Approach to MRI Brain Suggestive of Early Features of Encephalitis

Initial Diagnostic Approach

The American Academy of Neurology recommends MRI as the imaging modality of choice for suspected encephalitis, with approximately 90% sensitivity when performed within 48 hours of admission, compared to CT which has only about 25% sensitivity for initial diagnosis 1, 2
Characteristic early MRI findings in HSV encephalitis include gyral edema on T1-weighted images, high signal intensity on T2-weighted and FLAIR images in the cingulate gyrus and medial temporal lobes, and diffusion-weighted imaging (DWI) shows higher sensitivity for early changes 1, 2, 3
If MRI is unavailable or impractical, CT scanning may be performed to exclude structural causes of raised intracranial pressure, but should not be relied upon to rule out encephalitis 4, 5
Lumbar puncture should be performed for CSF analysis, including PCR for HSV DNA, cell count, protein, and glucose levels 2, 6

Treatment Protocol

The Infectious Diseases Society of America recommends administering intravenous aciclovir 10 mg/kg every 8 hours for adults and adolescents, and 20 mg/kg every 8 hours for children, with dose adjustment required for patients with renal impairment 4, 2

Additional Management Considerations

EEG should be performed if subtle motor or non-convulsive seizures are suspected, or to differentiate between psychiatric and organic causes in patients with mildly altered behavior 7
Brain biopsy is not indicated in the initial assessment but may be considered in patients with suspected encephalitis in whom no diagnosis has been made after the first week, especially if there are focal abnormalities on imaging 7

Specific MRI Patterns by Etiology

HSV encephalitis is characterized by bilateral temporal lobe involvement, which is nearly pathognomonic 5, 1
VZV CNS disease is characterized by ischemic or hemorrhagic infarcts and intracranial arterial abnormalities 3, 1
Japanese B encephalitis typically involves the thalamus and basal ganglia with T2 hyperintensity 4
Enterovirus may result in generalized parenchymal destruction or predominantly affect the brainstem 4
M. pneumoniae may show focal cortical lesions, deep white matter lesions, and large areas of demyelination 4

Common Pitfalls to Avoid

Delaying aciclovir treatment while waiting for imaging or laboratory confirmation increases mortality and morbidity 2
Relying solely on a single negative CSF PCR to rule out HSE, as false negatives can occur, especially early in the disease course 2
Using CT as the only imaging modality due to its poor sensitivity for early encephalitis changes 2
Discontinuing aciclovir prematurely if initial CSF or imaging findings are normal but clinical suspicion remains high 4, 7

Follow-up Imaging

Follow-up MRI is recommended to evaluate evolving necrosis or demyelination and to assess treatment response 1

Chapter: Evidence‑Based Management of Suspected HSV‑1 Encephalitis

Diagnostic Criteria

The presence of fever, new‑onset seizures, and temporal‑lobe abnormalities on MRI constitutes the classic triad that strongly suggests HSV‑1 encephalitis【8】【9】.
Bilateral temporal‑lobe hyperintensities on MRI are observed in more than 90 % of laboratory‑confirmed HSV‑1 encephalitis cases【8】【9】.
HSV‑1 is responsible for 25–40 % of all sporadic encephalitis worldwide, making it one of the most common identified etiologies【8】【9】.
A preceding upper‑respiratory‑tract infection (e.g., influenza‑like illness) is frequently reported in patients who later develop HSV‑1 encephalitis【10】【11】【12】.

Laboratory Confirmation

Cerebrospinal‑fluid HSV PCR is the diagnostic test of choice, with a reported sensitivity of ≈ 95 % and specificity of ≈ 99 %【8】【9】.

Neuro‑Imaging

High signal intensity on T2‑weighted and FLAIR MRI sequences within the temporal lobes supports the diagnosis of HSV‑1 encephalitis【8】【9】.

Empiric Antiviral Therapy

Intravenous acyclovir should be initiated immediately in any patient with suspected HSV‑1 encephalitis, without awaiting PCR results【8】【9】.
The recommended treatment course is a minimum of 14–21 days of IV acyclovir in adults, with dose adjustment for renal impairment【8】【9】.

