THE ANATOMY OF A HEAD INJURY

THE ANATOMY OF A HEAD INJURY
Injuries involving some type of blow to the head are among the most common in our society. Some 700,000 people in North America suffer traumatic head injuries each year, and between 70,000 and 90,000 are left permanently disabled. Head injuries can range from relatively minor damage to the scalp and face such as lacerations, abrasions and bruising to more serious consequences involving damage to the brain. While traumatic brain injury occurs much less frequently, it is important to know how it is identified and what to do for the person.

Loss of consciousness, even for a very brief period, is one of the clearest indications that the brain may have been affected by a blow to the head. A confusional state involving uncertainty about time, date, and location and/or a period of memory loss for the events surrounding the head injury are also indicators of trauma to the brain. Any of these symptoms following a blow to the head should be taken seriously.

With the most severe symptoms, loss of consciousness for more than a few minutes, the person should receive immediate medical attention. With less severe symptoms the person should be watched for a period of several hours after the head trauma. The person's state of consciousness, orientation to time and place and immediate memory function (e.g., remembering a series of four numbers) should be evaluated periodically during this time. Any evidence of deterioration may be a sign of the delayed effects of brain injury due to swelling or internal bleeding and require that the person receive medical attention as soon as possible. Some appreciation for how and why these symptoms arise will provide insight into why even a seemingly mild blow to the head may have very serious and potentially life-threatening consequences.

The effects of a blow to the head on brain function arise from the structural characteristics of the skull and the brain and the direction and size of the forces acting on the head. The brain, a rather soft tissue with the consistency somewhere between egg white and jello, is covered by three membrane layers. The outer-most layer, called the dura mater, is connected to the inside of the skull at various suture points which serve to suspend the brain within the skull. The brain sits atop the brain stem, an extension of the spinal cord which passes out the base of the skull through a hole called the foramen magnum. Brain injuries arise from three characteristics of this brain-skull anatomy: the rigidity and internal contours of the skull, the incompressibility of brain tissue and the susceptibility of the brain to shearing forces.

The first two characteristics give rise to contusions or hematomas (i.e., bleeding) on the surface of the brain, one of the most common injuries. There are usually two contusion sites in a brain injury. One occurs at the site of the blow to the brain and is called the coup injury. The other arises where the brain bounces off the skull when it has been moved away from the site of the original blow. The contusion here is termed the contre coup injury. Some bleeding may also arise at the suture points when the dura mater is torn away from the inside of the skull.

The third characteristic, susceptibility to shearing forces, plays a role primarily in injuries which involve rapid and forceful movements of the head, such as in motor vehicle accidents. In these situations rotational forces such as might occur in whiplash-type injuries are particularly important. These forces, associated with the rapid acceleration and deceleration of the head, are smallest at the point of rotation of the brain near the lower end of the brain stem and successively increase at increasing distances from this point. The resulting shearing forces cause different levels in the brain to move relative to one another. This movement produces stretching and tearing of axons (diffuse axonal injury) and the insulating myelin sheath, injuries which are the major cause of loss of consciousness in a head trauma. Small blood vessels are also damaged causing bleeding (petechial hemorrhages) deep within the brain.

Collectively these injuries can result in swelling of the brain. If the pressure within the skull is not relieved through surgery, cooling or medication, the brain will gradually be pushed down through the opening at the base of the skull, the foramen magnum. Nuclei in the brain stem controlling breathing and cardiac function will eventually be compressed resulting in death.

Returning now to the symptoms of head injury a great deal of work has been directed toward using these symptoms to classify the severity of head injury. Loss of consciousness or coma and post traumatic amnesia (difficulty remembering new information after waking up from the coma), are the two most common symptoms used. A mild head injury is one in which the period of unconsciousness is less than twenty minutes and post traumatic amnesia lasts for less than one hour, while a head injury in which the person is unconscious for at least one day and experiences post traumatic amnesia for more than twenty four hours is considered severe.

