Daily Bolus of LR: Encephalitis vs Meningitis

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February 7, 2011 by dailybolusoflr

Encephalitis, infection of the brain parenchyma, can present with similar signs and symptoms as meningitis, infection of the membranes of the brain and spinal cord (meninges).  However, focal neurological findings (ie, seizure, hallucinations, aphasia, motor weakness) are more commonly seen in encephalitis.
Etiologies of meningitis:
·         S. pneumonia -the most common bacterial etiology in adults
·         N. meningitides – the most common bacterial etiology in children and adolescents
·         Listeria monocytogenes must be considered in the elderly and immunocompromised population
·         Enterovirus- the most common viral etiology in adults
Etiologies of encephalitis:
·         Herpes simplex virus-the most common viral etiology
Diagnosis of both entities is based on lumbar puncture analysis.  Note the following:
·         bacterial meningitis is more likely to have a neutrophilic predominance of the white blood cells
·         cell count in viral or aseptic meningitis and encephalitis tends to be lower (< 500 cells/mm) and predominantly mononuclear
·         other indicators suspicious for bacterial meningitis include a CSF to serum glucose ration of less than 0.5 and CSF protein level > 150 mg/dL
In patients with meningitis:
·         most unenhanced CTs are normal 
·         contrast enhanced CT may show enhancement of the meninges, which is a non-specific finding 
·         MRI is more sensitive and may show leptomeningeal enhancement with distention of the subarachnoid space ( a finding more common in patients with severe disease) 
In patients with encephalitis:
·         more likely to have findings on CT, although MRI is still more sensitive
·         with herpes as the most common etiology of viral encephalitis, findings of hypodense areas on CT or hyperintensity on MRI will most likely be found in the temporal lobes
·         more advanced disease may show hemorrhage on either imaging modalit
Linda Regan, MD FACEP
Program Director, Emergency Medicine Residency
Johns Hopkins Medical Institutions

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