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Computed Tomography, Magnetic Resonance Imaging, and Electroencephalography

Performance of a head CT scan with and without contrast agent should be performed in virtually all patients with encephalitis. This should be done prior to LP if there are focal complaints or findings, signs to search for evidence of elevated intracranial pressure (ICP), obstructive hydrocephalus, or mass effect due to focal brain infection. Head CT scanning also helps exclude brain hemorrhage or infarction as a cause of an encephalopathic state. Magnetic resonance imaging (MRI) is more sensitive than CT scanning in demonstrating brain abnormalities earlier in the disease course.

In HSE, MRI may show several foci of increased T2 signal intensity in medial temporal lobes and inferior frontal gray matter. Head CT commonly shows areas of edema or petechial hemorrhage in the same areas. EEE and tick-borne encephalitis may show similar increased MRI signal intensity in the basal ganglia and thalamus.

In toxoplasmosis, contrast-enhanced head CT typically reveals several nodular or ring-enhancing lesions. Because lesions may be missed without contrast, MRI should be performed in patients for whom use of contrast material is contraindicated.

In HSE, electroencephalography (EEG) often documents characteristic paroxysmal lateral epileptiform discharges (PLEDs), even before neuroradiography changes. Eventually, PLEDs are positive in 80% of cases; however, the presence of PLEDs is not pathognomonic for HSE.

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