Page 928 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 71: Bacterial Infections of the Central Nervous System 659

Trauma is a third pathogenic mechanism in the development of lesion is in the dominant hemisphere. A visual field defect (eg, an upper
brain abscess, whether secondary to an open cranial fracture with dural homonymous quadrantanopia) may be the only presenting sign of a
breach, to neurosurgery, or (especially in children) to a foreign body temporal lobe abscess. Persons with brain stem abscesses usually pres-
injury. 52,54 The incidence of brain abscess formation after head trauma ent with facial weakness, fever, headache, hemiparesis, dysphagia, and
ranges from 3% to 17% in military populations, where it is usually sec- vomiting. Clues to the site of origin of the infection should also be
ondary to retained bone fragments or contamination of initially “sterile” sought; these include otorrhea, orbital cellulitis, purulent nasal dis-
missile sites with bacteria from skin, clothes, and the environment. charge, dental sepsis, and postoperative or posttraumatic cranial infec-
Predisposing traumatic conditions in the civilian population (incidence tion; such findings occur in about 60% of cases. It is important to note
of 2.5%-10.9%) include compound depressed skull fractures, dog bites, that the clinical presentation of brain abscess in immunosuppressed
rooster pecking, and, especially in children, injury from lawn darts and patients may be masked by the diminished inflammatory response.
pencil tips. Likely infective microorganisms after trauma include staphy-
lococci, streptococci, gram-negative bacilli, and anaerobes. ■ DIAGNOSIS
Brain abscess is cryptogenic in about 10% to 35% of patients. Many
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of these cases are secondary to unrecognized dental foci of infection; The diagnosis of brain abscess has been revolutionized by CT, which
not only is an excellent means to examine the brain parenchyma but
patent foramen ovale has also been suggested as a possible predisposing
factor. In this subgroup of patients, broad antimicrobial therapy is indi- also is superior to standard radiologic procedures for examination of
The sensitiv-
the paranasal sinuses, mastoid cells, and middle ear.
52,54
cated pending culture results (see the section Treatment below).
Overall, the most commonly isolated bacterial species in brain abscess ity of CT is 95% to 99% for brain abscess; it also yields information
are streptococci (aerobic, anaerobic, and microaerophilic), which are concerning the extent of surrounding edema, the presence or absence
present in approximately 70% of cases, and are frequently isolated in of a midline shift, the presence of hydrocephalus, and the possibility of
imminent ventricular rupture. The characteristic appearance of brain
mixed infection (30%-60% of cases). 52,54 These bacteria (especially the
Streptococcus milleri/anginosus/intermedius group) normally reside in abscess on CT is a hypodense center with a uniform peripheral ring
enhancement after the injection of contrast material; this is surrounded
the oral cavity, appendix, and female genital tract and have a procliv-
ity for abscess formation. Staphylococcus aureus, which was isolated in by a variable hypodense area of brain edema. A similar appearance
is seen with neoplasms, granulomas, cerebral infarction, or resolving
25% to 30% of cases in the era before antibiotics, currently accounts for
10% to 20% of isolates, although the frequency of isolation of S aureus hematoma. Contrast enhancement of the ependymal lining suggests
ventriculitis. Other CT findings include nodular enhancement and areas
is increased in certain clinical situations (eg, cranial trauma and endo-
carditis). Attention to proper culture techniques has increased the rate of low attenuation without enhancement; the latter finding is observed
during the early stage of cerebritis, before abscess formation; as the
of isolation of anaerobes, with Bacteroides species isolated in 20% to
40% of cases, often in mixed culture. Enteric gram-negative bacilli abscess progresses, contrast enhancement is observed. In later stages, as
the abscess becomes encapsulated, contrast no longer differentiates the
(Proteus species, E coli, Klebsiella species, and Pseudomonas species) are
isolated in 23% to 33% of patients. Other bacterial species occur less lucent center, and the CT appearance is similar to that in the early stage
of cerebritis. The use of delayed films may be helpful because the pres-
commonly (<1% of cases) and include H influenzae, S pneumoniae,
L monocytogenes, and Nocardia asteroides (Nocardia is more often ence of contrast material in the center of the lesion suggests cerebritis.
