Acute bacterial infections and bacterial abscesses

15


Acute bacterial infections and bacterial abscesses



ACUTE BACTERIAL MENINGITIS



NEONATAL BACTERIAL MENINGITIS



MACROSCOPIC APPEARANCES


The brain is swollen and congested. Hemorrhagic infarcts are common and may be extensive (Fig. 15.1), and a purulent exudate is seen over the cerebral hemispheres or the base of the brain or both (Fig. 15.2). The ventricles may appear compressed by the swollen cerebral hemispheres, or distended if the aqueduct has been obstructed by purulent material. There may be coexistent cerebral abscesses, particularly in association with Citrobacter diversus or Proteus mirabilis meningitis.





MICROSCOPIC APPEARANCES


The meningeal exudate contains large numbers of neutrophils, scattered macrophages, fibrin, and necrotic cellular debris. The exudate may be particularly prominent over the base of the brain. It extends along perivascular spaces into the brain parenchyma. Bacteria are usually demonstrable within the exudate.



image NEONATAL BACTERIAL MENINGITIS


In the UK, USA, and in many other developed countries, group B streptococci and Escherichia coli account for over 65% of cases of neonatal bacterial meningitis. However, the range of bacteria responsible for meningitis in neonates is wide (Table 15.1) and varies in different regions and even at different times in the same region.




image Outbreaks of neonatal meningitis due to group B streptococci or other bacteria may be caused by cross-contamination within hospital wards. Epidemics of listerial meningitis have been traced to contaminated batches of unpasteurized cheese and other uncooked foods.


image Infants of low birth weight are at particular risk. Other risk factors include prolonged rupture of the amniotic membranes and postpartum maternal pyrexia.


image The absence of alveolar macrophages, a relatively small pool of neutrophils, and low levels of secretory IgA and IgG and a limited capacity for their synthesis, all render neonates particularly susceptible to bacterial infection.


image The initial portal of entry is usually oral, from contaminated amniotic fluid, the maternal genital tract, or the hands of the mother or ward staff. The bacteria cross the respiratory or gastrointestinal mucosa, proliferate within the blood, and enter the CNS.



image NEONATAL BACTERIAL MENINGITIS




Inflammatory cells tend to infiltrate the walls of leptomeningeal and cortical arteries and veins (Fig. 15.3). Intimal accumulations of inflammatory cells may be a prominent feature. Venous thrombosis and infarction are often evident (Figs 15.1, 15.3) and may be extensive.



Purulent material is usually seen in the choroid plexus and may also adhere to ventricular walls. An acute, predominantly perivascular, inflammatory infiltrate is present in the adjacent tissue.




ACUTE BACTERIAL MENINGITIS IN CHILDREN AND ADULTS



MACROSCOPIC APPEARANCES


The brain is swollen and the leptomeninges are congested. A purulent exudate is seen in the subarachnoid space over the cerebral hemispheres and the base of the brain (Fig. 15.5a), and accumulating in the basal cisterns and cerebral sulci. The exudate associated with pneumococcal infections is often particularly prominent over the cerebral convexities (Fig. 15.5b). There may be little or no macroscopically discernible exudate in patients dying very acutely or those who have been partially treated with antibiotics. Sectioning of the brain may reveal edematous white matter and compressed ventricles or ventricular enlargement due to obstructive hydrocephalus. There is usually purulent fluid within the ventricles. The ventricular exudate may be pronounced in patients with ventricular shunt infection (Fig. 15.6), in whom the leptomeninges appear remarkably clear. Occasionally, the coagulopathy associated with meningococcal septicemia is complicated by ventricular hemorrhage. Small infarcts may result from thrombosis of penetrating arteries (Fig. 15.7). Venous thrombosis and hemorrhagic infarcts are less common than in neonatal meningitis.






image ACUTE BACTERIAL MENINGITIS IN CHILDREN AND ADULTS


The responsible organisms vary according to age:



image In neonates, meningitis is usually caused by Gram-negative bacilli (mostly Escherichia coli but also Klebsiella and Citrobacter species). Less frequent causes include Listeria monocytogenes, Staphylococcus epidermidis and Staphylococcus aureus.


image In children, especially those aged between 1 month and 5 years, Haemophilus influenzae type b and Neisseria meningitidis (meningococcus) are responsible for approximately 80% of cases. H. influenzae is a particularly common cause in children under 2 years of age. Streptococcus pneumoniae (pneumococcus) accounts for a further 10–20% of cases.


