ENCEPHALITIS

✓ Medically ReviewedDISEASEID: MDBT0127🚨 TRIAGE: EMERGENCYICD-10: G04.9

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Definition

CLINICAL DEFINITION

Encephalitis is inflammation (immune injury) of the brain parenchyma (working brain tissue). It causes fever with altered mental status (confusion or low alertness) plus focal deficits (local brain failure). Seizures (fits), aphasia (language loss) and behaviour change are common. Viruses are the main infectious trigger with rare bacterial or parasitic causes. Autoimmune disease occurs when antibodies (immune proteins) attack brain targets.

IN PLAIN ENGLISH

Brain tissue is swollen. Germs can cause the swelling. Fever starts fast. Thinking gets mixed up. Fits or speech loss may occur. Tests and care must start fast.

COMMON MISCONCEPTION / WHAT IT DOES NOT MEAN

Encephalitis is not meningitis alone, which affects meninges (thin brain coverings) without brain tissue invasion. It is not metabolic encephalopathy (brain fog from body upset) with normal scans and spinal fluid. It is not stroke (blocked brain vessel) with sudden focal loss without fever. It is not functional symptoms (stress-linked changes) without fever, imaging or spinal fluid signs.

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Word Breakdown

en-

=in or within

encephal

=brain

-itis

=inflammation

Literal Meaning: inflammation inside the brain

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Abbreviations

HSV=Herpes simplex virus - cold sore virus that can infect the brain

VZV=Varicella zoster virus - chickenpox virus that can inflame the brain

CSF=Cerebrospinal fluid - clear fluid around the brain and spine

MRI=Magnetic resonance imaging - detailed brain scan using magnets

EEG=Electroencephalogram - brain wave recording for seizures

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Synonyms & Variants

Brain inflammation›Acute encephalitis›Viral encephalitis›

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Category-Specific Context

Encephalitis is a Disease because it is a tissue-level process with causes and course. It is not a Symptom (patient complaint) such as headache alone. It is not a Sign (exam finding) such as fever alone. It spans infectious and autoimmune origins (germ and self-attack levels). Care targets cause, swelling and seizures rather than comfort alone.

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Clinical Context & Usage

CLINICAL SETTINGS

Emergency DepartmentNeurology Inpatient UnitIntensive Care UnitInfectious Disease ClinicPaediatric Acute Care

USAGE & DOCUMENTATION NOTES

Use encephalitis for fever with altered behaviour, seizure or focal signs suggesting brain invasion. Record exposure, vaccine status, suspected cause and consciousness score. Distinguish brain tissue disease from meningeal (covering) disease by focal features and scans. Escalate early for airway, seizure and pressure (swelling) concerns.

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Primary Causes & Pathophysiology

PATHOPHYSIOLOGY

Virus or autoantibody (self-attacking protein) crosses the blood-brain barrier (protective brain filter). Direct invasion kills neurons (brain nerve cells) and releases cytokines (inflammation signals). Immune influx causes oedema (swelling) with small-vessel injury. Temporal lobe (memory area) necrosis (tissue death) is typical of herpes infection. Severe swelling raises pressure and risks herniation (brain shift) with lasting harm.

PRIMARY CAUSES

Herpes simplex virus type 1

Invades temporal lobes (memory areas) through nerves and causes necrotising (tissue-killing) inflammation

Varicella zoster virus

Reactivates from nerve clusters and spreads to brain vessels and tissue with vasculitis (vessel inflammation)

Enteroviruses

Reach the brain through blood after gut infection and trigger immune inflammation

Arboviruses such as West Nile and Japanese encephalitis viruses

Pass by mosquito bite then cross the brain filter and infect neurons (nerve cells)

Anti-NMDA receptor autoimmune encephalitis

Antibodies (immune proteins) block brain receptors and disrupt signalling with psychosis (false beliefs) and movements

Acute disseminated encephalomyelitis

Post-infectious immune response attacks myelin (nerve coating) in brain and cord

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Drug-Induced Causes

💊 Cefepime and other beta-lactams ↗

Mechanism: Lowers seizure threshold (fit trigger level) and causes toxic brain dysfunction

→ Stop drug promptly with supportive care and neurology input

💊 Immune checkpoint inhibitors ↗

Mechanism: Unleash T cells (immune cells) that attack brain tissue as well as tumour

→ Hold therapy and give steroids (inflammation-calming drugs) per oncology and neurology guidance

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Epidemiology

Prevalence:Encephalitis is uncommon but seen worldwide in all settings. Cases occur sporadically with seasonal clusters linked to mosquito vectors (insect carriers). Young children and older adults face higher risk with worse outcomes. Autoimmune forms are increasingly recognised with better antibody (immune protein) testing.

Age Group:All ages can be affected with cause varying by group. Young children often have viral and post-infectious forms (after-infection immune reactions). Older adults often have herpes and West Nile types with severe course. Autoimmune types often affect young adults but can occur widely.

Geographic Distribution:Encephalitis occurs worldwide without strict borders. Mosquito-borne types cluster in warm seasons and endemic (locally common) regions. Japanese encephalitis concentrates in parts of Asia with rice and pig exposure. Tick-borne types occur in wooded temperate zones with tick activity.

