Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (115 page)

   Penetrating head trauma and postneurosurgical infections: staphylococci (coagulase positive and coagulase negative), aerobic gram-negative bacilli,
Propionibacterium acnes
(CSF shunts).

   Clinical Presentation

A significant proportion of adult patients with community-acquired ABM do not present with all of the classic clinical features (headache, fever, stiff neck, and altered mental status), but the majority will show at least two of the four. A significant minority of patients may be comatose on admission or show focal neurologic abnormalities. Seizures are present in approximately 5% of patients. Nonspecific symptoms are more frequent in infants and the elderly. Overall, the mortality rate is 20–25%; pneumococcal meningitis has a higher mortality rate than meningococcal (30% vs. 7%). Factors associated with increased mortality risk in patients with meningitis include:

   Age (>60 years)

   Otitis or sinusitis

   Absence of rash

   Low admission score on Glasgow Coma Scale

   Tachycardia (>120 beats/minute)

   Labs: Positive blood culture, increased ESR, decreased platelet count, low CSF WBC count (<1,000 cells/mm
³
)

ABM caused by invasive medical procedures or by trauma is associated with a different etiology of infecting organisms, including
S. aureus
and enteric gramnegative bacilli. Signs and symptoms depend on the infecting organism as well as the predisposing event; those related to the trauma may overlap with those of the subsequent infection and may delay diagnosis and intervention.

The risk of meningitis is approximately 5–10% following compound skull fractures. The risk is increased when the wound is heavily contaminated with external material. Basilar skull fractures, which result in communication of the subarachnoid space with sinus cavities, are associated with a risk of meningitis up to 25%, with onset in the 2nd week after trauma. Persistent CSF leak may be associated with recurrent bacterial meningitis.

Fewer than 2% of craniotomy procedures results in bacterial meningitis. Two thirds of these infections occur within the first 2 weeks after the procedure. Internal intraventricular catheters become infected in approximately 5–15% of cases, usually within the 1st month after placement, and usually represent intraoperative transmission. The incidence of infection of external CSF drainage catheters is <10%.

The risk of CNS infection caused by lumbar puncture is very low (approximately 1:50,000).

   Diagnosis and Laboratory Findings

When acute bacterial meningitis is suspected, appropriate laboratory testing and cultures should be collected, followed by empirical antibiotic therapy. HSV should be ruled out in patients if encephalitis is present.

   Diagnostic testing performed on CSF represents the primary approach to specific meningitis diagnosis. However, collection of CSF may be hazardous in patients with increased intracranial pressure (ICP). Cranial CT scan should be performed prior to lumbar puncture if clinical presentation suggests increased ICP. (Note: Imaging studies should not delay administration of antibiotics and dexamethasone therapy; blood cultures may be collected prior to imaging studies.) Clinical features significantly associated with increased ICP in adults include

   Positive history of CNS disease