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  The classic triad of meningitis includes fever, neck stiffness, and altered mental status. However, all 3 of these are present less than half of patients with bacterial meningitis.

  
  
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  Patients who are very young, very old, or immunocompromised may present with atypical signs and symptoms.

  
  
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  Empiric antibiotics should not be delayed while waiting for a computed tomography (CT) scan before a lumbar puncture (LP) if meningitis is a likely diagnosis. When a CT scan is necessary, draw blood cultures and administer steroids and appropriate antibiotics before the LP.

  
  
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  Consider the diagnosis of herpes simplex virus encephalitis in patients with focal neurologic findings or altered mental status and add intravenous acyclovir to the empiric antimicrobial regimen.

Bacterial meningitis and viral encephalitis are life-threatening causes of infection and inflammation within the central nervous system (CNS). In the early stages of illness the diagnosis can be very challenging, and evaluation is focused on identifying patients who require urgent diagnostic testing and treatment.

Until antibiotics became available at the beginning of the 20th century, bacterial meningitis was nearly 100% fatal. Morbidity and mortality still remain high even with appropriate treatment. Meningitis affects patients of all ages, but those at the extremes of age or immunosuppressed are at increased risk. Accurate diagnosis, timely administration of antibiotics, and other adjunctive therapies (eg, dexamethasone) are important for patients with suspected bacterial disease.

Meningitis is an inflammatory process of the membranes that surround the brain and spinal cord. The most common causative agents of bacterial meningitis are encapsulated organisms, namely Streptococcus pneumoniae and Neisseria meningitidis. Listeria monocytogenes more commonly infects older patients (>50 years old), infants (<3 months old), and immunocompromised or pregnant individuals. These pathogens often invade the host through the upper airway by infecting the mucosa and bloodstream and ultimately cross the blood–brain barrier, entering the CNS. CNS inoculation can also occur after trauma, surgery, or a contiguous infection such as sinusitis or otitis media.

Changes in epidemiology have mirrored vaccination practices in children and adults against Haemophilus influenzae, S. pneumoniae, and N. meningitidis. Routine childhood vaccination against H. influenza type b has helped decrease this pathogen as a cause of meningitis. Use of the pneumococcal vaccine in adults may be reducing the rate of S. pneumoniae disease. Given the success of routine childhood vaccination programs, over the past 25 years, the median age of a patient diagnosed with meningitis has risen from 15 months to 42 years of age.

Aseptic meningitis is due to inflammation from other causes such as drugs, rheumatologic conditions, or nonbacterial infections. Most cases are caused by viral (the most common overall causes of meningitis) or mycobacterial infections. Of the viral etiologies, enteroviruses and echoviruses are the most common, but herpes simplex virus (HSV) is an important pathogen.

Encephalitis is an infection of the brain parenchyma causing inflammation within the CNS. Viral pathogens include HSV, which is the most treatable cause of encephalitis. In the acute care setting, it can be difficult to distinguish encephalitis from severe cases of bacterial meningitis, as patients’ signs and symptoms may be similar.

History

The classic triad of meningitis includes fever, neck stiffness, and altered mental status, but all of these are present together in less than half of adult patients with bacterial meningitis. Patients may also experience seizure. Many of the symptoms are nonspecific, such as headache, nausea and vomiting, and neck pain, making accurate diagnosis challenging. Patients at the extremes of age and those who are immunocompromised can be further difficult to diagnose, as they can have more subtle presentations or even lack fever. Infants may present with only irritability, lethargy, poor feeding, rash, or a bulging fontanelle. Seizures may be present in up to one third of pediatric patients with bacterial meningitis. Geriatric patients can often present with confusion or altered mental status. The clinical presentation of patients with encephalitis can be similar to patients with meningitis, including fever, headache, or stiff neck, but the diagnosis of encephalitis is characterized by the presence of altered mental status or neurologic symptoms.

Physical Examination

Although not all patients with meningitis will have fever, it is a common physical finding. Classically described meningeal findings include nuchal rigidity (severe neck stiffness due to meningeal irritation), Kernig sign (flexing the hip and extending the knee to elicit pain in the back and the legs) and Brudzinski sign (passive flexion of the neck elicits flexion of the hips). However, these findings cannot be relied on exclusively, as they have relatively poor sensitivities. Neck stiffness may only be present 30% of the time in patients with meningitis. Petechiae and purpura are classically associated with meningococcal meningitis; however, these skin findings can be present with other bacterial causes or may be absent. Altered mental status and focal neurologic findings should raise concern for encephalitis as a possible diagnosis.