Subdural haemorrhages result from rupture of cortical bridging veins. These connect the venous system of the brain to the large intradural venous sinuses and lie relatively unprotected in the subdural space.
Acute subdural haemorrhage is usually associated with severe brain injury following trauma. It can occur at any age.
Chronic subdural haemorrhage may be traumatic or may arise spontaneously. It is most common in infants and the elderly. Blood slowly accumulates in the subdural space, usually overlying the frontal and parietal lobes. It becomes encysted between an outer wall of highly vascularised, granulation tissue adherent to the dura, and a thinner, inner wall of fibrous tissue. It is frequently bilateral.