Meningitis & Encephalitis

  • Author: Elizabeth W. Kelly, MD, Assistant Professor, Department of Emergency Medicine, Wake Forest School of Medicine
  • Author: Michael T. Fitch, M.D., Ph.D. Professor and Vice Chair for Academic Affairs, Department of Emergency Medicine, Wake Forest School of Medicine
  • Editor: Rahul Patwari, MD. Rush University and Medical College. Chicago, Illinois.
  • Last update: 2015


Bacterial meningitis and viral encephalitis are life-threatening causes of infection and inflammation within the central nervous system (CNS).  When patients present to the Emergency Department (ED) it is often in the early stages of illness when the diagnosis is challenging due to he presence of common non-specific signs and symptoms.  The Emergency Medicine (EM) provider is challenged to quickly identify patients who require urgent diagnostic testing and treatment.


  • List the classic triad of meningitis.
  • Interpret CSF findings typical for viral or bacterial meningitis.
  • Recognize the limitations of some of the classically described signs and symptoms of CNS infection
  • Compare appropriate empiric medications recommended for patients with differing ages and risk factors.

Until antibiotics became available at the beginning of the 20th century, bacterial meningitis was nearly 100% fatal.  And although the disease is treatable with antibiotics, morbidity and mortality remain high even with appropriate and timely treatment.  Meningitis affects patients of all ages but those at the extremes of age or those who are immunosuppressed are at increased risk.  An accurate diagnosis and timely administration of antibiotics and other adjunctive therapies are important for patients with suspected bacterial disease.

Meningitis is an inflammatory process of the membranes that surround the brain and spinal cord.  It is classified as bacterial meningitis when caused by a bacterial pathogen, the most common of which are encapsulated organisms. These pathogens often invade the host through the upper airway by infecting the mucosa, bloodstream, and ultimately crossing the blood-brain barrier and entering the CNS.  CNS inoculation can also occur after trauma, surgery, or a contiguous infection such as sinusitis or otitis media.  The most common bacterial pathogens include include Streptococcus pneumoniae and Neisseria meningitidis. Changes in epidemiology have mirrored vaccination practices in adults and children against H. influenzaeS. pneumoniae, and N. meningitidis. Routine childhood vaccination against Haemophilus influenza type b has helped decrease this pathogen as a common cause of meningitis, and use of the pneumococcal vaccine may be reducing the rate of disease in adults by herd immunity. Listeriamonocytogenes meningitis is a disease more common in older patients (greater than 50 years old), infants (less than 3 months old), and immunocompromised or pregnant individuals. 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.

Meningitis is classified as aseptic meningitis when the inflammation is due to other causes such as drugs or non-bacterial infections.  Most cases of aseptic meningitis are caused by viral infections (the most common overall causes of meningitis) or mycobacterial infections.  Enteroviruses and echoviruses are the most common causes of aseptic meningitis, and herpes simplex virus (HSV) can also be a causative pathogen.

Encephalitis is an infection of the brain parenchyma causing inflammation within the CNS and is often viral in origin.  It can be caused by a variety of viral pathogens including 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.

Patients diagnosed with meningitis or encephalitis have a broad differential diagnosis for the underlying causes of the inflammatory responses within the CNS.  Life threatening causes include bacterial meningitis or viral encephalitis from a variety of potential pathogens.  Aseptic meningitis can be caused by drugs, rheumatologic conditions, viruses, parasitic infections, fungal infections, malignancy, HIV, HSV, syphilis, Lyme disease, Rocky Mountain Spotted Fever, Ehrlichiosis, or autoimmune diseases.

Initial Actions and Primary Survey

Ill-appearing patients with a suspected CNS infection should be promptly evaluated, monitored, and treated appropriately. Patients with rapidly declining consciousness, ongoing seizures, or airway compromise, should have appropriate critical care interventions completed immediately.  Prioritize evaluation and stabilization of airway, breathing, and circulation as with any critically ill patient.  If bacterial meningitis is a likely diagnosis, antibiotics should be given immediately after a prompt lumbar puncture (LP), or soon after blood cultures are drawn if there is an anticipated delay in obtaining the LP. Typically in the Emergency Department the pathogen is unknown when the patient initially presents, and therefore broad empiric antimicrobial coverage is recommended.

