Arachnoid cyst with bony erosion of the posterior frontal sinus wall as cause of a subdural empyema after pansinusitis

Intracranial arachnoid cysts (AC) can lead to a bony thinning of the surrounding skull. We present a young man, in whom a frontal AC has caused a thinning of the frontal calvaria and of the posterior wall of the frontal sinus. This bony vulnerability allowed a frontal sinusitis to spread intracranially and finally led to a subdural empyema with neurological deterioration. The treatment consisted of an emergency craniotomy with evacuation of the empyema and appropriate antibiotic treatment of the causing pathogen (Streptococcus intermedius).