Lining the inside of the skull is a tough fibrous tissue called Dura. Between the Dura and brain is a space filled with Cerebrospinal Fluid (CSF). Young people have a very thin subdural space, but as we age and the brain shrinks because of loss of neurons and, as a result, this space grows larger. Blood is supplied to the brain by two carotid and two vertebral arteries that travel up the neck and cross this space. Veins bridge this space along the midline of the head and at the temples, to drain eventually into the Jugular Veins on either side of the neck.
When the head is struck by an object (either deceleration or acceleration) the brain moves within the skull and tugs on these bridging veins. A strong enough tug will tear a vein, causing bleeding into the subdural space. This results in a clot which is called a subdural hematoma. Because older people have smaller, atrophic brains, there is more space for the brain to move on impact, even with relatively minor trauma, such as falls. For this reason, subdural hematomas are more common in seniors.
The pressure a subdural hematoma exerts on the brain may cause weakness on the opposite side of the body. However, the mass may not cause enough symptoms for the patient to seek medical evaluation. Over time, the center of the clot liquefies while the outer layers form a tough fibrous capsule of scar tissue and the clot changes from an acute, to sub-acute, to a chronic subdural hematoma.
CT or MRI scans easily show the mass.
The treatment is surgical removal. But this can become problematic in chronic cases where the membranes are well formed and rich in small blood vessels, thus potentially causing more problems than leaving them undisturbed.