This extra-axial collection has characteristics of subdural empyema, likely secondary to acute sinusitis. Parenchymal hypodensity in the left inferior frontal lobe adjacent to the collection without enhancing focus likely represents cerebritis. 

The patient proceeded to urgent neurosurgery, and pus was drained from the sinuses and subdural space. 

Microscopy and culture

GRAM STAIN

• leukocytes ++
• gram-positive cocci ++
• gram-negative bacilli ++

CULTURE

Mixed anaerobes including:

• Prevotella oris +
• Slackia exigua +
• Fusobacterium nucleatum ++
• Parvimonas micra ++

Sinusitis is the most common cause of intracranial subdural empyema with anaerobic organisms common.