Subdural effusions in association with acute bacterial meningitis are:

Subdural effusions are a common complication of acute bacterial meningitis, occurring in 30-50% of cases. The exact pathogenesis is unknown, but the effusions are more frequent in young infants, usually sterile, and not related either to inadequate therapy or to a bleeding disorder. They are frequently asymptomatic and rarely fatal. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 32. Infections of the Nervous System (Bacterial, Fungal, Spirochetal, Parasitic) and Sarcoidosis. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.