EM providers should consider the diagnosis of encephalitis in cases of suspected CNS infection accompanied by altered mental status or a focal neurologic deficit. Antiviral therapy is recommended if HSV encephalitis is suspected.


Commonly reported descriptions of patients with meningitis typically include the classic clinical triad of fever, neck stiffness, and altered mental status.  However, studies have shown that all of these components are present together in less than half of adult patients with bacterial meningitis.  Many early symptoms of meningitis are nonspecific, such as headache, nausea and vomiting, and neck pain, which can make for a diagnostic challenge. Patients may also experience confusion, altered mental status, or seizure as the disease progresses.  Patients at the extremes of age and those who are immunocompromised can be particularly difficult to diagnose as they can have vague symptoms, more subtle presentations, or lack fever. Chief complaints can also be nonspecific for infants and include irritability, lethargy, poor feeding, rash, or a bulging fontanelle.  Seizures may be a presenting symptom 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.

Common physical exam findings include a febrile patient, although this finding is not always present. Classically described meningeal findings include nuchal rigidity (severe neck stiffness due to meningeal irritation), Kernig’s sign (flexing the hip and extending the knee to elicit pain in the back and the legs) and Brudzinski’s sign (passive flexion of the neck elicits flexion of the hips). Unfortunately these findings cannot be relied upon 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 causes of bacterial meningitis or may be absent.  Altered mental status and focal neurologic findings should raise concern for encephalitis as a possible diagnosis.

Diagnostic Testing

A prompt lumbar puncture (LP) is the preferred diagnostic procedure in patients with suspected bacterial meningitis or encephalitis. A CT scan of the brain before LP should be considered under the following circumstances: altered mental status, new onset seizures, an immunocompromised state, focal neurologic signs, or papilledema.  Neuroimaging is intended to identify patients with possible contraindications to LP such as an occult mass from infection or brain tumor, or signs of brain shift or herniation.

When an LP is performed, four tubes of CSF, each containing about 1mL of fluid, are typically obtained.  Common initial CSF laboratory studies include cell count and differential, protein and glucose levels, and a Gram’s stain and bacterial culture. Additional CSF studies may be ordered for immunocompromised patients, or if a CNS infection is suggested based on initial laboratory testing.  Other CSF studies to consider may include HSV or enterovirus PCR, bacterial antigen testing, or specialized fungal testing. Patients being evaluated for possible CNS infections often have other studies completed such as a complete blood count (CBC), serum glucose and electrolytes, serum urea nitrogen (BUN) and creatinine.  Most laboratory studies are nonspecific for meningitis or encephalitis, although a blood culture drawn before empiric antibiotics when meningitis is likely can be helpful to identify bacterial pathogens.

The initial history and physical exam are important for guiding diagnostic decisions and initiating emergent therapy when indicated for patients with suspected bacterial meningitis or encephalitis., Elevated numbers of white blood cells in the CSF obtained by lumbar puncture are diagnostic for meningitis or encephalitis, although it can be a challenge to determine whether the cause of such a CNS inflammatory response is more likely bacterial or non-bacterial in etiology.

CSF findings suggestive of bacterial meningitis include the following:

  • Positive Gram’s stain with identified organism
  • Glucose less than 40 mg/dL or ratio of CSF/blood glucose less than 0.40
  • Protein greater than 200 mg/dL
  • WBC greater than 1000/mL
  • Greater than 80% polymorphonuclear neutrophils
  • Elevated opening pressure of CSF during LP (pressure reading must be obtained with patient in the lateral decubitus position)


Table 1: Classically described CSF findings in bacterial, viral, and fungal meningitis.
CSF findingsBacterialViralFungal
Opening PressureElevatedNormalElevated
White Blood Cell count1,000-10,000< 300< 500
Neutrophils>80%1% – 50%1% – 50%
Gram StainBacteriaNothingNothing
*There can be considerable overlap in CSF white blood cell counts between the various causes of meningitis, and cell counts should be interpreted in the context of other CSF findings and individual patients’ clinical presentations.

