A case of Streptococcus pneumoniae meningitis showing a marked contrast enhancement effect of whole arachnoid membrane on MRI.

We have experienced the adult case of meningitis caused by the Streptococcus pneumoniae, which showed the most aggressive clinical course. The peculiar and unique magnetic resonance images are reported. The whole arachnoid membrane was prominently enhanced following the contrast material injection, however, no contrast enhancement was observed in subarachnoid space, ependyma, intraventricular cerebrospinal fluid, and dura mater. This implies the superacute phase of the bacterial meningitis, in which inflammation is localized in the whole arachnoid membrane. The contrast enhancement effect in the meningitis may vary depending on the phase and severity, because it is a time-progressive disease.