Natural history of chronic subdural haematoma.

This review will clarify the natural history of chronic subdural haematoma (SDH). Chronic SDH has dual origins, one from subdural hygromas (SDG) and the other from acute SDHs. It occurs only in patients with a suitable pre-morbid condition, i.e. sufficient potential subdural space (PSS). In unresolved SDGs, proliferation of dural border cells produces the neomembrane. Unresolved SDGs become chronic SDHs by repeated micro-haemorrhages from fragile new vessels, which were grown into the neomembrane. When PSS is sufficient, acute SDHs may become chronic SDHs. Chronic SDHs enlarge when rebleeding exceeds absorption and they become symptomatic. When the neomembrane is matured, the neocapillary is no longer fragile. If absorption exceeds rebleeding, the haematoma will disappear. Maturation of the neomembrane and stabilization of the neovasculature eventually result in spontaneous resolution. The fate of chronic SDH depends on the pre-morbid status, the dynamics of absorption-expansion and maturation of the neomembrane.