The Origin of Chronic Subdural Hematoma considered on the Basis of Hematoma Membrane Findings and Contained Fluid Findings

The authors divided chronic subdural hematoma into 5 stages, from occurrence to disappearance, mainly according to computerized tomography (CT) findings. The origin was determined on the basis of the findings from the hematoma membrane and the contained fluid obtained at the surgery in each stage. The initial stage (about 7-14 days after head injury) was characterized by the presence of communication between the subdural low density area and the subarachnoid space, shown by the CT scan. In this stage, a thin fibroblastic membrane was recognized beneath the dura mater. The contained fluid consisted of cerebrospinal fluid and a minimal amount of blood. The premature stage (about 15-30 days after head injury) was characterized by insufficient communication between the subdural low density area and the subarachnoid space in the CT scan. In this stage, the subdural membrane was thicker and consisted of fibroblastic cells, capillaries, and infiltrating cells. The contained fluid was rich in protein and showed increased plasmin activity. In the mature stage (about 30-60 days after head injury), CT change into iso- or high-density without mass sign was noted. In this stage, the subdural membrane consisted of fibroblastic cells, well-developed capillaries, and many infiltrating cells. Intramembranous bleeding was seen. Moreover, the contained fluid was very similar to fresh blood, and increased in fibrin degradation products. The progressive stage showed the typical chronic subdural hematoma with mass sign. The subdural membrane and the contained fluid were typical, as reported previously by other authors. In the resolving stage, the hematoma was relatively low in CT density and gradually disappearing. Mass sign was not found. In this stage, the subdural membrane became fibrous and consisted of well-developed collagen fibers. From these findings, the authors postulated the following: The hematoma begins as a subdural fluid collection which is a mixture of a minimal amount of blood and cerebrospinal fluid. The fluid collection produces an active fibroblastic membrane beneath the dura mater (initial and premature stages). Immature capillaries in the membrane are made to bleed by physical factors, such as variation of the intracranial pressure. The intramembranous bleeding breaks the membrane to form the subdural hematoma without mass sign yet (mature stage). The subdural hematoma is expanded by repeated bleeding from the membrane (perhaps due to topical fibrinolysis,