Infratentorial abscess after bacterial meningitis

Presents with a subacute meningitis and and neck stiffness and decreased consciousness.  It occurs after surgery for otitis, mastoiditis, and sinusitis.  Delay in detection occurs due to CT missing the diagnosis, leading to a high mortality.  MRI with DWI can differentiate this condition from reactive subdural effusion,  which is important due to the risk of hydrocephalus, the need for antibiotics and sometimes surgery (should have low threshold to explore). 
See van de Beek, et al.  Neurology 2007, and Wong et al., AJNR 2004.