image In adults, S. pneumoniae is the commonest cause of bacterial meningitis, accounting for up to 50% of all cases. People at particular risk are the elderly or debilitated, and those with an immunodeficiency disorder or a dural fistula (usually post-traumatic) or those who have had a splenectomy. A dural fistula may be associated with recurrent bacterial meningitis. N. meningitidis meningitis is slightly less common overall and tends to occur in outbreaks, being facilitated by crowded living conditions and poor hygiene.


image The risk of infection with any of the three main pathogens is reduced by the presence of circulating specific anticapsular antibodies and increased by high rates of nasopharyngeal carriage in contacts. The carriage rate tends to be much lower for H. influenzae type b than for N. meningitidis or S. pneumoniae, but is increased for all these bacteria among the contacts of infected patients, in closed populations, and in crowded living conditions. Patients with H. influenzae or S. pneumoniae meningitis have often experienced a preceding upper respiratory tract infection or otitis media. Up to 50% of patients with pneumococcal meningitis have an associated pneumonia.


image Vaccination is highly effective in preventing Haemophilus influenzae type b pneumonia and meningitis in children. Current meningococcal vaccines offer a high level of protection against N. meningitidis serogroups A, C, W-135 and Y, for up to 3 years in most recipients. Pneumococcal vaccination provides >50% protection against invasive infection by S. pneumoniae (including meningitis), for several years.


image Gram-negative bacilli are responsible for approximately 65% of cases of meningitis complicating neurosurgery, and a smaller but significant proportion occurring after head injury. E. coli, Klebsiella species, and Pseudomonas aeruginosa are most often involved. Gram-negative bacterial meningitis and occasionally meningitis due to S. aureus may also occur in the context of severe debilitation, diabetes mellitus, or chronic alcoholism.


image Patients with ventricular or lumbar shunts have a risk of developing ventriculitis and meningitis, due particularly to slime-producing strains of S. epidermidis. Less commonly, S. aureus or Gram-negative bacilli are responsible. The infection is usually introduced at the time of surgery, although the manifestations may be delayed for weeks or even months.


image Bacillus anthracis has been responsible for several outbreaks of industrial, bioweapons- or bioterrorism-related meningitis. This large, Gram-negative bacillus primarily infects herbivores such as cattle, sheep, horses, and goats. The spores can remain dormant in soil for prolonged periods. The route of entry in human infections may be cutaneous, gastrointestinal or inhalational. The risk of meningitis after cutaneous or gastrointestinal exposure is only ~ 5% but can be as high as 50% after inhalational exposure (as in bioweapons- and bioterrorism-related outbreaks).


Subdural effusions are an occasional complication of meningitis in children and appear as accumulations of clear fluid over the cerebral convexities or in the interhemispheric fissure.



image BACTERIAL MENINGITIS IN CHILDREN AND ADULTS




image Bacterial meningitis typically presents with pyrexia, headache, vomiting, nuchal rigidity, and photophobia. Patients may develop seizures, cranial nerve palsies, or focal neurologic deficits. These are particularly common in pneumococcal meningitis.


image Initial symptoms and signs may progress rapidly over a few hours to obtundation and coma or, most notably in some children with Haemophilus influenzae meningitis, may evolve over several days.


image Bacterial meningitis commonly causes symptoms and signs of raised intracranial pressure. Occasionally, these are due to the development of a subdural effusion (an accumulation of albumin-rich fluid). This is much more frequently seen in infants and children, in whom the effusion sometimes becomes infected, resulting in a subdural empyema.


image Over 50% of patients with meningococcal meningitis develop a rash. This is initially maculopapular, but later becomes petechial or purpuric. In a small proportion of children with meningococcal meningitis, overwhelming septicemia results in bilateral adrenal hemorrhage and circulatory collapse (Waterhouse–Friderichsen syndrome).


image The mortality of bacterial meningitis varies between about 5–40% and depends on the age of the patient, the responsible agent and the presence of specific risk factors. Mortality tends to be highest in infants, the elderly and the debilitated. S. pneumonia and Gram-negative bacilli carry a relatively high risk of morbidity and mortality. A few patients have survived anthrax meningitis after cutaneous or gastrointestinal exposure but no survivors have been reported of meningitis complicating inhalational exposure.

< div class='tao-gold-member'>

Related posts:

Hydrocephalus Meningiomas Ataxic disorders Thalamic and pallidal degenerations, neuroaxonal dystrophy, and autonomic failure Prion diseases Fetal and neonatal hypoxic–ischemic lesions

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 20, 2016 | Posted by in NEUROLOGY | Comments Off on Acute bacterial infections and bacterial abscesses

Full access? Get Clinical Tree
Get Clinical Tree app for offline access