Microbiology · Year 2 · from Microbiology

Case 1: Herpes Simplex Encephalitis

Presentation

A 52-year-old woman is brought to the emergency department by her husband because of 3 days of fever, headache, and progressive confusion. Today she had a generalized seizure at home. Her husband reports she has been behaving strangely, with difficulty finding words and personality changes. She has no history of immunocompromise. On examination, temperature is 39.2°C, she is disoriented to time and place, and has difficulty naming common objects. MRI brain shows T2 hyperintensity and swelling involving the left temporal lobe and insular cortex. CSF analysis shows 85 WBC/μL (90% lymphocytes), protein 78 mg/dL, glucose 55 mg/dL (serum glucose 100 mg/dL).

Clinical Image

MRI brain demonstrating T2/FLAIR hyperintensity involving the temporal lobe and insular cortex, characteristic of herpes simplex encephalitis.

Image Source: Lecture image - HSV encephalitis imaging

Questions

  1. What is the most likely diagnosis and causative agent?
  1. Why does this infection characteristically involve the temporal lobes?
  1. What is the most important diagnostic test, and how quickly should treatment begin?
  1. What is the prognosis, and what factors affect outcome?

Answers

  1. Diagnosis and causative agent: This is herpes simplex encephalitis (HSE), most commonly caused by HSV-1 (over 90% of adult cases). HSE is the most common cause of sporadic fatal encephalitis in the United States. The presentation with fever, altered mental status, seizures, focal neurologic deficits (especially aphasia), and characteristic temporal lobe involvement on imaging is classic.
  1. Temporal lobe predilection: The temporal lobe predilection is thought to relate to the virus's route of entry into the CNS. HSV-1 may access the brain via:
  • Olfactory nerve from the nasal mucosa to the olfactory bulb and then to the medial temporal lobe
  • Trigeminal nerve (site of latency) with retrograde spread along meningeal branches to the temporal lobe

The limbic system (which includes the temporal lobes, insula, and cingulate gyrus) is characteristically affected, explaining the personality changes, memory disturbances, and psychiatric symptoms often seen.

  1. Diagnostic test and treatment timing: CSF PCR for HSV DNA is the gold standard for diagnosis, with sensitivity >95% and specificity >99%. Results are typically available within hours to a day.

However, treatment with IV acyclovir (10 mg/kg every 8 hours) must begin IMMEDIATELY upon clinical suspicion - do not wait for PCR results. HSE has a mortality rate of 70% if untreated, reduced to approximately 20-30% with prompt acyclovir. Every hour of delay increases the risk of death and permanent neurologic damage. Treatment duration is typically 14-21 days.

  1. Prognosis and prognostic factors: Even with treatment, mortality is 20-30%, and significant neurologic sequelae occur in up to 50% of survivors, including:
  • Memory impairment (especially anterograde memory from hippocampal damage)
  • Personality changes
  • Epilepsy
  • Aphasia

Factors affecting outcome:

  • Time to treatment initiation (most critical factor)
  • Age (worse outcomes in very young and elderly)
  • Level of consciousness at presentation
  • Extent of brain involvement on imaging

All cases for this lecture as Markdown