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 men ­

ingeal 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 t ime

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.

DIAGNOSTIC STUDIES

.... Laboratory

Evaluation of the CSF is most critical. Initial CSF laboratory studies include cell count and differential, protein and

glucose levels, and a Gram stain and bacterial culture.

Other CSF studies to consider may include HSV or enterovirus polymerase chain reaction, bacterial antigen testing,

or specialized fungal testing. Additional CSF studies may

be ordered for immunocompromised patients.

Other studies include a complete blood count, serum

glucose and electrolytes, blood urea nitrogen and creatinine, and C-reactive protein. 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.

.... Imaging and Procedures

A prompt lumbar puncture (LP) is the preferred diagnostic

procedure in patients with suspected bacterial meningitis or

encephalitis. A computed tomography 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 infec ­

tion, brain tumor, or signs of brain shift or herniation.

MEDICAL DECISION MAKING

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 LP are diagnostic for meningitis or encephalitis,

although it can be a challenge to determine whether the cause

is more likely bacterial or nonbacterial in etiology.

CSF findings suggestive of bacterial meningitis include

the following:

• Positive Gram stain with identified organism

• Glucose <40 mgldL or ratio of CSF/blood glucose

<0.40

• Protein >200 mgldL

• WBC > 1 ,000/mL

• >80% polymorphonuclear neutrophils

• Elevated opening pressure of CSF during LP (pressure reading must be obtained with patient in the

lateral decubitus position)

See Table 35- 1 for classically described CSF findings

in bacterial, viral, and fungal meningitis. Although these

Table 35-1. Classically described CSF findings

in bacterial, viral, and fungal mening itis.

CSF findings Bacterial Viral Fungal

Opening pressure High Normal High

White blood cell count 1,000-10,000 <300 <500

Neutrophils >80% 1-50% 1-50%

Glucose Low Normal Low

Protein High Normal High

Gram stain +

Positive Gram stain

OR

Pattern of CSF neutrophil ia,

low g lucose level, elevated

protein

Bacterial meningitis

MENINGITIS AND ENCEPHALITIS

Suspect meningitis/encephalitis

(fever, headache, nuchal

rigidity, AMS)

Obtain blood cultures

Administer IV antibiotics

CT (if indicated), followed

by LP

CSF with mononuclear

predominance, normal glucose

level and mild-moderately

elevated protein. If hemorrhagic,

treat as HSV and consider

subarachnoid hemorrhage

Viral meningitis

CSF with mononuclear

predominance, norma l or

decreased g l ucose level,