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Emergency And Acute Medicine - Encephalitis
Basics Overview
Encephalitis is an acute inflammatory process involving the brain parenchyma and accounts for approximately 20,000 cases annually in the United States, with an overall mortality near 10%. The inflammatory response leads to neuronal destruction, cerebral edema, and petechial hemorrhages. Central nervous system infection most commonly occurs via hematogenous spread from respiratory or gastrointestinal sources, blood transfusion, or organ transplantation. Certain pathogens, including rabies virus, herpes simplex virus (HSV), and varicella zoster virus (VZV), may reach the CNS through neural migration.
Etiologic Factors
Viral infections represent the most frequent cause, although noninfectious etiologies such as autoimmune, paraneoplastic, and collagen vascular diseases are increasingly recognized. In nearly half of cases, no specific cause is identified. HSV accounts for approximately 10–20% of all encephalitis cases, and early antiviral therapy significantly improves outcomes. Arboviruses cause 10–15% of cases and are transmitted by mosquitoes or ticks during warmer months. Eastern equine encephalitis is notable for its fulminant course and hippocampal involvement, while Western equine encephalitis tends to follow a nonspecific prodrome. Japanese encephalitis, the most common arboviral encephalitis worldwide, often progresses from mild systemic symptoms to severe neurologic dysfunction.
Flaviviruses such as West Nile virus have increased in incidence in North America and are associated with higher morbidity and mortality in elderly patients, with possible flaccid paralysis and respiratory failure. Enteroviruses primarily affect children and usually follow a benign course. Measles-related encephalitis may occur acutely or years after infection and often involves seizures and rapid neurologic decline. HIV-associated encephalitis is linked to low CD4 counts and presents with white matter degeneration, motor spasticity, and dementia. Rabies causes isolated CNS infection without systemic illness.
Nonviral infectious causes include Mycoplasma pneumoniae, Toxoplasma gondii, Rickettsia rickettsii, Mycobacterium tuberculosis, Borrelia burgdorferi, and Bartonella henselae. In immunocompromised patients, pathogens such as Cryptococcus neoformans, CMV, HHV-6, Listeria monocytogenes, and VZV are common. Autoimmune etiologies include anti-LGI1 and anti–NMDA receptor encephalitis.
Clinical Features And Symptoms
Encephalitis often begins after a brief flulike prodrome characterized by fever, headache, myalgias, sore throat, and reduced appetite. Progressive neurologic involvement includes altered mental status ranging from confusion to coma, personality changes, hallucinations, psychosis, agitation, or delirium. Seizures are common, as are focal neurologic deficits such as tremor, ataxia, cranial nerve palsies, and autonomic instability. Rash may accompany certain infectious causes. Papilledema may be present, and the disease course may be indolent or rapidly progressive, potentially resulting in permanent neurologic injury or death.
Diagnostic Evaluation
A lumbar puncture is essential and typically reveals lymphocytic pleocytosis, elevated protein, and normal glucose in viral etiologies. CSF should undergo Gram stain, culture, and targeted antigen or antibody testing based on clinical suspicion. PCR testing of CSF is critical for confirming viral causes, particularly HSV, VZV, enteroviruses, and West Nile virus. Blood studies include CBC, metabolic panel, liver function tests, and blood cultures. Imaging begins with CT to exclude mass lesions or hemorrhage, while MRI is more sensitive for detecting temporal lobe abnormalities typical of HSV encephalitis. EEG is useful in patients with seizures or unexplained altered mental status.
Differential Considerations
Conditions that may mimic encephalitis include meningitis, brain abscess, sepsis, ischemic or hemorrhagic stroke, head trauma, subarachnoid hemorrhage, metabolic encephalopathies, epilepsy, acute disseminated encephalomyelitis, neoplasms, and toxic exposures such as carbon monoxide.
Initial Management And Stabilization
Immediate priorities include airway protection, seizure control, and stabilization of vital functions. Patients with depressed consciousness or absent protective reflexes require intubation. Empiric treatment for altered mental status includes glucose, thiamine, and naloxone when indicated. Signs of increased intracranial pressure warrant hyperventilation and osmotic therapy with mannitol, along with neurosurgical consultation. Intravenous fluids should be administered cautiously to avoid worsening cerebral edema.
Emergency Department Care
Seizures are treated promptly with benzodiazepines, followed by longer-acting antiepileptic agents if recurrent. Although no specific therapy exists for most viral encephalitides, intravenous acyclovir must be initiated immediately when HSV encephalitis is suspected, based on clinical presentation and preliminary CSF or imaging findings. In immunocompromised patients, ganciclovir or foscarnet should be considered for CMV or HHV-6. Broad-spectrum antibiotics should be administered when meningitis cannot be excluded, especially in the presence of rash or systemic toxicity.
Pharmacologic Therapy
Recommended agents include intravenous acyclovir for suspected HSV, benzodiazepines for acute seizure control, fosphenytoin or phenobarbital for seizure prophylaxis, and mannitol for elevated intracranial pressure. Antiviral therapy should not be delayed while awaiting confirmatory testing.
Disposition And Follow-Up
All patients with suspected or confirmed encephalitis require hospital admission for close neurologic monitoring and definitive management.
Clinical Insights And Common Errors
Empiric treatment with acyclovir should be started immediately in any patient with unexplained encephalitis, as delays significantly increase morbidity and mortality. Failure to initiate early antiviral therapy remains one of the most critical and preventable errors in management.
