A. Central nervous system (CNS) infections of major interest in the intensive care unit (ICU) include:

1. Bacterial meningitis.

a. Clinical hallmark is stiff neck secondary to inflammation of the leptomeninges.

b. Usually also have alteration of cerebral function (meningoencephalitis).

2. Encephalitis.

a. Disturbance of cerebral function in conjunction with cerebral spinal fluid (CSF) pleocytosis.

1. Community-acquired meningitis.

a. Streptococcus pneumoniae: most common in all age groups.

b. Neisseria meningitidis: peak incidence in teenage years, outbreaks.

c. Listeria monocytogenes: infants (<3 months), older adults (>50 years), alcoholism, immunosuppression, general debility.

d. Haemophilus influenzae type B: formerly the most common cause in young children.

2. Nosocomial meningitis.

a. Skin or hospital flora: staphylococci, aerobic gram-negative bacilli.

1. Herpes simplex virus (HSV): most common; one of the few etiologies for which a specific anti-infective agent exists.

2. Other causes include arboviruses (West Nile virus), Rickettsiae (Rocky Mountain spotted fever), and spirochetes (Lyme disease, syphilis).

A. Mechanisms of pathogen entry.

1. Bloodstream.

2. Contiguous extension.

3. Viral reactivation (HSV).

B. Meningeal irritation causes headache and neck stiffness.

C. Metabolic and circulatory disturbances may cause altered mental status.

Clinical evaluation should be expedited to avoid treatment delays. The goals are to recognize the diagnosis and define the likely pathogen.

1. Consider CNS infection in any patient with altered consciousness.

2. Classical meningitis presentation is acute-onset fever with headache, photophobia, and/or stiff neck.

3. Alcohol use, previous head trauma, recent antibiotics, ill contacts, and immunosuppression influence risk, etiology, and yield of diagnostic tests.

1. Nuchal rigidity and altered consciousness are suggestive, but not always present in meningitis.

2. Papilledema or focal neurologic deficits: delay lumbar puncture (LP) until mass lesion is excluded.

3. Petechiae: suggests meningococcal meningitis, but nonspecific.

1. Blood cultures: Collect before initiation of antibacterial therapy!

a. Positive in 30% to 80% patients with community-acquired meningitis.

2. CSF: Collect promptly, except when mass lesion suspected.

a. Bacterial meningitis.

i. White blood cell (WBC) count: typically >1,000 cells/mm3, neutrophil predominance.

ii. Glucose: <20 mg/dL highly suggestive; normal in up to 50% of patients.

iii. Protein: usually >100 mg/dL.

iv. Gram stain.

v. CSF culture.

b. Encephalitis.