ABIM & Internal Medicine Shelf Exam Review Question of the Week: Differentiating Bacterial and Viral Meningitis
As we begin #InfDiseaseWeek in our preparation of the ABIM and Internal Medicine Shelf exam review, here is a question directly out of the Knowmedge QVault. Give it your best shot and then see below for the answer.
45-year-old female presents to the ED with fever and headaches, a condition she has been experiencing for the past two days. Physical exam reveals neck stiffness concerning for meningitis. Lumbar puncture is performed and cerbrospinal fluid (CSF) studies are obtained. Opening pressure is noted at 300mmH2O. CSF analysis reveals: 2,000 WBCs/µL, PMN predominance, no RBCs, glucose 24mg/dL and protein 106mg/dL. Serum glucose is 86mg/dL. Which of the following is the most likely diagnosis?
A. HIV encephalitis B. HSV encephalitis C. Early bacterial meningitis D. West Nile virus encephalitis E. Late bacterial meningitis
ABIM Exam Practice Question Explanation
Meningitis may result from:
● Infectious or non-infectious causes
● Acute or chronic in timeframe
● Bacterial or non-bacterial in etiology
Of the different types of meningitis, the one that requires the most urgent treatment is acute bacterial meningitis. This is also the most common form of meningitis. Two-thirds of patients with bacterial meningitis present with fever, neck stiffness (nuchal rigidity) and altered mental status.
To determine the cause of meningitis, a lumbar puncture with cerebrospinal fluid (CSF) analysis is performed. CSF opening pressure is between 200-500mmH2O in bacterial meningitis while it is less than 250mmH2O in viral cases. Leukocyte count is also higher in bacterial cases: 1,000-5,000/µL versus 50-1,000/µL in viral cases.
The differential is a neutrophil-dominant WBC count in bacterial meningitis and a lymphocytic-dominant WBC count in viral meningitis. A mnemonic for remembering that CSF glucose is lower in bacterial infections (less than 40mg/dl versus greater than 45mg/dL) is that the bacteria eat up the glucose, lowering its level. Similarly, protein levels are lower in viral infections but this is not as definitive as there is overlap in the respective ranges of 100-500mg/dL for bacterial and less than 200mg/dL for viral.
Like most other viral-caused infections, patients with meningitis will have a negative Gram stain and culture while the majority of patients with bacterial meningitis will have positive results.
This patient’s CSF glucose of 24mg/dL and elevated leukocytes point to a bacterial cause. To distinguish the timeframe of bacterial meningitis, a CSF-to-serum glucose ratio is calculated. If this ratio is less than 0.4, early bacterial meningitis is the likely diagnosis. As this patient’s CSF-to-serum glucose ratio is 0.3, the best answer is Choice C (Early bacterial meningitis).
Choice B (HSV encephalitis) would be likely if RBCs were noted in the CSF.
You can view all the previous ABIM Exam Review Questions of the Week at the Knowmedge Blog. You can also find additional topics and questions directly from the Knowmedge Internal Medicine ABIM Board Exam Review Questions QVault.
