Background Acute bacterial meningitis is an inflammation of the meninges, which results from bacterially mediated recruitment and activation of inflammatory cells in the cerebrospinal fluid (CSF). Bacterial meningitis was an almost invariably fatal disease at the start of the 20th century. With the development of and advancements in antimicrobial therapy, however, there has been a significant reduction in the mortality rate, although this has remained stable during the past 20 years (1). One large study of adults with community-acquired bacterial meningitis reported an overall mortality rate of 21%, including a 30% mortality rate associated with Streptococcus pneumoniae meningitis and a 7% mortality rate for Neisseria meningitidis (2). In adults, the most commonly identified organisms are S. pneumoniae (40–50%), Neisseria meningitidis (14–37%), and Listeria monocytogenes (4–10%) (2-4).
Bacterial meningitis is a serious illness that often progresses rapidly. The classic clinical presentation consists of fever, nuchal rigidity, and mental status change (3). One large review of 10 critically appraised studies showed that almost all (99–100%) of the patients with bacterial meningitis presented with at least one of these clinical findings; and 95% of the patients had at least 2 of the clinical findings (5). In contrast, less than half of the patients presented with all 3 findings. Thus, in the absence of all 3 of these classic findings, the diagnosis of meningitis can virtually be dismissed, and further evaluation for meningitis need not be pursued. Individually, fever was the most common presenting finding, with a sensitivity of 85%. Nuchal rigidity had a sensitivity of 70%, and mental status change was 67%. While these physical examination findings may be of value in determining the diagnosis of bacterial meningitis, the accuracy of the clinical history including features such as headache, nausea and vomiting, and neck pain was too low to be of use clinically.
Signs of meningeal irritation may be of benefit in the clinical diagnosis of bacterial meningitis. Kernig’s and Brudzinski’s signs were first described nearly a century ago and have been used by most clinicians in the clinical realm; however, their diagnostic utility has been evaluated only in a limited number of studies. Kernig’s sign is positive when a patient in the supine position with his/her hips flexed at 90 degrees develops pain in the lower back or posterior thigh during an attempt to extend the knee. Brudzinski’s sign is positive when a patient in the supine position whose neck is passively flexed responds with flexion of his/her knees and hips. Recently, a bedside maneuver called jolt accentuation of headache was found to be potentially useful. In this maneuver, the patient is asked to turn his/her head horizontally 2–3 times per second, and a worsening headache is considered a positive sign. A small study showed that this maneuver had 97% sensitivity and 60% specificity for patients with CSF pleocytosis (6).
Other clinical manifestations in patients with bacterial meningitis include photophobia, seizure, rash, focal neurologic deficits, and signs of increased intracranial pressure. While these various findings may be present in many patients with bacterial meningitis, their sensitivities have been found to be low. Thus, their clinical utility in ruling out the diagnosis of bacterial meningitis is limited (5).
Any patient who presents with a reasonable likelihood of having bacterial meningitis should undergo a lumbar puncture (LP) to evaluate the CSF as soon as possible. The initial CSF study should measure the opening pressure. One study demonstrated that 39% of patients with bacterial meningitis had opening pressures greater than 300 mg H20 (3). Other CSF laboratory studies should be sent for analysis in 4 sterile tubes filled with approximately 1 mL of CSF each. The first tube is typically reserved for gram stain and culture. The gram stain is positive in about 70% of patients with bacterial meningitis, and the culture will be positive in about 80% of cases. The second tube is sent for protein and glucose levels. Patients who have markedly elevated CSF protein counts (>500 mg/dL) and low glucose levels (<45 mg/dL, or ratio of serum: CSF glucose levels <0.4) are likely to have bacterial meningitis. The third tube is sent for cell count and differential. Patients with bacterial meningitis are likely to have >10 WBC/μL that are predominantly polymorphonucleocytes and have few or no red blood cells in the absence of a traumatic LP. We recommend the fourth tube be used for any viral, fungal, or other miscellaneous studies. In addition to the CSF studies, other diagnostic evaluations should include blood cultures, complete blood count with platelets and differential (CBCPD), and basic chemistry labs.