See Table 1 for classically described CSF findings in bacterial, viral, and fungal meningitis.  While these general guidelines may be helpful to broadly characterize CSF findings in many cases, several studies have demonstrated that no single laboratory finding can accurately categorize the cause of CSF pleocytosis in all patients.

CSF studies for patients with encephalitis will lead to similarly abnormal results with increased numbers of white blood cells in the CSF, generally with a lymphocytic predominance.  Results may also reveal increased numbers of red blood cells in the CSF due to neuronal cell death leading to edema, hemorrhage, and necrosis when encephalitis is present.


For patients with suspect bacterial meningitis, empiric intravenous antibiotic therapy and admission to the hospital is recommended. See Table 2 for age-based antibiotic recommendations. Patients with severe disease may require ICU level care depending on the clinical circumstances.

Table 2: Empiric therapy for patients with suspected bacterial meningitis.
Patient ageIntravenous empiric therapy*


<1 month

  • Ampicillin and Cefotaxime
  • Alternative: Ampicillin and Gentamicin


1-3 months

  • Ampicillin
  • and Cefotaxime


>3 months

  • Dexamethasone (consider initiating before first dose of antibiotics)
  • and (Cefotaxime or Ceftriaxone)
  • and Vancomycin


<50 years

  • Dexamethasone (consider initiating before first dose of antibiotics)
  • and (Ceftriaxone or cefotaxime)
  • and Vancomycin


>50 years

  • Dexamethasone (consider initiating before first dose of antibiotics)
  • and (Ceftriaxone or cefotaxime)
  • and Vancomycin
  • and Ampicillin
*Consult with your local Infectious Disease experts for local regimens.  Remember to add Acyclovir in cases where HSV encephalitis is suspected

The treatment for most cases of encephalitis is supportive care. HSV encephalitis is the only cause of this disease with a specific treatment, and intravenous acyclovir is recommended for patients suspected to have this infection.

Studies have demonstrated that adjunctive corticosteroid treatment started before or concurrently with the first dose of antibiotics may decrease mortality and neurologic sequelae for some subsets of patients with bacterial meningitis. Therefore, consider intravenous dexamethasone every 6 hours for 4 days in adults and children 3 months and older when it is initiated before or at the same time antibiotics are given. See Table 2 for recommended empiric therapy for suspected bacterial meningitis.

In patients who need a CT scan of the brain prior to LP, blood cultures should be drawn and empiric therapy including broad-spectrum antimicrobial agents administered prior to CT to avoid additional delays to beginning treatment.

Patients who are diagnosed with possible bacterial meningitis based on CSF results should be admitted to the hospital for empiric IV antibiotics and further evaluation. Patients with encephalitis will also require inpatient care. Some well-appearing patients with suspected viral meningitis may be suitable for outpatient treatment with careful return precautions and close outpatient follow-up plans in place. However, if the diagnosis is unclear after emergency evaluation and bacterial meningitis remains a likely possibility, then the patient should be admitted to the hospital for observation and possible empiric antibiotic treatment.

If the lumbar puncture results are negative for CSF infection, additional inpatient or outpatient diagnostic testing may be appropriate to identify an alternative diagnosis as appropriate for the clinical circumstances.

Pearls and Pitfalls

  • Patients who are very young, very old, or immunocompromised may present with atypical signs and symptoms.
  • The classic triad of meningitis includes fever, neck stiffness, and altered mental status but all three of these are present less than half of the time in patients with bacterial meningitis.  The absence of all three makes the diagnosis of meningitis unlikely.
  • When the diagnosis of bacterial meningitis is likely, treat with broad spectrum empiric antibiotics and consider adjunctive treatments while awaiting the results of diagnostic studies.
  • Empiric antibiotics should not be delayed while waiting for a CT scan prior to an 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.
  • Consider the diagnosis of HSV encephalitis in patients with focal neurologic findings or altered mental status and add IV acyclovir to the empiric antimicrobial regimen.
  • Remember that initial CSF laboratory findings do not always yield classic results, so consider empiric treatment for possible bacterial meningitis (and/or HSV encephalitis) if CSF results are inconclusive.


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