Acute Bacterial Meningitis

[Section Outline]

Clinical Features
Laboratory Evaluation
Differential Diagnosis
Prognosis

Pathogens most frequently involved in immunocompetent adults are Streptococcus pneumoniae (“pneumococcus,” ∼50%) and Neisseria meningitidis (“meningococcus,” ∼25%). Predisposing factors for pneumococcal meningitis include infection (pneumonia, otitis, sinusitis), asplenia, hypogammaglobulinemia, complement deficiency, alcoholism, diabetes, and head trauma with cerebrospinal fluid (CSF) leak. Listeria monocytogenes is a consideration in pregnant women, individuals >60 years, alcoholics, and immunocompromised individuals of all ages. Enteric gram-negative bacilli and group B Streptococcus are increasingly common causes of meningitis in individuals with chronic medical conditions. Staphylococcus aureus and coagulase-negative staphylococci are causes following neurosurgical procedures, especially shunting procedures for hydrocephalus.

Clinical Features

Presents as an acute fulminant illness that progresses rapidly in a few hours or as a subacute infection that progressively worsens over several days. The classic clinical triad of meningitis is fever, headache, and nuchal rigidity (“stiff neck”). Mental status changes occur in >75% of pts and vary from lethargy to coma. Nausea, vomiting, and photophobia are also common. Seizures occur in 20-40% of pts. Raised intracranial pressure (ICP) is the major cause of obtundation and coma. The rash of meningococcemia begins as a diffuse maculopapular rash resembling a viral exanthem but rapidly becomes petechial on trunk and lower extremities, mucous membranes and conjunctiva, and occasionally palms and soles.

Laboratory Evaluation

The CSF profile is shown in Table 194-1 Cerebrospinal Fluid (CSF) Abnormalities in Bacterial Meningitis. CSF bacterial cultures are positive in >80% of pts, and CSF Gram's stain demonstrates organisms in >60%. There are a number of “CSF pathogen panels” available that use specific bacterial primers to detect the nucleic acid of S. pneumoniae, N. meningitidis, Escherichia coli, L. monocytogenes, Haemophilus influenzae, and Streptococcus agalactiae (Group B streptococci). The latex agglutination (LA) test for detection of bacterial antigens of S. pneumoniae, N. meningitidis, H. influenzae type b, group B streptococcus, and E. coli K1 strains in the CSF is being replaced by the CSF bacterial polymerase chain reaction (PCR) assay. The Limulus amebocyte lysate assay rapidly detects gram-negative endotoxin in CSF and thus is useful in diagnosis of gram-negative bacterial meningitis; false-positives may occur but sensitivity approaches 100%. Petechial skin lesions, if present, should be biopsied. Blood cultures should always be obtained.

Differential Diagnosis

Includes viral meningoencephalitis, especially herpes simplex virus (HSV) encephalitis (see next); rickettsial diseases such as Rocky Mountain spotted fever (immunofluorescent staining of skin lesions); focal suppurative CNS infections including subdural and epidural empyema and brain abscess (see next); subarachnoid hemorrhage (Chap. 20 Subarachnoid Hemorrhage); and the demyelinating disease acute disseminated encephalomyelitis (ADEM, Chap. 192 Multiple Sclerosis).

TREATMENT
Acute Bacterial Meningitis Recommendations for empirical therapy are summarized in Table 194-2 Antibiotics Used in Empirical Therapy of Bacterial Meningitis and Focal Central Nervous System Infectionsa . Therapy is then modified based on results of CSF culture (Table 194-3 Antimicrobial Therapy of Central Nervous System Bacterial Infections Based on Pathogena ). In general, the treatment course is 7 days for meningococcus, 14 days for pneumococcus, 21 days for gram-negative meningitis, and at least 21 days for L. monocytogenes. Adjunctive therapy with dexamethasone (10 mg IV), administered 15-20 min before the first dose of an antimicrobial agent and repeated every 6 h for 4 days, improves outcome from bacterial meningitis; benefits most striking in pneumococcal meningitis. Dexamethasone may decrease the penetration of vancomycin into CSF, and thus higher vancomycin doses may be needed. In meningococcal meningitis, all close contacts should receive prophylaxis with rifampin (600 mg in adults [10 mg/kg in children >1 year]) every 12 h for 2 days; rifampin is not recommended in pregnant women. Alternatively, adults can be treated with one dose of azithromycin (500 mg), or one IM dose of ceftriaxone (250 mg).

Prognosis

Moderate or severe sequelae occur in ∼25% of survivors; outcome varies with the infecting organism and can include decreased intellectual function, memory impairment, seizures, hearing loss and dizziness, and gait disturbances.

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Acute Bacterial Meningitis

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