++
KEY POINTS
Clinically encephalitis is characterized by the acute onset of fever, altered mental status, new-onset seizures, and focal neurologic deficits.
Microorganisms can cause encephalitis by direct invasion of brain parenchyma, but inflammation can also result from immune-mediated mechanisms triggered by systemic infection in the absence of direct nervous system involvement.
There is overlap between the clinical features of infectious and autoimmune encephalitis. In general, subacute onset, lag between fever and onset of neurologic symptoms, and prominent psychiatric symptoms suggest autoimmune cause rather than infection as the etiology.
Once infection has been identified as a likely etiology, it is important to narrow the differential. Age, geography, seasonality, and psychosocial factors define the range of potential pathogens.
Unless contraindicated, all patients suspected of having encephalitis should have a lumbar puncture performed.
Herpes simplex virus type 1 is the most common cause of sporadic encephalitis worldwide.
Intravenous acyclovir has markedly reduced the mortality associated with herpes simplex virus encephalitis and should be started empirically on all patients with suspected encephalitis.
Arboviruses are the most common cause of seasonal encephalitis worldwide. Treatment is largely supportive.
Doxycycline should be started empirically in patients with a history of tick exposure presenting with encephalitis and a rash suggestive of Rocky Mountain spotted fever.
A history of recreational water activities could suggest exposure to Naegleria fowleri in patients presenting with acute meningoencephalitis.
++
Encephalitis is defined as inflammation of brain parenchyma resulting in neurologic dysfunction. Historically, encephalitis has been almost synonymous with direct infection, but autoimmune causes are increasingly recognized.1 Infection can result in brain injury through direct invasion of neural structures and downstream inflammation. However, inflammation can also result from immune-mediated mechanisms triggered by systemic infection in the absence of direct nervous system involvement by the pathogen. When this occurs contemporaneously with acute infection, the term “parainfectious” is used, whereas postinfectious encephalitis refers to cases where neurologic symptoms arise weeks after systemic infection. Acute disseminated encephalomyelitis (ADEM), an inflammatory demyelinating disorder on the central nervous system (CNS), is generally considered to be postinfectious. Autoimmune encephalitis can also be triggered by systemic malignancy as paraneoplastic phenomena, or it can occur idiopathically. Clinically, infectious encephalitis is characterized by the acute to subacute onset of fever, altered mental status, focal neurologic deficits, and generalized or focal seizures. Headache, photosensitivity, and nuchal rigidity suggest coexisting meningeal involvement (meningoencephalitis). Limb weakness with or without bladder sphincter dysfunction can indicate spinal cord involvement (encephalomyelitis). Prompt and thorough investigation of patients suspected of having encephalitis is important as severe impairment can accrue rapidly without a clear diagnostic and treatment plan. Here we review common and rare infections that can cause encephalitis, emphasizing practical aspects of diagnosis and early management.
+++
DIAGNOSIS AND INITIAL MANAGEMENT
++
There is overlap between the clinical features of infectious and immune-mediated encephalitis (Box 75-1). Familiarity with well-described clinicoradiographic syndromes characteristic of ...