The CSF studies described above are the primary tools in diagnosing bacterial meningitis; however, there are other studies that may be helpful in certain clinical settings. Latex agglutination tests for bacterial antigens may be used in cases in which bacterial meningitis remains a possible diagnosis despite negative CSF studies. This test is available for S. pneumoniae, N. meningitidis, H. influenzae type B, group B Streptococcus, and E. coli. The polymerase chain reaction (PCR) test of the CSF has been developed for some bacterial pathogens including S. pneumoniae, N. meningitidis, H. influenzae type B, and Mycobacterium tuberculosis. The limulus amebocyte lysate assay is a very sensitive test for gram-negative endotoxins, which may aid in identifying gram-negative organisms as potential pathogens in the CSF. While these alternative CSF diagnostic tests are available, many laboratories do not perform the tests on site and require send-out to a specialty laboratory. The time required for this may negate the clinical utility of these tests.
Role of Brain Imaging
The decision to obtain a brain imaging study prior to performing an LP has been a controversial issue for both patient safety and medical-legal reasons. Two large studies have been published in an attempt to derive a clinically useful decision analysis tool (7,8). In summary, the studies found that 5 clinical features were associated with an abnormal head cranial tomography (CT) scan. These were:
- Age >60 years
- Immunocompromised state
- Any history of central nervous system (CNS) disease
- A history of seizure within 1 week prior to presentation
- Presence of a focal neurologic abnormality, including altered level of consciousness, inability to answer or follow 2 consecutive requests, gaze palsy, abnormal visual fields, facial palsy, arm or leg drift, and abnormal language.
In patients with none of these findings, there was a 97% negative predictive value of having an abnormal CT scan, with the few patients with positive scans nonetheless tolerating LP without adverse effects. Thus, in patients with none of these findings, it appears that an LP can safely be performed without obtaining a CT scan. One study also demonstrated that patients who underwent a CT scan prior to their LP waited, on average, 2 hours longer to get an LP; with antibiotic administration delayed by an average of 1 hour (8). Antibiotic administration should not be delayed in any patient suspected of having bacterial meningitis, whether brain imaging is performed or not.
Given the severe nature of this disease, the diagnosis of bacterial meningitis must be differentiated from other conditions that may present in similar ways. Infectious causes that may present similarly to bacterial meningitis include other types of meningitis (viral, tuberculous, Lyme disease, syphilitic), viral encephalitis, Rocky Mountain spotted fever, fungal meningitis, parasitic causes, brain abscess, and epidural and subdural empyema. Other infectious etiologies not originating from the CNS may be mistaken for bacterial meningitis when these patients present with concomitant mental-status changes. This is especially common in elderly patients with pneumonia and urinary tract infections. Other noninfectious considerations include a CNS bleed such as a subarachnoid hemorrhage, drug-induced aseptic meningitis, and CNS vasculitis.
When the patient’s presentation is suggestive of bacterial meningitis, empiric antibiotics should be administered without delay, while awaiting diagnostic evaluation. The initial dose of antibiotics should not alter the results of the diagnostic studies significantly. The choice of antibiotics is based upon the most likely offending organism from epidemiologic data and underlying predisposing conditions. S. pneumoniae and N. meningitidis are the 2 most common causes of bacterial meningitis in adults.
The development of antibiotic resistance by S. pneumoniae to penicillin and cephalosporins has been one of the major developments in the past 20 years. Due to this resistance, the recommended empiric therapy is a combination of a third-generation cephalosporin (ceftriaxone or cefotaxime) and vancomycin. For special cases, additional or alternative therapy should be given. Ampicillin should be added for patients at risk for Listeria monocytogenes; and postsurgical or post-trauma patients should have expanded coverage to include staphylococcal and gram-negative infections. Table 1 lists the recommended antibiotic therapy for patients with possible bacterial meningitis, along with the most commonly associated organisms.
Once the offending organism has been identified, antibiotic therapy should be narrowed to target the bacteria based on laboratory minimal inhibitory concentrations (MIC). The antibiotic should also have excellent CSF penetration and bactericidal activity. For S. pneumoniae that are susceptible to penicillin, penicillin G and ampicillin remain the therapy of choice (9). The increasing trend toward antibiotic resistance by S. pneumoniae has increased the use of vancomycin as therapy. In patients with resistant strains of S. pneumoniae, however, vancomycin should not be used alone. Vancomycin should be used in combination with a third-generation cephalosporin while keeping the serum vancomycin levels in the range of 15–20 μg/mL (10). It is imperative that the treatment course outlined be completed through its full duration. Table 2 lists specific antibiotic therapy with dosages and recommended duration of therapy based on isolated organisms.
