During injury the blood vessels around the brain can tear and form a pool of blood between the dura and arachnoid membranes. This is called a subdural hematoma. This pooling of blood around the brain takes up space and causes increased pressure within the skull.
After a period of time the blood in the hematoma becomes coagulated and hardened. This is the chronic subdural hematoma. By the time the thin fibrous neo-membrane has formed over the surface of the hematoma, it has been quite some time since this injury occurred.
When a medical examiner finds both a fresh subdural hematoma and a chronic hematoma, it shows that the child has been repeatedly injured.
When injured the brain can become swollen and fill the sub arachnoid space. The ventricles, normally filled with cerebrospinal fluid can become flattened and compressed.