Head Injury
(Brain Injury)

Head injuries are one of the most common causes of death and disability in the United States. Every year, almost a quarter million people are hospitalized because of traumatic injuries to the brain, and 50,000 people die. As well, 80-90,000 people sustain long-term or lifelong disabilities because of a brain injury each year. Children are not excluded, with more than 2,500 deaths and almost a half million emergency department visits per year for head injuries.

Blows to the head most often cause brain injury, it is important to remember that the face and jaw are located in the front of the head. Brain injury may also be associated with injuries to these structures. It is also important to note that a head injury does not always mean that there is also a brain injury.

The brain is a rather soft, pliable material almost jelly-like in feel, and is surrounded by a thin layer of cerebrospinal fluid (CSF). The brain is surrounded by thin layers of tissue called the meninges; 1) the pia mater, 2) the arachnoid mater, and 3) the dura mater. The CSF is present in the space beneath the arachnoid layer called the subarachnoid space.

The dura mater is very thick and has septae, or partitions, that help support the brain in the skull. The septae attach to the inner lining of the bones of the skull. The dura mater also helps support the large veins that return blood from the brain to the heart.

The spaces between the meninges are usually very small but they can fill with blood when trauma occurs, and this build-up of blood can potentially press into the brain tissue and cause damage.

The skull protects the brain from trauma but it does not absorb any of the impact from a blow. Direct blows may cause fractures of the skull; there can be a contusion or bruising and bleeding to the brain tissue directly beneath the injury site. However, the brain can bounce around inside the skull if it is subject to significant force and because of this, the brain injury may not necessarily be located directly below the trauma site. A contre-coup injury describes the situation in which the initial blow causes the brain to bounce and is damaged by hitting the skull directly opposite the trauma site. Acceleration/deceleration and rotation are the common types of forces that can cause injuries away from the area of the skull that received the trauma.

Picture of the brain and potentially brain injury areas

• Epidural hematoma: With an epidural hematoma, the bleeding is located between the dura mater and the skull. This is often present along the side of the head where the middle meningeal artery runs in a groove along the temporal bone. This bone is relatively thin and offers less protection than other parts of the skull. As the bleeding continues, the hematoma or clot expands. There is little space in the skull for the hematoma to grow and as it expands, the adjacent brain tissue is compressed. As the pressure increases, the whole brain begins to shift and becomes compressed against the bones of the skull. The pressure tends to build quickly because the septae that attach the dura to the skull bones create small spaces that trap blood. Symptoms of head injury and decreased level of consciousness occur as the pressure increases.
  
  
• Subdural hematoma: A subdural hematoma is located beneath the dura mater (sub=below), between it and the arachnoid mater. Blood in this space is able to dissipate into a larger space because there are no septae limiting the blood flow. However, after a period of time, the amount of bleeding may cause increased pressure and cause symptoms similar to those seen with an epidural hematoma.
  
  
• Subarachnoid bleed: Subarachnoid bleeding occurs in the space beneath the arachnoid layer where the CSF is located. Often there is intense headache and vomiting with subarachnoid bleeding. Because this space connects with the spinal canal, pressure build-up tends not to occur. However, this injury often occurs in combination with the other types of bleeding in the brain, and the symptoms can be compounded.
  
  
• Intracerebral bleed: Intracerebral bleeding occurs within the brain tissue itself. Sometimes the amount of bleeding is small, but like bruising in any other part of the body, swelling or edema may occur over a period of time, causing a progressive decrease in the level of consciousness and other symptoms of head injury.
  
  
• Sheer injury: Sometimes, the damage is due to sheer injury, where there is no obvious bleeding in the brain, but instead the nerve fibers within the brain are stretched or torn. Another term for this type of injury is diffuse axonal injury.
  
  
• Edema: All injuries to the brain may also cause swelling or edema, no different than the swelling that surrounds a bruise on an arm or leg. However, because the bones of the skull cannot stretch to accommodate the extra volume caused by swelling, the pressure increases inside the skull and causes the brain to compress against the skull.
  
  
• Scull fracture: The bones of the skull are classified as flat bones, meaning that they do not have an inside marrow. It takes a significant amount of force to break the skull, and the skull does not absorb any of that impact. It is often transmitted directly to the brain.
  

Skull fractures are described by which bone is broken, whether there is an associated laceration of the scalp, and whether the bone is depressed and potentially pushed into the brain tissue.

Brain injuries often occur in combination with one another. The effects of brain injury depend upon the amount of brain tissue damaged and the level of pressure within the skull and its effects on the brain.

Source: MedicineNet.com
http://www.medicinenet.com/head_injury/article.htm