In a study of febrile infants with a bulging fontanelle, only one out of 153 infants had bacterial meningitis. Other diagnoses included aseptic meningitis (26.7%), upper respiratory tract infection (18.3%), viral disease NOS (15.6%), roseola infantum (8.5%) and acute otitis media (6.5%).
In this study, most infants with fever and a bulging fontanelle had benign and self-limited disease.
Bulging fontanelle in febrile infants: is lumbar puncture mandatory?
Arch Dis Child. 2009 Sep;94(9):690-2