Non Traumatic brain abscess
The most frequent intracranial locations (in descending order of frequency) are: frontal-temporal, frontal-parietal, parietal, cerebellar, and occipital lobes.
There are approximately 1500 to 2000 cases of brain abscess diagnosed in the United States annually, with an estimated 1 in 10,000 being hospitalized for a brain abscess. The infection tends to occur in young men, although infection can occur in all age groups; the male-to-female ratio varies between 2 : 1 and 3 : 1. In some series, children account for up to 25% of cases.
The epidemiology of brain abscess has changed with the increasing incidence of this infection in immunocompromised patients, particularly solid organ and bone marrow transplant recipients, and the decreasing incidence of brain abscess related to sinusitis and otitis 1).
There have been several trends in the epidemiology of brain abscess over recent decades. One trend is that there appears to be a trend toward a decreasing incidence of brain abscess. In a population-based study of residents of Olmstead County, Minnesota, the incidence rate was 1.3 per 100,000 patient-years from 1935 to 1944 compared with 0.9 per 100,000 patient-years from 1965 to 1981 2).
Brain abscess commonly occurs secondary to an adjacent infection (mostly in the middle ear or paranasal sinuses) or due to hematogenous spread from a distant infection or trauma.
Development of brain abscess after an infarction is a rare clinical condition. There have been 11 cases in the literature. Many patients were treated with potent antibiotics only and did not survive. Wang et al., present 2 cases in which patients received aggressive surgical resection of brain abscess and survived. The analysis of the literature confirmed this finding that surgical intervention of brain abscess in patients after stroke is advisable.
Secondary abscess formation after significant ischemic stroke is a rare condition that carries potential for high morbidity/mortality. The limited body of literature with the addition of our 2 cases supports aggressive management with surgical evacuation of brain abscess to increase survival. 4).
Until recently, post-radiotherapy brain abscess was considered rare, but it has become an increasingly important etiology. Discussions of the relationship between bacterial brain abscess and radiotherapy (RT) are rare in the literature. Further study based on a proper patient cohort is warranted 7).
Now hematogenous spread is the most common vector.
Generally occurring after septic episodes in immunodeficient patients or complicating neurosurgical procedures. Even though they are known complications of surgically treated intracerebral haemorrhages (ICH), the presence of a brain abscess at the site of an untreated ICH is a rare event.
Such cases may result from haematogenous spread from distant foci or contiguous sites and are often preceded by episodes of sepsis and local infection. Immunodeficiency, AIDS, age, diabetes mellitus and vitamin-K deficiency are predisposing factors.
Multiple abscesses have been noted in 10 to 50% of these patients 9).
In patients with bronchiectasis and with new neurological manifestations, infected lesions in the central nervous system should be excluded.
Infections of the abdomen – such as peritonitis (an infection of the bowel lining).
Pelvic infections – such as infection of the bladder lining (cystitis).
Is an uncommon complication of severe cystic fibrosis lung disease 11).
Streptococcus is most common, up to 60 % are polymicrobial.
Similar to any other mass lesion but tend to progress rapidly.
Abscess formation should be considered in case of clinical deterioration, headache, and any neurological deficit after febrile episodes.
Specimens obtained during surgery or stereotactic computerized axial tomography (CT) guided aspiration should be sent for aerobic, anaerobic, mycobacterial and fungal culture and, when indicated, for protozoa.
Routine laboratory studies are not helpful for the diagnosis of brain abscess. Leukocytosis may be absent; in some series about 40% of patients have a normal peripheral white blood cell count. Acute-phase reactants are moderately helpful but nonspecific. The C reactive protein level is elevated in almost all patients, but the sedimentation rate can be only moderately elevated and sometimes is normal. Samples for blood cultures should be obtained in all suspected cases; although the yield is low, a positive result can be extremely valuable.
CT scanning with contrast during stage 1 (early cerebritis) may show only edema—an area of hypodensity—which may or may not enhance with contrast. If done very early in the course of infection, a contrast-enhanced CT may be normal. During later stages, there is the development of a space-occupying lesion with a hypodense center and later a ring-enhancing rim, which is often surrounded by a large area of edema. Occasionally, ring enhancement can be seen with late cerebritis; delayed administration of contrast that fills in the central hypodensity is suggestive that the lesion is still in the cerebritis stage. Although contrast-enhanced CT scanning is considered sensitive for the detection of brain abscesses, it is not specific. Brain abscesses tend to have smooth thin-walled capsules, whereas tumors tend to have more irregular capsules. There are additional characteristics of brain tumors, but some overlap exists with brain abscesses. It is important to note that brain abscesses and brain tumors may have an identical appearance on the CT scan.
