There is a large extra-axial (subdural) left parafalcine collection with extension along the tentorium cerebelli on both sides. There is significant mass effect with rightward subfalcine herniation, partial effacement of left lateral ventricle (more on the occipital horn). There is also left cerebral hemisphere sulcal effacement. Importantly, the left ambient cistern is obliterated with evidence of compression of the left cerebral crus associated with left sided uncal herniation.
There is subtle hypodensity in the left hemispheric white matter, more conspicuous in the left frontal lobe.
Evidence of left frontal and bilateral ethmoid sinus opacification.
Interpretation:
Given the lack of trauma, a subdural empyema is highly suspected, likely from paranasal sinusitis.