1. Blood accumulates between the dura and arachnoid as a result of shearing of bridging veins. Subdural hemorrhage is often associated with blunt trauma without skull fracture and results from rotation of brain.
2. The rate of progression is variable - subdural hemorrhage may be classified as acute or chronic and (and sometimes subacute) depending on the rate of accumulation of blood and thus rate of progression of symptoms. Acute and chronic conditions will be discussed separately.
Acute subdural hemorrhage is associated with obvious trauma and is usually accompanied by contusion and cerebral artery tearing (leading to subarachnoid hemorrhage and bloody CSF). Symptoms develop within a few days (the onset of symptoms is slower than for epidural hemorrhage). The hemorrhage acts as a space-occupying lesion and symptoms may be focal and/or those of increased intracranial pressure. Consequences depend on the rapidity of surgical drainage and severity of concomitant damage to the brain.