rings. On MRI the three regions (necrotic central region, surrounding rim and oedema) are depicted with better accuracy. Interestingly the necrotic central region shows a different signal as compared to CSF, higher on both T2- and T1-weighted images, allowing a differential diagnosis from cystic neoplasias. The additional contribution of MRI to the diagnosis of abscess is related to its better sensitivity for the initial cerebritis phase or for smaller satellite lesions. Nuclear medicine studies with 99-m Tc HPAO leucocyte scintigraphy may provide a specific test to differentiate bacterial abscesses from other rim enhancing lesions.
 
GS-FS

FS

To view high resolution images,
please register first.

Click  here to register.

Already registered? Enter your e-mail in the window below.
Re-register

Fig.1

a. Axial CT, following contrast medium injection. An oval hypodense lesion of the right frontal pole is seen, with a thin regular enhancing rim. Mild hypodensity of the white matter of the frontal lobe, representing oedema. b. MR, T1-weighted axial image. Hypointense lesion (with a faint fluidfluid level) with a thin isointense capsule and white matter oedema. c. MR, T2-weighted axial image. Hyperintensity of the content of the abscess and isointense rim. d, e. MRI following contrast injection. Marked enhancement of the capsule of the abscess. In the most cranial portion (e) a forming daughter abscess is seen.
Abscess, cerebral, Fig.1 (a)
Abscess, cerebral, Fig.1 (b)
Abscess, cerebral, Fig.1 (c)
Abscess, cerebral, Fig.1 (d)
Abscess, cerebral, Fig.1 (e)