neurologic disorders
Encephalitis
Etiology of Encephalitis
Viruses causing primary encephalitis directly invade the brain. These infections may be Epidemic (eg, due to arbovirus, echovirus, coxsackievirus, or Poliomyelitis [in some underdeveloped countries]) Sporadic (eg, due to Cytomegalovirus (CMV) Infection or to Mosquito-borne arboviral encephalitides infect people during the spring, summer, and early fall when the weather is warm (see table Some Arboviral Encephalitides ). Incidence in the US varies from 150 to In the US, the most common sporadic encephalitis is caused by Herpes Simplex Virus (HSV) Infections (HSV); hundreds to several thousand cases occur yearly. Most are due to HSV-1, but HSV-2 may be more common among immunocompromised patients. HSV encephalitis occurs at any time of the year, tends to affect patients Rabies remains a significant cause of encephalitis in developing countries and still causes a few cases of encephalitis in the US. Encephalitis can also occur as a late reactivation of latent or subclinical viral infection. The best known types are HIV-Associated Dementia None Subacute Sclerosing Panencephalitis (SSPE) (which occurs years after a measles infection and is thought to represent reactivation of the original infection; it is now rare in Western countries) Progressive Multifocal Leukoencephalopathy (PML) (which is caused by reactivation of JC virus; particularly in AIDS or immunosuppressed patients) Herpes simplex type 1 and herpes zoster encephalitis ; and live-virus vaccines (eg, the older rabies vaccines prepared from sheep or goat brain). In the US, most cases now result from Influenza A or B virus, Rarely, apparent encephalitis has developed in patients with COVID-19 Encephalopathies caused by autoantibodies to neuronal membrane proteins (eg,N-methyl-d-aspartate receptors [NMDAR] ) may mimic viral encephalitis. Studies indicate that anti-NMDAR encephalitis is more common than viral encephalitides. Anti-NMDAR encephalitis was first described in young women with ovarian teratomas but can occur in both sexes and at any age. Anti-NMDAR encephalitis also occurs as a postinfectious complication of HSV encephalitis, resulting in clinical decline within weeks of the HSV infection. 