The absence of contrast material likely indicates a well-encapsulated
isolated in patients with T lymphocyte or mononuclear phagocyte
defects). Nocardial brain abscesses have increased in incidence with lesion. This difference is important therapeutically because cerebritis
may respond to medical therapy alone, whereas most encapsulated
the increasing numbers of immunosuppressed patients, although up to
48% of patients with nocardiosis have no underlying conditions. Brain lesions require surgical intervention. CT is also useful for following the
course of brain abscess, although, after aspiration, improvement in the
abscesses due to Actinomyces species are commonly associated with pul-
monary and odontogenic infections. Multiple organisms are cultured in CT appearance may not be seen for up to 5 weeks or longer. Complete
resolution may take 4 to 5 months.
14% to 28% of cases; the incidence of negative cultures ranges from 0%
MRI has an important role in the diagnosis of brain abscess, for which
to 43%, with an increased frequency often correlated with use of prior it has several advantages over CT, and has now become the first imaging
antimicrobial therapy. modality of choice in patients with brain abscess. MRI is more effec-
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■ CLINICAL PRESENTATION tive for the early detection of cerebritis. Also, in cases of cerebral edema,
MRI shows a stronger contrast between an edematous and a normal
The clinical course of brain abscess may be indolent or fulminant; 75% brain and more clearly shows the spread of inflammation into the ven-
of patients have symptoms for less than 2 weeks. Most of the clini- tricles and subarachnoid space. MRI also permits earlier detection of
cal manifestations are due to the presence of space-occupying lesions satellite lesions. T1-weighted images characteristically demonstrate a
within the brain. 52,54 The most common symptom is headache, present peripheral zone of mild hypointensity (representative of edema forma-
in an average of 70% to 75% of patients. The headache is usually mod- tion) related to adjacent brain, which surrounds a central zone of more
erate to severe and hemicranial, but it may be generalized; sudden marked hypointensity (indicative of the necrotic center of the abscess);
worsening with the onset of new-onset meningismus may signify these two regions are separated by a capsule that appears as a discrete
intraventricular rupture of the abscess. Other findings include nausea rim, which is isointense to mildly hyperintense. On T2-weighted images,
and vomiting (∼50% of cases), nuchal rigidity (25%), and papilledema there is an area of marked hyperintensity in the zone of edema when
(∼25%). Mental status changes ranging from lethargy to coma occur in compared with adjacent brain, whereas the central core is isointense
the majority of cases. Seizures, usually generalized, occur in 13% to 35% to hyperintense compared with gray matter; the capsule appears as a
of patients. Fever appears in only 32% to 79% of cases; afebrile patients well-defined hypointense rim at the margin of the abscess. Contrast-
tend to be older and to have a longer duration of illness and a higher enhanced MRI, using the paramagnetic agent gadolinium diethylene-
mortality rate. Fewer than 50% of patients present with the classic triad triamine pentaacetic acid, has the advantage of clearly differentiating
of fever, headache, and focal neurologic deficit. Patients with frontal the central abscess, the surrounding contrast-enhancing rim, and the
lobe abscess often present with headache, drowsiness, inattention, and cerebral edema surrounding the abscess. MRI is the current diagnostic
deterioration in mental status; the most common focal neurologic procedure of choice for detection of brain abscess, although it is not
signs are hemiparesis, with unilateral motor signs, and a motor speech always feasible in critically ill patients.
disorder. The clinical presentation of cerebellar abscess may include A major advance in the use of CT is the availability of stereotaxic
ataxia, nystagmus, vomiting, and dysmetria. Persons with abscess of the CT-guided aspiration of the abscess to facilitate bacteriologic diagnosis.
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temporal lobe may present with ipsilateral headache and aphasia, if the However, aspiration during the early cerebritis stage may be complicated

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