Chronic subdural hematoma (SDH) starts as a flat blood clot between the dura and the arachnoid membrane. Initially, it is not attached to the dura. Fibroblasts, growing from the dura into the clot, organize it. In 5-6 days, fibroblast growth causes the blood clot to be loosely attached to the dura. In 10-20 days, a loose fibrous membrane is formed between the dura and the clot (outer membrane). Fibrous tissue then grows around the edges of the hematoma and along its inner surface (inner membrane), encapsulating it completely. Maturation of connective tissue results, after several weeks or months, in the formation of a sac with a fibrous wall (chronic SDH). Blood in this sac is absorbed to a variable degree, and the cavity contains clear or hemorrhagic fluid and a loose, vascular connective tissue. Rupture of delicate vessels may cause repeated bleeding in the sac. Fluid may also leak into the cavity from immature capillaries. If a large amount of cerebrospinal fluid enters the subdural space during the traumatic event, it washes off the blood, and no clotting or organization takes place. The histological appearance of the sac is helpful in estimating the duration of the SDH ¹⁾