Red Flags & Triage ★

TRIAGE LEVEL

🚨 EMERGENCY

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

Herpes simplex encephalitis needing immediate antiviral
Bacterial meningitis mimicking encephalitis needing urgent antibiotics
Brain herniation from swelling needing airway and pressure care
Autoimmune encephalitis needing immunotherapy not antivirals alone
Nonconvulsive status epilepticus needing brain wave testing

URGENT EVALUATION TRIGGERS

Fever with confusion, agitation or low responsiveness
New seizure, one-sided weakness, language loss or vision change
Severe headache with vomiting, stiff neck or rash
Rapid decline with breathing difficulty or pressure signs

Contraindications & Precautions

CONTRAINDICATIONS

Do not delay acyclovir (herpes antiviral) while awaiting tests when herpes is suspected
Do not perform lumbar puncture (spinal tap) if imaging shows mass effect (dangerous shift)
Do not give sedatives that mask consciousness without monitoring
Do not use live vaccines during acute illness with severe immune suppression

SPECIAL PRECAUTIONS

Monitor airway with breathing, seizures and brain pressure closely
Isolate per infection control for suspected contagious causes
Adjust drug doses in kidney and liver dysfunction with pharmacy review
Screen for autoimmunity (self-attack) when infection tests are negative or relapse occurs

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Seizures with status epilepticus (prolonged fits)
Brain oedema with raised pressure and herniation (brain shift)
Memory loss with cognitive deficits (thinking problems) and behaviour change
Weakness with speech loss and movement disorders
Aspiration pneumonia from inhaled secretions with intensive care harm
Death in severe or late-treated disease

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Confused With (LASA & DDx) ★

LEXICAL (LASA) — SIMILAR NAMES

Encephalopathy

Both cause confusion but encephalopathy is brain dysfunction without inflammation while encephalitis is true inflammation

Meningitis

Both cause fever with headache but meningitis affects meninges (brain coverings) while encephalitis affects brain tissue itself

Encephalomyelitis

Names overlap but encephalomyelitis involves spinal cord plus brain while encephalitis is brain only

CLINICAL DIFFERENTIAL DIAGNOSES (DDx)

Bacterial meningitis

Key Distinguishing Feature: Neck stiffness with spinal fluid pus cells and no focal brain lesions on scan

Metabolic encephalopathy

Key Distinguishing Feature: Diffuse dysfunction with normal scans and abnormal salts or liver failure

Ischaemic stroke

Key Distinguishing Feature: Sudden focal deficit in vessel territory without fever with diffusion lesion on scan

Seizure with postictal state

Key Distinguishing Feature: Brief altered awareness then gradual recovery with seizure patterns on brain waves

Brain abscess

Key Distinguishing Feature: Focal ring-enhancing spot with fever but localised pus collection

Toxic delirium

Key Distinguishing Feature: Drug exposure with fluctuating attention and clear scans without spinal fluid inflammation

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Clinical Pearls

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Start acyclovir (herpes antiviral) at once when herpes is possible without waiting for virus DNA results. Delay raises necrosis (tissue death) and lasting harm.

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Check spinal fluid with opening pressure, cells, protein, glucose and virus PCR (virus DNA test) once scans exclude shift. Pair with blood cultures and autoimmune panel when negative.

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Use brain wave recording early for silent fits when confusion persists after treatment. Nonconvulsive status (hidden ongoing seizures) is easily missed without it.

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Consider autoimmune cause when virus tests are negative with strange movements or dominant psychosis (false beliefs). Escalate for steroids (calming drugs) with neurology input.

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Protect airway with intensive care review for coma (unresponsiveness), swelling or repeated fits. Monitor pressure signs with frequent nerve checks.

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Example Usage & SOAP

CLINICAL SENTENCE

""26-year-old with fever, confusion and seizure has temporal encephalitis likely from herpes.""

IN A SOAP NOTE

S: 27-year-old with 2 days fever, headache and confusion plus one witnessed fit, no head injury. O: Temp 38.7C, BP 128/80, HR 106, RR 22, SpO2 96 percent on air, drowsy with disorientation to time, neck supple without rash. A: Suspected acute encephalitis likely viral with seizure. P: Start antiviral, arrange brain scan with spinal tap if safe, brain wave test, intensive care review, seizure and airway watch.

DIFFERENTIAL DIAGNOSIS CONTEXT

Fever with seizure and behaviour change favours encephalitis over stroke without fever. Neck stiffness with cloudy spinal fluid favours bacterial meningitis. Diffuse fog with abnormal salts favours metabolic cause. Sudden focal loss in vessel territory favours stroke. Check fever course with scans and spinal fluid.

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Classifications & Scoring

GRADING SYSTEM

Glasgow Outcome Scale

Grades brain injury recovery from death to good recovery for prognosis tracking

GRADING SYSTEM

Modified Rankin Scale

Grades disability from no symptoms to severe dependence for function review

SCORING TOOL

Glasgow Coma Scale

Scores eye, verbal and motor responses to track consciousness level

SCORING TOOL

Clinical Assessment Scale for Autoimmune Encephalitis

Scores seizures, memory, behaviour and other domains to track autoimmune activity

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Prognosis

SHORT-TERM OUTCOME

Short term course ranges from full recovery to rapid decline. Early herpes therapy with swelling control improves survival. Coma (deep unconsciousness), seizures and intensive care need predict worse course. Delay in recognition raises risk of lasting harm.

LONG-TERM PROGNOSIS

Many survivors keep memory, attention or mood problems. Epilepsy (recurrent fits), hearing loss and motor deficits can persist. Rehabilitation (recovery therapy) improves function over months to years. Autoimmune cases may relapse (return) without ongoing immune control.

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References & Coding

ICD-10 DIAGNOSTIC CODE

G04.9

LITERATURE CITATIONS & GUIDELINES

[1]WHO ICD-10 G04 Encephalitis

[2]NICE guideline on meningitis and encephalitis recognition and management

[3]Harrison's Principles of Internal Medicine acute encephalitis

[4]Cochrane reviews on viral encephalitis therapies

MEDICAL DISCLAIMER — This content is intended for educational purposes and qualified healthcare professionals only. Always refer to current clinical diagnostic criteria, institutional protocols, and official guidelines. Medobo is not responsible for clinical decisions made based on this content.