An intracranial or rarely intraspinal suppurative process invading the space between the inner surface of the DURA MATER and the outer surface of the ARACHNOID. Bacteria and other pathogenic organisms may gain entrance to the subdural space from the FRONTAL SINUS; ETHMOID SINUS; middle ear (EAR, MIDDLE); MASTOID; or as the result of $CRANIOCEREBRAL TRAUMA$ or NEUROSURGICAL PROCEDURES. This condition may be associated with intracranial sinus thrombosis (SINUS THROMBOSIS, INTRACRANIAL). Circumscribed collections of purulent material in the subdural space are referred to as subdural abscesses. (From Adams et al., Principles of Neurology, 6th ed, p709).