Basics Overview
Encephalitis is an acute inflammatory process involving the brain parenchyma and accounts for approximately 20,000 cases annually in the United States, with an overall mortality near 10%. The inflammatory response leads to neuronal destruction, cerebral edema, and petechial hemorrhages. Central nervous system infection most commonly occurs via hematogenous spread from respiratory or gastrointestinal sources, blood transfusion, or organ transplantation. Certain pathogens, including rabies virus, herpes simplex virus (HSV), and varicella zoster virus (VZV), may reach the CNS through neural migration.
Etiologic Factors
Viral infections represent the most frequent cause, although noninfectious etiologies such as autoimmune, paraneoplastic, and collagen vascular diseases are increasingly recognized. In nearly half of cases, no specific cause is identified. HSV accounts for approximately 10–20% of all encephalitis cases, and early antiviral therapy significantly improves outcomes. Arboviruses cause 10–15% of cases and are transmitted by mosquitoes or ticks during warmer months. Eastern equine encephalitis is notable for its fulminant course and hippocampal involvement, while Western equine encephalitis tends to follow a nonspecific prodrome. Japanese encephalitis, the most common arboviral encephalitis worldwide, often progresses from mild systemic symptoms to severe neurologic dysfunction.
Flaviviruses such as West Nile virus have increased in incidence in North America and are associated with higher morbidity and mortality in elderly patients, with possible flaccid paralysis and respiratory failure. Enteroviruses primarily affect children and usually follow a benign course. Measles-related encephalitis may occur acutely or years after infection and often involves seizures and rapid neurologic decline. HIV-associated encephalitis is linked to low CD4 counts and presents with white matter degeneration, motor spasticity, and dementia. Rabies causes isolated CNS infection without systemic illness.
Nonviral infectious causes include Mycoplasma pneumoniae, Toxoplasma gondii, Rickettsia rickettsii, Mycobacterium tuberculosis, Borrelia burgdorferi, and Bartonella henselae. In immunocompromised patients, pathogens such as Cryptococcus neoformans, CMV, HHV-6, Listeria monocytogenes, and VZV are common. Autoimmune etiologies include anti-LGI1 and anti–NMDA receptor encephalitis.
Clinical Features And Symptoms
Encephalitis often begins after a brief flulike prodrome characterized by fever, headache, myalgias, sore throat, and reduced appetite. Progressive neurologic involvement includes altered mental status ranging from confusion to coma, personality changes, hallucinations, psychosis, agitation, or delirium. Seizures are common, as are focal neurologic deficits such as tremor, ataxia, cranial nerve palsies, and autonomic instability. Rash may accompany certain infectious causes. Papilledema may be present, and the disease course may be indolent or rapidly progressive, potentially resulting in permanent neurologic injury or death.
Diagnostic Evaluation
A lumbar puncture is essential and typically reveals lymphocytic pleocytosis, elevated protein, and normal glucose in viral etiologies. CSF should undergo Gram stain, culture, and targeted antigen or antibody testing based on clinical suspicion. PCR testing of CSF is critical for confirming viral causes, particularly HSV, VZV, enteroviruses, and West Nile virus. Blood studies include CBC, metabolic panel, liver function tests, and blood cultures. Imaging begins with CT to exclude mass lesions or hemorrhage, while MRI is more sensitive for detecting temporal lobe abnormalities typical of HSV encephalitis. EEG is useful in patients with seizures or unexplained altered mental status.
Differential Considerations
Conditions that may mimic encephalitis include meningitis, brain abscess, sepsis, ischemic or hemorrhagic stroke, head trauma, subarachnoid hemorrhage, metabolic encephalopathies, epilepsy, acute disseminated encephalomyelitis, neoplasms, and toxic exposures such as carbon monoxide.
Initial Management And Stabilization
Immediate priorities include airway protection, seizure control, and stabilization of vital functions. Patients with depressed consciousness or absent protective reflexes require intubation. Empiric treatment for altered mental status includes glucose, thiamine, and naloxone when indicated. Signs of increased intracranial pressure warrant hyperventilation and osmotic therapy with mannitol, along with neurosurgical consultation. Intravenous fluids should be administered cautiously to avoid worsening cerebral edema.
Emergency Department Care
Seizures are treated promptly with benzodiazepines, followed by longer-acting antiepileptic agents if recurrent. Although no specific therapy exists for most viral encephalitides, intravenous acyclovir must be initiated immediately when HSV encephalitis is suspected, based on clinical presentation and preliminary CSF or imaging findings. In immunocompromised patients, ganciclovir or foscarnet should be considered for CMV or HHV-6. Broad-spectrum antibiotics should be administered when meningitis cannot be excluded, especially in the presence of rash or systemic toxicity.
Pharmacologic Therapy
Recommended agents include intravenous acyclovir for suspected HSV, benzodiazepines for acute seizure control, fosphenytoin or phenobarbital for seizure prophylaxis, and mannitol for elevated intracranial pressure. Antiviral therapy should not be delayed while awaiting confirmatory testing.
Disposition And Follow-Up
All patients with suspected or confirmed encephalitis require hospital admission for close neurologic monitoring and definitive management.
Clinical Insights And Common Errors
Empiric treatment with acyclovir should be started immediately in any patient with unexplained encephalitis, as delays significantly increase morbidity and mortality. Failure to initiate early antiviral therapy remains one of the most critical and preventable errors in management.
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