There are two hypotheses on how chronic subdural hematoma causes hemiparesis and other focal neurologic symptoms without, in most cases, leaving permanent neurologic deficits. One hypothesis is that the reduced regional cerebral blood flow (rCBF) caused by either compression or distortion of the local or remote brain tissue is responsible for hemiparesis. The other hypothesis is that the reduction in rCBF alone is not sufficient to cause permanent hemiparesis.