A traumatic brain injury is an injury to the brain caused by trauma. Trauma is the most common cause of brain injury in the United States. In the U.S. an estimated 1.5 to 2 million people suffer traumatic brain injury each year. Approximately 70,000 to 90,000 of those individuals incur a TBI resulting in a long-term, substantial loss of functioning. Additionally, there are approximately 300,000 hospital admissions annually for persons with mild or moderate TBI, and unknown number of traumatic brain injuries which are not diagnosed but may result in long-term disability.
Traumatic injuries to the brain are divided into two categories: open head injuries and closed head injuries. The most frequent type of traumatically acquired brain injury is the closed head injury. A closed head injury is defined as an injury to the brain without penetration or breach of the skull.
Medical professionals generally classify brain injuries as either mild, moderate or severe. These classifications may be misleading because they are based on an initial assessment of the potential for the injury to result in death of the injured person, not the long term consequences of the injury to the individual. The Glascow Coma Scale (GCS) was developed to enable medical professionals to quantify brain injury in acute trauma patients. The scale is based on a separate assessment of eye, verbal and motor responsiveness. The GCS generally provides a good indicator of long term prognosis, particularly in cases of severe brain injury.
A “mild” brain injury is defined as an injury resulting in unconsciousness of less than 30 minutes or an initial Glascow Coma Scale (GCS) of 13-15 (15 being “normal”). It includes an injury that causes the injured person to become dazed or disoriented but not to totally lose consciousness. It is now recognized that an individual may suffer brain injury resulting in long term cognitive deficits without loss of consciousness. A “moderate” brain injury is one resulting in unconsciousness lasting from 30 minutes to 6 hours or a GCS 9-12. If the initial GCS is less than 9 or the period of unconsciousness is greater than 6 hours the injury is classified as “severe”.
It is essential to note that while these general classifications of brain injury provide some indication of the patient’s ultimate prognosis, they do not reflect the severity of the post concussive symptoms a patient may experience, nor do they reflect the extent to which the injury may eventually disable the patient. Patients with “mild” brain injuries may be so severely debilitated by the injury that they are unable to return to any gainful employment. Conversely, patients with a “moderate” or even “severe” brain injury may recover sufficient mental functioning to return to employment and relatively normal, productive lives. Many factors, such as educational level, coping skills, employment skills, family support, and the presence of other disabling injuries will contribute to the final outcome.
Mechanism of Injury: How a TBI occurs
1. Impact with Skull
The consistency of the brain is often compared to that of Jell-O. It sits within the skull surrounded by cerebral spinal fluid which circulates over the brain and spinal cord and cushions the brain from shock. The inside of the skull is not smooth. There are many bony prominences or ridges which the brain rests upon, particularly in the lower half of the skull.
Several basic principles of physics explain how the brain is injured when rapidly accelerated or decelerated. One such principle, is Newton’s First Law of Motion, which states that once an object is in motion, it tends to remain in motion at a constant velocity until acted upon by sufficient force in the opposite direction to stop it. When the head stops moving suddenly, such as when it strikes an object within the interior of an automobile, the brain continues to move within the skull at the original velocity of the car until it strikes the inside of the skull. Because of its soft consistency, the impact of the brain against the skull and the bony ridges within the skull, causes bruising and microscopic bleeding within the brain tissue.
If the bleeding is severe, such as rupture of a blood vessel, it may require surgical intervention or evacuation. These injuries are known as mass lesions. The brain is surrounded by the dura, a tough, leathery membrane. A epidural hematoma is a lesion that occurs between the dura and the skull. A subdural hematoma is a lesion between the dura and the brain.
Cavitation is the formation of microscopic bubbles within brain tissue as it is pulled away from the skull when the head suddenly stops or accelerates. Cavitation occurs when an object moves rapidly through a liquid, such as when the brain moves through cerebral spinal fluid. The formation and collapse of these bubbles causes disruption of brain tissue. Cavitation injuries occur on the opposite side of the brain from the point of impact. They are sometimes referred to as contrecoup injuries.
3. Rotational Acceleration
The brain is composed of billions of nerve cells called neurons. A neuron is a specialized cell which conducts electrochemical impulses. A neuron consists of a cell body and cell extensions called processes. There are two types of processes: long single processes known as axons and short, branching processes known as dendrites. An axon can be up to three feet in length. Some axons are covered by a white fatty substance called myelin. The surface of the cerebral cortex appears gray, because the nerve cell bodies are not covered with myelin (grey matter). The brain is made of several layers or meninges. In the lower areas of the brain the nerve cells are covered by myelin (white matter). These layers are of varying consistency or viscosity. When the head stops suddenly the brain rotates on the brain stem where the stem exits the skull in a forward and downward motion. The layers of the brain farthest from the brain stem move faster and farther than the layers which are closest. As the brain moves the layers stretch and pull at different rates.
Diffuse axonal injury occurs when the layers of the brain slide over one another causing the axons to be stretched, torn and twisted. This may damage the myelin covering, which in turn affects electrochemical impulse transmission. Nerve impulses are transmitted from one nerve cell to another by electrochemical transmissions across synapses at the end of the axons. If the myelin is sufficiently damaged the nerve impulse is not transmitted to the adjacent neuron. This causes a loss of brain function. It is important to note that when the body of a neuron is sufficiently damaged, the cell will die. Unlike many types of cells within the body, neurons do not regenerate. Brain damage resulting from the destruction of large numbers of neurons is permanent. It is now thought that most of the loss of brain function following acceleration/deceleration trauma is the result of diffuse axonal injury.
In extreme cases of diffuse axonal injury the grey matter is virtually disconnected from the white matter in parts of the brain. This disconnection causes the victim to remain completely unresponsive in a persistent vegetative state. A victim may remain in such a state for many years without ever regaining consciousness.
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