Acute bleeds often develop after high-speed acceleration or deceleration injuries. They are most severe if associated with cerebral contusions. Though much faster than chronic subdural bleeds, acute subdural bleeding is usually venous and therefore slower than the arterial bleeding of an epidural hemorrhage. Acute subdural hematomas due to trauma are the most lethal of all head injuries and have a high mortality rate if they are not rapidly treated with surgical decompression. The mortality rate is higher than that of epidural hematomas and diffuse brain injuries because the force required to cause subdural hematomas tends to cause other severe injuries as well.