[Chronic subdural hematoma--changed its density from low to high on follow-up CT (author's transl)].

Recently much attention has been paid once again to etiology of chronic subdural hematoma since the appearance of computed tomography (CT). Authors examined 1824 head injury cases by CT from January 1977 to September 1979. Among them, there are 40 cases (they were all over 16 years old) whose CT showed frontal extracerebral low density area. The low density area which is considered to be so-called fluid collection could be classified into four types (type I to IV). In two cases of those 40 cases (5%), the low density area changed into high density area which seemed to be hemorrhage. The 2 cases were diagnosed to be chronic subdural hematoma and proved surgery. The change of density on CT developed only from type I (the brain surface looks smooth and sulci are not recognized beneath the low density area). The change of density, occurred in 2 cases among 24 cases of type I (8.3%), was recognized about 2 months after the head injury. This fact is compatible with the report by Yamada et al. (1979). Thus, when CT after the head injury shows frontal extracerebral low density area of type I, the case may develop to chronic subdural hematoma about 2 months after the head injury. Therefore, careful observation should be needed especially during this period after the head injury.