A Subdural Hematoma is a collection of blood that pools under the dura. The dura is a relatively tough connective tissue (collagenous) membrane, about the thickness of parchment paper. It is firmly attached to the under surface of the skull, and in the spinal canal, it is separated from the bony structure by a layer of fatty tissue. The inner underside of the dura is applied to a much thinner, transparent membrane, the arachnoid, which overlies the brain and subarachnoid space. This interface is easily separated, forming the subdural space. The subdural space is referred to as a "potential space" because a space is not generally created unless a subdural hematoma or another space occupying mass is formed.

When a subdural hematoma forms, it is generally an indicator of a broken vein on the underlying surface of the brain. Veins draining the surface of the brain pass through the subarachnoid space and then the dura on their way into the sagittal sinus and other intradural venous sinuses that carry the venous blood eventually to the jugular system. If one or more of these veins that "bridge" the dura are injured, bleeding occurs into the subdural "space," causing a subdural hematoma (clot). For whatever reason, most subdural hematomas in "Shaken Baby" cases, as well as nonaccidental trauma cases in infants, are bilateral. An early study by Gulthkelch found that subdural hematomas were bilateral in children 78% of the time. This differs from the rates found in adults. A study by Nashelsky found bilateral subdural hematomas in only 50% of adult cases.

Contra Coup Injuries- Contra Coup injuries occur when there is an injury to the opposite side of the head from the impact site. Contra coup injuries are generally thought to be an indicator of a moving head hitting a stationary, unyielding force or object (Spitz and Fisher III ed.).