A subdural hematoma is a collection of blood below the dura but above the arachnoid.  The classical source of the bleeding is a tear in the cortical bridging veins that traverse the subarachnoid space as they travel from the cortical surface to the meninges and eventually the dural sinuses.  However, subdural hematomas have been known to arise from intraparenchymal hemorrhages and even aneurysm rupture. 

The most common cause of a subdural hematoma is trauma (rapid acceleration/deceleration coupled with torsion).

There are three distinct classes of subdural hematomas, based on the age of the blood.  The age of the blood is easily differentiated on CT imaging.  The classical appearance of an acute subdural hematoma on CT imaging is a hyperdense, crescent-shaped lesion between the brain and inner table of the skull.  Subdural hematomas do not cross the dural reflections and do cross suture lines (as opposed to epidural hematomas that do cross dural reflections and (typically) do not cross suture lines).

The acute subdural hematoma occurs from the time of injury to seven days after injury.
The subacute subdural hematoma is seen from seven days to 21 days after injury.
The chronic subdural hematoma is seen 21 days or more after injury.

Surgical Guidelines for Acute Subdural Hematoma (1)

INDICATIONS
- Greater than 10mm thick or midline shift greater than 5mm should be surgically evacuated, regardless of GCS score.
- All patients with a GCS score less than 9 should have ICP monitoring.
- A patient with GCS score less than 9, with a hematoma less than 10mm thick and a shift less than 5mm should have surgery if the GCS score between injury and hospital admission decreased by 2 or more points, and/or patient presents with asymmetric or fixed and dilated pupils and/or ICP greater than 20mmHg.

TIMING
- Surgical evacuation should be performed as soon as possible.

PROGNOSIS (2,3)

- Mortality: GCS 3 to 100%
Bilateral, non reactive pupils 88%
Bilateral, non reactive pupils 7-25% functional recovery (GOS scale)
Unilateral, non reactive pupil 48%

References:
1.  Bullock MR, Chesnut R, Ghajar J, Gordon D, Hartl R, Newell DW, Servadei F, Walters BC, Wilberger JE, Surgical Management of Traumatic Brain Injury Author Group. Surgical management of acute subdural hematomas. Neurosurgery 2006 Mar;58(3 Suppl):S2-16-S2-24.
2.  Wilberger JE Jr, Harris M, Diamond DL: Acute subdural hematoma: morbidity and mortality related to timing of operative intervention. J Trauma 1990 Jun; 30(6): 733-6
3.  Phuenpathom N, Choomuang M, Ratanalert S: Outcome and outcome prediction in acute subdural hematoma. Surg Neurol 1993 Jul; 40(1): 22-5