The release and production of inflammatory cytokines in bacterial meningitis is thought to be a major cause of adverse outcomes. To counteract this inflammatory process, use of adjunctive steroids in patients with bacterial meningitis has been evaluated. Initial data from children with bacterial meningitis, mostly due to H. influenzae and S. pneumoniae, demonstrated improved neurologic outcomes, with significant reductions in deafness, in patients treated with dexamethasone as an adjunctive therapy to antibiotics (11). In adults with bacterial meningitis, a recent major trial demonstrated that treatment with adjunctive steroids, along with antibiotics, led to significant improvement in mortality and morbidity in patients with meningitis due to S. pneumoniae (12). Among patients with meningococcal meningitis, there was a trend toward improved outcomes. Patients with suspected pneumococcal meningitis should receive their first dose of dexamethasone 20–30 minutes prior to or at the same time as the initial antibiotic administration. The recommended dose and duration is 0.15 mg/kg every 6 hours for 2 to 4 days. The use of dexamethasone appears to have no benefit if administered after antibiotics have already been given, and data are lacking for patients with meningitis due to organisms other than S. pneumoniae. Most experts recommend against the use of adjunctive corticosteroids in these cases (10-13). Several questions, however, remain unanswered with regard to adjunctive corticosteroid use. These include the optimal duration of treatment, whether the penetration of vancomycin into the CSF is significantly decreased by dexamethasone, and whether they should be administered to immunocompromised patients (14).
Currently, prevention of some types of bacterial meningitis can be accomplished by appropriate use of vaccines, or through antibiotic chemoprophylaxis in certain situations. For adults, vaccines are available against the 2 most common causes of bacterial meningitis. The 23 polyvalent pneumococcal vaccine is recommended for all adults >65 years of age and for anyone age >2 with a compromised immune status. The meningococcal vaccine is available as a quadravalent vaccine (serotypes A, C, Y, and W-135) and should be administered to anyone with functional asplenia, terminal complement deficiencies, those traveling to endemic areas of meningococcal meningitis, and any college freshman requesting the vaccine who will be living in college dormitories (15).
Antibiotic chemoprophylaxis can be administered to individuals who have had close contact with an index patient with meningococcal meningitis. Antibiotics should be administered as soon as exposure has been determined. There are several options available for meningococcal meningitis exposure. Ciprofloxacin is probably the simplest regimen due to its 1-time 500-mg oral dose. Other options include rifampin 600 mg every 12 hours ×4 doses and ceftriaxone 250 mg IM as a 1-time dose. Pregnant women should avoid ciprofloxacin and rifampin due to their potential teratogenic effects.
Prognosis and Follow-up
Prognosis of bacterial meningitis is closely linked to the causative organism, the severity of disease at the time of presentation, and the speed at which the disease progresses. One large retrospective study demonstrated in-hospital mortality rates of 25% for S. pneumoniae, 10% for N. meningitidis, and 21% for L. monocytogenes. Conditions associated with an increased risk of mortality included age >60, state of obtundation on admission, and development of seizure within 24 hours of admission. This study also showed that 21% of patients developed some type of neurologic deficits, and, overall, 9% had persistence of these deficits at time of discharge (3). Another study showed that baseline features of hypotension, mental status changes, and seizures were associated with increased mortality and neurologic morbidity (16). A more recent large study evaluating the efficacy of adjunctive corticosteroids reported a mortality rate of 15% in the control arm, with mortality of 34% in patients infected with S. pneumoniae (12). Another study suggested that if patients had a rapid progression of their disease, this seemed to correlate with worse outcomes. These investigators found an uncertain correlation between antibiotic timing and unfavorable outcomes (16).
Patients discharged from the hospital should have close follow-up with their primary care physician or infectious disease specialist. Evaluation in the short-term should focus on any complications that may have developed as a result of the bacterial meningitis; such as mental status change, seizure, focal neurologic deficits, and hearing loss. Long-term evaluations should also address cognitive functioning and the neuropsychiatric well-being of the patient, in addition to those issues addressed during short-term follow-up (11, 12).
Dr. Kim may be reached at [email protected].
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