What is the defining feature of primary diffuse injury?

The defining feature of primary diffuse injury is indeed widespread axonal injury due to brain movement. This type of injury occurs when there is rapid acceleration and deceleration of the brain within the skull, which can happen during events such as helmet-to-helmet collisions or falls. As the brain moves, it can cause stretching and tearing of axons, leading to diffuse axonal injury (DAI). This form of injury is particularly concerning because it can affect multiple areas of the brain simultaneously and is often not visible on standard imaging, making it harder to diagnose than localized injuries.

Other types of injuries, such as localized injuries, typically occur at the direct site of impact where a force is applied, leading to contusions or lacerations. These injuries tend to be more easily identifiable on imaging studies. Additionally, primary diffuse injury does not solely affect cranial structures; it involves the entire brain's network of axons, affecting communication between different brain regions. Lastly, while diffuse injuries can result from trauma, they are not always the result of a direct blow to the head. Rather, they can occur in scenarios where there is significant movement or impact, regardless of whether there is an apparent direct hit.