What are the clinical features and diagnostic criteria for encephalitis and meningitis?

Clinical Features and Diagnostic Criteria for Encephalitis and Meningitis

The diagnosis of encephalitis and meningitis requires a combination of clinical features, laboratory findings, and neuroimaging, with altered mental status being the cardinal feature of encephalitis and meningism being the hallmark of meningitis.

Encephalitis

Clinical Features

Major Clinical Features

• Altered mental status (required feature) - includes decreased or altered consciousness, lethargy, confusion, disorientation, or personality changes lasting ≥24 hours 1
• Fever (typically ≥38°C within 72 hours before or after presentation) 1
• Seizures (generalized or focal) 1
• Focal neurological deficits 1
• Speech disturbances (dysphasia, aphasia) 1
• Behavioral changes (can be mistaken for psychiatric illness) 1

Additional Features

• Headache, often severe 1
• Nausea and vomiting 1
• Cognitive dysfunction 1
• Movement disorders, including Parkinsonism 1

Diagnostic Criteria

According to the International Encephalitis Consortium, diagnosis requires 1:

1. Major criterion (required): Altered mental status lasting ≥24 hours with no alternative cause identified

2. Minor criteria (≥2 required for possible encephalitis; ≥3 required for probable or confirmed encephalitis):

   • Documented fever ≥38°C within 72 hours before/after presentation
   • Generalized or partial seizures not attributable to pre-existing seizure disorder
   • New focal neurological findings
   • CSF pleocytosis (≥5 WBC/mm³)
   • Abnormal brain parenchyma on neuroimaging suggestive of encephalitis
   • Abnormal EEG consistent with encephalitis

3. Confirmed encephalitis requires one of:

   • Pathologic confirmation of brain inflammation
   • Evidence of acute infection with a microorganism associated with encephalitis
   • Laboratory evidence of an autoimmune condition associated with encephalitis

Essential Diagnostic Tests 1, 2

• CSF analysis:

  • Opening pressure
  • Cell count with differential (typically <1000 WBC/mm³ with lymphocyte predominance)
  • Protein (often elevated)
  • Glucose (usually normal)
  • PCR for HSV-1/2, VZV, enteroviruses
  • Gram stain and bacterial culture
  • Cryptococcal antigen
  • Oligoclonal bands and IgG index

• Neuroimaging:

  • MRI preferred over CT (more sensitive for detecting early changes)
  • Temporal lobe abnormalities suggest HSV encephalitis

• EEG:

  • May show generalized slowing or specific patterns (e.g., temporal lobe periodic discharges in HSV)

• Serology:

  • Acute and convalescent serum samples for paired antibody testing

Meningitis

Clinical Features

Major Clinical Features

• Meningism (neck stiffness, headache, photophobia) 1
• Fever (not always present) 1
• Altered consciousness (suggests complicated meningitis or meningoencephalitis) 1

Additional Features

• Nausea and vomiting 1, 3
• Non-specific symptoms: diarrhea, muscle pain, sore throat (particularly in viral meningitis) 1
• Rash (may be present in certain etiologies like meningococcal disease) 1

Diagnostic Criteria

No formal consensus criteria exist, but diagnosis typically requires:

1. Clinical features of meningism
2. CSF analysis showing:
   • For bacterial meningitis: Elevated WBC (typically neutrophil predominance), elevated protein, decreased glucose
   • For viral meningitis: Mild to moderate pleocytosis (lymphocyte predominance), mildly elevated protein, normal glucose 1

Essential Diagnostic Tests 1

• CSF analysis:

  • Opening pressure
  • Cell count with differential
  • Protein and glucose levels
  • Gram stain and culture
  • PCR for enteroviruses, HSV-1/2, VZV

• Blood tests:

  • Complete blood count
  • Blood cultures
  • C-reactive protein and procalcitonin (may help distinguish bacterial from viral causes)

Distinguishing Features and Diagnostic Pitfalls

Encephalitis vs. Meningitis

• Encephalitis: Primary brain parenchymal inflammation with altered mental status and focal neurological signs
• Meningitis: Primary meningeal inflammation with meningism but typically preserved consciousness
• Meningoencephalitis: Combined features of both conditions 1

Important Diagnostic Pitfalls

1. Absence of fever does not rule out infection, especially in immunocompromised patients 1

2. Absence of CSF pleocytosis does not exclude encephalitis, particularly:

   • Early in the disease course
   • In immunocompromised patients 1

3. Normal neuroimaging does not exclude encephalitis, especially early in the disease course 1

4. Subtle presentations of HSV encephalitis may include:

   • Low-grade fever rather than high fever
   • Behavioral changes mistaken for psychiatric illness
   • Speech disturbances 1

5. Metabolic, toxic, and systemic causes of encephalopathy must be excluded 1

Special Considerations

Pediatric Patients

• May present with non-specific symptoms (feeding difficulties, respiratory symptoms) 1
• Febrile seizures are common and don't necessarily indicate encephalitis 1
• Concomitant respiratory (54%) or gastrointestinal (21%) symptoms are common 1

Immunocompromised Patients

• May have atypical presentations with minimal inflammatory response 1, 2
• Broader differential diagnosis including opportunistic infections 1

Immediate Management

While not the focus of the question, it's critical to note that both conditions are medical emergencies requiring:

• Encephalitis: Immediate empiric aciclovir (10 mg/kg IV every 8 hours) 2, 4
• Bacterial meningitis: Early administration of appropriate antibiotics (typically ceftriaxone plus vancomycin) 4

Early treatment significantly reduces mortality and neurological sequelae in both conditions 4, 3.