Viral meningitis usually does not cause somnolence; Bacterial meningitis usually progresses to confusion; encephalitis usually results in bizarre behaviour.
Difficult meningitis scenarios were reviewed. The best resource to read is Harrisons textbook of internal medicine.
Acute meningitis presentations - bacterial, viral and encephalitis vs
subacute presentations of bacterial brain abscess
Viral meningitis does not cause somnolence / drowsiness
Bacterial meningitis progresses to confusion
Encephalitis often presents with bizarre behaviour
Meningism, kernigs and neck stiffness are late signs and only present in <20% of adults and fewer children. mortality once meningism is present is 20-30% -- so don't use this to rule out meningitis!!!
Best resource for difficult types of meningitis eg TB, cryptococcal, syphillis is Nelsons textbook of paediarics or UTD.
see tables attached below