The meningeal pathogen can be predicted by the patient’s age. In neonates, the most common pathogens are group B streptococci (Streptococcus agalactiae), gram-negative bacilli, and Listeria monocytogenes. In children, adolescents, and adults, the most common causative organisms of community-acquired bacterial meningitis are Neisseria meningitidis and Streptococcus pneumoniae. Listeria monocytogenes is a causative organism of meningitis in individuals with impaired cell-mediated immunity due to organ transplantation, chronic illness, pregnancy, acquired immunodeficiency syndrome, malignancy, immunosuppressive therapy, or age. When meningitis complicates acute otitis media, mastoiditis, or sinusitis, the causative organisms are Streptococci spp., gram-negative anaerobes, S. aureus, Haemophilus sp., or Enterobacteriaceae. Meningitis in the postneurosurgical patient and the patient with a ventriculostomy or other indwelling catheter may be due to staphylococci, gram-negative bacilli, or anaerobes.
Clinical Manifestations. The signs and symptoms of meningitis in the neonate include irritability, lethargy, poor feeding, vomiting, diarrhea, temperature instability, respiratory distress, apnea, seizures, and a bulging fontanel. The signs and symptoms of bacterial meningitis in children, adolescents, and adults include fever, vomiting, photophobia, headache, nuchal rigidity (meningismus), and a decreased level of consciousness ranging from lethargy to stupor, obtundation, or coma.
On physical examination, the classic sign of bacterial meningitis is meningismus, but this sign is not invariably present. The neck resists passive flexion. Kernig sign and Brudzinski sign are also signs of meningeal irritation (see Plate 11-2). Both signs are elicited with the patient in the supine position. To elicit Kernig sign, the thigh is flexed on the abdomen with the knee flexed. Attempts to passively extend the leg elicit pain and are met with resistance when meningeal irritation is present. Brudzinski sign is positive when passive flexion of the neck results in spontaneous flexion of the hips and knees. The presence of a petechial rash on the trunk and lower extremities, in the mucous membranes and conjunctiva, and occasionally on the palms and soles is typical of the rash of meningococcemia. A petechial rash is not seen in all cases of meningococcal meningitis, and a petechial rash is rarely seen in H. influenzae, pneumococcal, and staphylococcal meningitis. Patients with enteroviral meningitis may also have a rash, but this is an erythematous maculopapular rash that involves the face and neck early in infection.
Diagnosis. The gold standard for the diagnosis of bacterial meningitis is analysis of the cerebrospinal fluid. A computed tomography (CT) scan should be obtained in the patient with any of the following: an altered level of consciousness, papilledema, a focal neurologic deficit, new-onset seizure activity, immunocompromised state, a dilated or poorly reactive pupil, signs of a posterior fossa mass lesion (cranial nerve abnormalities, cerebellar deficit, and a wide-based ataxic gait), or a risk for neurocysticercosis. The classic abnormalities in bacterial meningitis on examination of the cerebrospinal fluid are the following: (1) an opening pressure greater than 180 mm H2O, (2) an increased white blood cell count with a predominance of polymorphonuclear leukocytes, (3) a decreased glucose concentration (less than 40 mg/dL), (4) an increased protein concentration, and (5) a positive Gram stain and bacterial culture. Gram stain is positive in identifying the organism in 60% to 90% of cases of bacterial meningitis. The probability of detecting bacteria on a Gram-stained specimen depends on the number of organisms present. The cerebrospinal fluid (CSF) 16S ribosomal ribonucleic acid (rRNA) conserved-sequence broad-based bacterial polymerase chain reaction (PCR) detects bacterial nucleic acid in CSF. There are also meningeal pathogen–specific PCRs available to identify the nucleic acid of a specific meningeal pathogen. The PCR is most useful in rapidly distinguishing between bacterial and viral meningitis. The PCR will not replace bacterial culture because culture is essential for antimicrobial sensitivity testing.
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