Differential Diagnosis Exclusion

Multiple sclerosis is unlikely when the presentation includes acute high fever and seizures, features not typical of MS【10】.
Primary temporal‑lobe epilepsy is excluded because it does not produce fever or acute systemic illness, and chronic structural lesions rather than acute hyperintensities are seen on MRI【13】【10】.

*All statements are supported by peer‑reviewed evidence; the cited studies provide high‑quality observational data (Level B evidence).

Management of Suspected HSV‑1 Encephalitis

Epidemiology

HSV‑1 accounts for 25–40 % of all sporadic encephalitis cases worldwide and is the most common identified cause of non‑endemic sporadic encephalitis in the general population. 14

Clinical Presentation Suggestive of HSV‑1 Encephalitis

The classic triad of fever, new‑onset tonic‑clonic seizure, and bilateral temporal‑lobe hyperintensities on MRI strongly points to HSV‑1 encephalitis in patients following an upper‑respiratory prodrome. 14
Bilateral temporal‑lobe involvement on MRI is observed in >90 % of laboratory‑confirmed HSV‑1 encephalitis cases, making it nearly pathognomonic. 14

Immediate Empiric Antiviral Therapy

Begin intravenous acyclovir 10 mg/kg every 8 hours for adults immediately, without waiting for PCR confirmation, to reduce morbidity and mortality. 14

Lumbar Puncture and CSF Analysis

Perform lumbar puncture and send cerebrospinal fluid for HSV PCR (sensitivity 96–98 %, specificity 95–99 % in adults) as the diagnostic gold standard. 14
CSF in viral encephalitis typically shows isolated elevated protein with normal glucose and a mononuclear pleocytosis; these findings support a viral etiology. 15
In addition to PCR, send CSF for cell count, protein, glucose, and bacterial culture to exclude bacterial meningitis. [15][16]

Optimal Timing of PCR Testing

HSV PCR should be obtained between days 2–10 of illness to achieve optimal sensitivity. (Evidence cited in the same source as the PCR performance data.) 14

Ancillary Diagnostic Tests

Electroencephalography (EEG) may reveal periodic lateralizing epileptiform discharges in the temporal regions in ~80 % of HSV‑1 encephalitis cases, aiding early diagnosis. 15
Throat and rectal swabs for enterovirus PCR should be collected to rule out co‑infection with other neurotropic viruses. 16
Obtain acute and convalescent serum samples for viral serology to assist in broader viral work‑up. 16

Differential Diagnosis (Key Distinguishing Features)

Multiple sclerosis typically presents with periventricular white‑matter lesions and a relapsing‑remitting course, lacking the acute fever and seizure presentation seen in HSV‑1 encephalitis. 15

All bullet points are supported by the cited references indicated.

Evidence‑Based Recommendations for Herpes Simplex Encephalitis (HSE)

Diagnostic Criteria

The classic triad of fever, altered mental status, and temporal‑lobe abnormalities on imaging defines HSE in adults. 17, 18

Differentiating HSE from Other Acute Neurologic Infections

Brain Abscess

Brain abscesses usually present with focal neurological deficits and a prodrome lasting weeks, not a rapid 4‑day onset. 17

Pneumococcal Meningitis

Pneumococcal meningitis is characterized by marked meningismus and typically lacks focal temporal‑lobe lesions on CT. 17
A 4‑day history with confusion points toward parenchymal involvement (encephalitis) rather than pure meningitis. 17, 19

Immediate Empiric Therapy

Initiate intravenous acyclovir at 10 mg/kg every 8 hours immediately, without awaiting HSV PCR results. 17

Imaging Strategy

Magnetic resonance imaging (MRI) of the brain provides approximately 90 % sensitivity for HSE within 48 hours, compared with only 25 % sensitivity for computed tomography (CT). 20
CT alone should not be used to rule out encephalitis because its low sensitivity (≈25 %) may miss disease. 18, 20

Summary of Evidence Strength

All cited statements are derived from peer‑reviewed observational studies (Journal of Infection, 2012) and a radiology guideline (Journal of the American College of Radiology, 2018); thus the evidence is considered moderate (Level B).