Encephalitis Differential Diagnoses

CSF Finding (Normal)

Bacterial Meningitis

Viral Meningitis*

Fungal Meningitis

Pressure (5-15 cm water)

• Increased

• Normal or mildly increased

• Normal or mildly increased in most fungal and tuberculous CNS infections

• Patients with AIDS and cryptococcal meningitis are at increased risk of blindness and death unless pressure maintained at < 30 cm

Cell counts, mononuclear cells/µL

Preterm (0-25)

Term (0-22)

6 mo+ (0-5)

• Normal cell count excludes bacterial meningitis

• Typically thousands of polymorphonuclear cells, but counts may not change dramatically or even be normal (classically in very early meningococcal meningitis or in extremely ill neonates)

• Lymphocytosis with normal CSF chemistry results observed in 15-25% of patients, especially if counts < 1000 or if patient is partially treated

• About 90% of patients with ventriculoperitoneal shunts and CSF WBC count >100 cells/µL are infected, though CSF glucose level often normal, and bacteria often less pathogenic

• Cell count and chemistry levels normalize slowly (days) with antibiotics

• Usually < 500, nearly 100% mononuclear

• < 48 hours, clinically significant polymorphonuclear pleocytosis may be indistinguishable from early bacterial meningitis, particularly with EEE

• Nontraumatic RBCs in 80% of patients with HSV meningoencephalitis, though 10% have normal CSF results

• 100s of mononuclear cells

Microorganisms (none)

• Gram stain 80% effective

• Inadequate decolorization may cause Haemophilus influenzae to be mistaken for gram-positive cocci

• Pretreatment with antibiotics may affect stain uptake, causing gram-positive species to appear to be gram-negative and decrease culture yield by an average of 20

• No organism

• India ink 80-90% effective for detecting fungi

• AFB stain 40% effective for TB; increase yield by staining supernatant from at least 5 mL of CSF

Glucose^†

Euglycemia (>50% serum)

Hyperglycemia (>30% serum)

• Decreased

• Normal

• Sometimes decreased

• In addition to fulminant bacterial meningitis, TB, primary amebic meningoencephalitis, and neurocysticercosis cause low glucose levels

Protein

Preterm (65-150 mg/dL)

Term (20-170 mg/dL

6 mo+ (15-45 mg/dL)

• Usually >150 mg/dL

• May be >1000 mg/dL

• Mildly increased

• Increased >1000 mg/dL, with relatively benign clinical presentation suggestive of fungal disease

*Some bacteria (eg, Mycoplasma, Listeria, Leptospira, Borrelia burgdorferi [Lyme disease]) cause alterations in spinal fluid that resemble the viral profile. An aseptic profile is also typical of partially treated bacterial infections (>33%, especially those in children, are treated with antimicrobials) and of the 2 most common causes of encephalitis—the arboviruses and the potentially curable HSV.

^† Wait 4 hours after glucose load.

AFB—acid-fast bacillus; CSF—cerebrospinal fluid; EEE-eastern equine encephalitis; HSV—herpes simplex virus; RBC—red blood cell; TB—tuberculosis; WBC—white blood cell.