NEJM Clinical Pearls
Common Organisms in Bacterial Meningitis
The most common bacteria causing meningitis in adults are Streptococcus pneumoniae (about half of cases), Neisseria meningitidis (about one fifth of cases), Listeria monocytogenes (about one tenth of cases), and Haemophilus influenzae (about one tenth of cases). The otitic meningitis in this patient (whose ear infection may have caused bony erosion that led to a cerebrospinal fluid fistula) prompted the clinicians to consider more aggressive organisms such as Staphylococcus aureus, gram-negative organisms, and group A streptococcus. In fact, blood and cerebrospinal cultures became positive for group A streptococcus within a few hours.
Group A Streptococcus Meningitis
Infections with group A streptococcus are a rare cause of bacterial meningitis, accounting for 0.5 to 1.5% of community-acquired cases; the mortality rate (27%) is similar to that for pneumococcal meningitis (30%). Otitis media due to group A streptococcus is associated with high rates of local invasion, including tympanic perforation and mastoiditis, and this form of otitis media is the most important risk factor for group streptococcal meningitis among adult patients. Most patients with group A streptococcal meningitis do not have the clinical features of septic shock associated with invasive streptococcal disease, and the incidence of group A streptococcal meningitis has not increased, despite an increasing incidence of other forms of invasive disease.
Morning Report Question
Q: An important clinical challenge is how to recognize when fluid emanating from the ear or the nose is, in fact, cerebrospinal fluid. How might this be accomplished?
A: Otorrhea and rhinorrhea may be recognized as containing cerebrospinal fluid if the fluid has a relatively low protein level (less than 2 g/L) and should be clinically suspected when a handkerchief or cloth soaked in the fluid does not stiffen when dry. A fluid glucose level of greater than 40 mg per deciliter (2.2 mmol per liter) also suggests that the fluid is cerebrospinal fluid. The most accurate test is a measurement of β2-transferrin, a protein that is found only in cerebrospinal fluid. Of note, cerebrospinal fluid otorrhea may be serosanguineous and mistaken for blood.