The severity of head injury can also be indexed on the basis of neurological and neuroradiological tests. Increased reflexes and muscle tone (e.g., spasticity), abnormal movements (e.g., tremors), difficulty in swallowing and slurring of speech are all indicators on a neurological examination of a moderate to severe head injury. Findings from neuroradiological tests using computer assisted brain scans have proven useful in visualizing the damage caused to the brain. Computerized Axial Tomography (CAT) and Magnetic Resonance Imaging (MRI) provide scans of the structural integrity of the brain and may reveal physical changes such as hematomas and diffuse axonal injuries. Positron Emission Tomography examines brain function as opposed to structure and provides a view of more subtle effects of trauma to the brain which might not be seen by the CAT or MRI scans.

Attempts have also been made to predict the outcome for persons who has suffered a head injury and to assess the stages in recovery following their emergence from coma. The Glasgow Coma Scale is one of the most widely used scales for describing the severity of head injury and predicting the person's likelihood of recovery. This scale rates the severity of person's injury based on his/her ability to open his/her eyes, move and speak. The more severe the injury the lower is the performance as reflected in the score on the scale. A very low score suggests a very severe injury and little likelihood of total recovery.

The Glasgow Coma Scale is very useful for predicting early outcome from a head injury, for example, whether the person will survive. It is not as useful for estimating how someone will eventually function in daily living nor what degree of independent living they might achieve. Other scales such as the Ranchos Los Amigos Scale of Cognitive Functioning have proven more valuable for predicting these later outcomes. This scale, divided into eight stages which progress from coma to appropriate behaviour and cognitive functioning, is useful in following the recovery of the head injury survivor and in determining when he/she is ready to begin a structured rehabilitation program. However, many changes in cognitive, memory and motor functions predictive of whether the person can return to gainful employment or to school are not identified with this scale. More detailed assessments by neuropsychologists, speech pathologists, and physical and occupational therapists are needed to identify these deficits.

The effects of head injury most often observed in these assessments can be classified generally into three categories: physical, cognitive and behavioural. The physical effects of head injuries include such symptoms as seizures, loss of motor speed and coordination and the presence of abnormal movement such as tremors and spasticity. Cognitive changes involve disorders of attention, concentration and memory, problems with understanding or producing speech, difficulties with initiating and planning daily activities, and poor reasoning and judgement. The behavioural effects include agitation and irritability, verbal and physical aggressiveness, impulsivity, depression and suicidal thoughts, and an egocentric or self-centred orientation in interpersonal relationships.

While the physical and behavioural effects of head injury present significant challenges for rehabilitation, the cognitive deficits are often the most difficult for the caregivers, family members and prospective employers to deal with. The relative "invisibility" of these deficits in comparison to the more obvious physical and behavioural effects is one of the key reasons for this fact. The relative impact of cognitive deficits is the greatest in the case of mild to moderate head injuries where there may be negligible physical symptoms. In such cases the head injury survivor looks "normal" and people around him/her are often unable to understand why he/she cannot, for example, act appropriately or remember instructions.

Identification of these effects of head injury is a very important first step in helping the person and his/her family. Too often, though, this assessment and early treatment stage is where the process stops. Until recently less than ten percent of head injury survivors received the more long term rehabilitation programs needed to enable them to attain the maximal possible recovery. This situation has been due largely to the lack of long term care programs and the difficulty on the part of the head injury survivor and his/her family to find the funds to pay for these services. In the past several years, however, this situation has improved substantially. The Ministry of Health in Ontario has developed a coordinated network of trauma care centres and several ministries have amalgamated to provide long term care services. Many private companies have also been formed offering a variety of rehabilitation services some of which (community re-entry programs) are aimed at returning the head injury victim to the community. This proliferation of services for head injury victims has been accompanied by new no fault insurance laws which increase the funds available for rehabilitation services for those who have suffered a head injury as a result of a motor vehicle accident.

The Ontario Head Injury Association has worked diligently over the past decade to increase the visibility of head injury victims and their needs. Local chapters such as the Brain Injury Association of Waterloo-Wellington offer a number of educational, recreational and support group programs and have developed a directory outlining the assessment and rehabilitation services available in this region(1).

These developments hold promise for improving the lifestyle and potential for recovery of those who have suffered a brain injury. They have also served to increase the awareness of the general public to effects of head injury which has lead to the development of programs to prevent head injuries in the first place. Prevention addressed through seat belt and helmet laws is clearly the best way to treat head injuries.

http://www.ahs.uwaterloo.ca/~cahr/headfall.html