Additional testing, such as serology for various arboviruses and HIV testing, may also be performed based on the individuals history and symptoms. Brain biopsy and body fluid specimen cultures and PCR may also be useful in some cases. Electroencephalography (EEG) is abnormal in more than 80% of viral encephalitis cases, including those who are experiencing seizures, and may need to be monitored continuously to identify non-convulsive status. Lack of testing resources may prevent accurate diagnosis.Test results specific to certain viruses include:
For herpes simplex virus encephalitis, a CT scan may show low-density lesions in the temporal lobe. These lesions usually appear 3 to 5 days after the start of the infection. Japanese encephalitis often has distinct EEG patterns, including diffuse delta activity with spikes, diffuse continuous delta activity, and alpha coma activity. Differential diagnosis
A broad differential diagnosis can be performed that looks at many potential causes of the encephalitis, infectious and noninfectious. Potential alternatives to viral encephalitis include malignancy, autoimmune or paraneoplastic diseases such as anti-NMDA receptor encephalitis, a brain abscess, tuberculosis or drug-induced delirium, exposure to certain drugs or toxins, neurosyphilis, vascular disease, metabolic disease, or encephalitis from infection caused by a bacterium, fungus, protozoan, or parasitic worm. In children, differential diagnosis may not be able to distinguish between viral encephalitis and immune-mediated inflammatory CNS diseases, such as acute disseminated encephalomyelitis, as well as immune-mediated encephalitis, so other diagnostic methods may need to be used.
Epstein-Barr virus encephalitis is usually accompanied by enlargement of the lymph nodes and enlargement of the spleen
Herpes zoster encephalitis may be accompanied by rash and skin vesicles, and because it involves the frontal lobe and temporal lobe, is often characterized by psychiatric features, memory deficits, and loss of language faculties. Many arboviral encephalitides, such as Japanese encephalitis, primarily affect the basal ganglia, sometimes causing motor symptoms such as involuntary movements and movements similar to those observed in Parkinsons disease. Nipah virus may produce brainstem and cerebellar signs, hypertension, and segmental myoclonus, or twitching of a group of connected muscles. Zika virus characteristically may cause microcephaly among newborn children if a pregnant woman is infected. Histology
The brain histology of viral encephalitis shows dead neurons with nuclear dissolution and elevated eosinophil count, called hypereosinophilia, within cells cytoplasm when viewed with an optical microscope. Because encephalitis is an inflammatory response, inflammatory cells situated near blood vessels, such as microglia, macrophages, and lymphocytes, are visible. Virions within neurons are visible via electron microscopes. Clinical evaluation
Neuroimaging and lumbar puncture (LP) are both essential methods of diagnosing viral encephalitis. Computed tomography (CT) or magnetic resonance imaging (MRI) help identify increased intracranial pressure and the risk of uncal herniation before performing an LP. Cerebrospinal fluid (CSF), if analyzed, should be analyzed for opening pressure, cell counts, glucose, protein, and IgG and IgM antibodies. CSF testing should also include polymerase chain reaction (PCR) testing for herpes simplex viruses 1 and 2 and enteroviruses. About 10% of patients have normal CSF results.