Clinical Scenario:
A 6-year-old girl presents with a 3-day history of fever, headache and progressive drowsiness. She has had a brief episode of left-sided facial twitching. On examination, she is drowsy but responds appropriately to simple commands. Neck stiffness is present. Her pupils are equal and reactive to light, and there is no focal limb weakness.
Cerebrospinal fluid examination shows:
- WBC: 55 cells/mm³
- Differential: 90% lymphocytes
- RBC: 2,500 cells/mm³
- Protein: 90 mg/dL
- Glucose: 60 mg/dL
- Blood glucose: 90 mg/dL
Which of the following is the most likely diagnosis?
A. Acute bacterial meningitis
B. Herpes simplex encephalitis
C. Tuberculous meningitis
D. Viral meningitis
E. Brain abscess
Correct answer & Explanation:
Correct answer: B. Herpes simplex encephalitis
Explanation
The clinical picture is most consistent with herpes simplex encephalitis (HSE).
The important clues are the acute onset, progressive drowsiness indicating encephalopathy, and a focal neurological seizure manifestation (facial twitching). HSV encephalitis commonly presents with fever, headache, altered consciousness and focal seizures or neurological deficits.
The CSF typically demonstrates a lymphocytic pleocytosis with elevated protein and usually preserved glucose. Importantly, RBCs may be present because HSV encephalitis can produce haemorrhagic necrosis.
In this case:
CSF glucose : blood glucose = 60 : 90 = 0.67
Thus, CSF glucose is relatively preserved, which is more compatible with viral encephalitis than classic TB meningitis.
The 2,500 RBCs/mm³ is an important examination clue. It should not be interpreted as diagnostic by itself, because RBCs can also result from a traumatic lumbar puncture, but in the appropriate clinical setting it supports HSV encephalitis.
Why the other options are incorrect
A. Acute bacterial meningitis — Incorrect
Bacterial meningitis generally produces a predominantly neutrophilic CSF pleocytosis, often with substantially elevated protein and reduced glucose. The lymphocyte predominance and focal encephalitic features make HSV more likely.
C. Tuberculous meningitis — Incorrect
TB meningitis typically has a more subacute presentation, with lymphocytic pleocytosis, elevated protein and reduced CSF glucose. Cranial nerve palsies can occur, but the acute 3-day presentation with focal facial twitching and RBC-containing CSF favours HSV in this case.
D. Viral meningitis — Incorrect
Simple viral meningitis generally produces fever, headache and lymphocytic CSF but does not typically cause progressive encephalopathy and focal seizures. Once altered consciousness or focal neurological features are prominent, encephalitis should be considered.
E. Brain abscess — Incorrect
A brain abscess can cause seizures and focal neurological deficits, but the CSF findings and acute meningoencephalitic presentation are more consistent with HSV encephalitis.
Exam Pearl
Acute encephalopathy + focal seizure/neurological deficit + lymphocytic CSF ± RBCs → think HSV encephalitis.
The diagnostic test of choice is CSF HSV DNA PCR. It has very high diagnostic accuracy, although an early negative PCR—particularly within the first 72 hours—does not completely exclude HSE when clinical suspicion remains high.
References:
- Kliegman RM, St Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, editors. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia: Elsevier; 2023.
- Long SS, Prober CG, Fischer M, editors. Principles and Practice of Pediatric Infectious Diseases. 6th ed. Philadelphia: Elsevier; 2023.
- Tunkel AR, Glaser CA, Bloch KC, Sejvar JJ, Marra CM, Roos KL, et al. The management of encephalitis: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2008;47(3):303-27.
