West Nile Encephalitis Differential Diagnoses

Diagnostic Considerations

The most important infection to exclude in the differential is herpes simplex virus type 1 (HSV-1) encephalitis, because it is the only treatable viral encephalitis. HSV-1 encephalitis is suggested by temporal lobe abnormalities on electroencephalograms and later on computed tomography (CT) and magnetic resonance imaging (MRI) scans.

Early in the course of HSV-1 encephalitis, cerebrospinal fluid (CSF) may initially show a polymorphonuclear predominance, frequently has red blood cells (RBCs), and may be associated with a decreased CSF glucose level in contrast to the CSF in WNE, which does not have any of these features.

HSV-1 encephalitis is the most common cause of non–arthropod-borne (nonseasonal) encephalitis in the United States. HSV-1 infection usually manifests as encephalitis, uncommonly as meningoencephalitis, or rarely as aseptic meningitis. Differential diagnoses of meningoencephalitis, including the HSV-1 type, are listed in the chart below.

Encephalopathy due to systemic illnesses

Encephalopathy is a feature of many systemic illnesses that the clinician should consider in patients presenting with encephalitis. Most of these patients have extra-CNS findings that suggest the underlying disease process. Common disorders with CNS manifestations that may mimic West Nile encephalitis (WNE) include subacute bacterial endocarditis, Legionnaires disease, Rocky Mountain spotted fever, Epstein-Barr virus infectious mononucleosis, human herpesvirus type 6 infection, and systemic lupus erythematosus cerebritis.

Other arthropod-borne viral encephalitides

The clinical presentation of WNE is not dissimilar from other causes of arthropod-borne viral encephalitis (eg, Japanese equine encephalitis, St. Louis encephalitis), including mental confusion, stupor, or coma. However, the clinical presentation of arthropod-borne viral encephalitis is characterized by rapid onset and severe headache. Arthropod-borne viral encephalitis has some distinctive features that indicate a prospective clinical diagnosis (as seen

in the charts below).

Enteroviral aseptic meningitis

The most common cause of aseptic meningitis encountered during the summer months is enteroviral meningitis. Enteroviral aseptic meningitis is most commonly due to coxsackieviruses but may also be due to enterocytopathogenic human orphan virus or nonparalytic strains of poliovirus. Enteroviral meningitis may occur after water exposure in swimming pools, lakes, streams, or oceans, as well as after contact with infected individuals.

Acute enteroviral CNS infections usually manifest as aseptic meningitis, uncommonly as meningoencephalitis, or, rarely, as encephalitis. Nonexudative pharyngitis, maculopapular extremity rash, loose stools, and even diarrhea often accompany enteroviral aseptic meningitis, which provides clues to its presence.

Excluding enterovirus 71, enteroviral meningitis is not accompanied by paralysis or prolonged and/or profound lymphopenia.

Nonsteroidal anti-inflammatory drugs

In patients who present with aseptic meningitis, consider drug-induced aseptic meningitis, most commonly due to nonsteroidal anti-inflammatory drugs (NSAIDs). A patient presenting with aseptic meningitis with no predisposing risk factors may have recently been taking NSAIDs or may be currently taking NSAIDs. Discontinue NSAIDs if the patient is taking them.

Differential Diagnoses

• Encephalopathy, Hypertensive
• Epidural Abscess