MR imaging is more sensitive than CT, and MRI can usually detect infection in the early cerebritis stage. The MRI scan in focal cerebritis usually shows an area of hypointensity on T1- and T2-weighted imaging. The characteristic appearance of a mature brain abscess on MRI is a focal lesion with low intensity on a T1-weighted image, with a smoothly marginated capsule that enhances with IV gadolinium.
On T2WI, the central abscess is hyperintense and the surrounding capsule is hypointense. There is extensive surrounding edema in most cases. The finding of a capsule that is hypointense on a T2-weighted image and mildly hyperintense on a T1-weighted image is suggestive of an abscess capsule.
The DWI shows often hyperintensity (restriction), (not reliable).
Diffusion-weighted imaging (DWI) is widely appreciated as an indispensable tool in the examination of the central nervous system. It is considered useful not only for the detection of acute ischemic stroke but also for the characterization and differentiation of brain tumors and brain abscess.
Proton magnetic resonance spectroscopic imaging (PMRS) has high sensitivity and specificity for the detection of pyogenic brain abscess and the categorization of bacteria. But the metabolite patterns failed to evaluate the etiology of disease when the culture results are sterile.
Based on metabolite resonances, PMRS can detect slow growing and fastidious organisms and classify them into aerobic and anaerobic bacteria which are difficult to culture by conventional method. It can categorize microorganisms even in culture sterile samples with rational sensitivity and specificity which may allow early choice of targeted therapy 12).
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
satellite lesions favour infection
abscesses may have low intensity capsule
rCBV elevated in high grade gliomas, reduced in abscesses
low intensity SWI rim of GBM
incomplete and irregular in 85%
within (rather than overlapping) the peripheral enhancement
absent dual rim sign
subacute infarction, haemorrhage or contusion
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 diagnosis:
Strongly influences poor outcome in patients with cyanotic heart disease. The key to decreasing poor outcomes may be the prevention and management of IVROBA. To reduce operative and anesthetic risk in these patients, abscesses should be managed by less invasive aspiration methods guided by computed tomography. Abscesses larger than 2 cm in diameter, in deep-located or parieto-occipital regions, should be aspirated immediately and repeatedly, mainly using computed tomography-guided methods to decrease intracranial pressure and avoid IVROBA. IVROBA should be aggressively treated by aspiration methods for the abscess coupled with the appropriate intravenous and intrathecal administration of antibiotics while evaluating intracranial pressure pathophysiology 13).
Once an abscess has formed, surgical excision or drainage combined with prolonged antibiotics (usually 4-8 weeks) remains the treatment of choice.
Early diagnosis with neuroradiological imaging, infection blood markers and microbiological identification of the causative pathogen is crucial for treatment with surgical drainage (needle drainage for some) or excision and specific antibiotic therapy, which guarantee good outcome and long-term survival. In fact, while prompt diagnosis and treatment guarantee good outcome and long-term survival, morbidity and mortality are very high in case of misdiagnosis 14).
The antibiotic must also have a bactericidal effect on the pathogen.
There have been advances in microbial definition of bacterial brain abscess. The identification of Bacteroides fragilis as a pathogen in certain brain abscesses has established a role for a newly available antibiotic, metronidazole. The study of the pathological distinction between cerebritis and frank abscess is clarifying two clinical characteristics of brain abscess: the limited success of antibiotic treatment and the increase in intracranial pressure 15).
There has been a gradual improvement in the outcome of patients with brain abscess over the past 50 years, which might be driven by improved brain imaging techniques, minimally invasive neurosurgical procedures, and protocoled antibiotic treatment. Multicenter prospective studies and randomized clinical trials are needed to further advance treatment and prognosis in brain abscess patients.
Our understanding of brain abscesses has increased by meta-analysis on clinical characteristics, ancillary investigations, and treatment modalities. Prognosis has improved over time, likely due to improved brain imaging techniques, minimally invasive neurosurgical procedures, and protocoled antibiotic treatment 16).
Current evidences suggest that for encapsulated brain abscess in superficial non-eloquent area, abscess resection compared to abscess aspiration had lower post-operative residual abscess rate; lower re-operation rate; higher rate of improvement in neurological status within 1 month after surgery, shorter duration of post-operative antibiotics and average length of hospital stay. There was no statistically significant difference in the rate of improvement in neurological status at 3 months post-operative and the mortality 17).