How Can the Brain Be Injured?
In the United States traumatic brain injury (TBI) is a leading cause of death for persons under age 45. TBI occurs every 15 seconds. Approximately 5 million Americans currently suffer some form of TBI disability. The leading causes of TBI are motorvehicle accidents, falls, and sports injuries. While the brain is by far the most complex object on earth, it is soft and vulnerable with a consistency of firm pudding.
Coup - Contrecoup Injury
Two image illustration showing coup caused by the primary impact
and the secondary impact or contrecoup injury.
A concussion is a sudden trauma-induced alteration of the alert state. The person may be unable to concentrate or be confused for a few seconds, or completely lose consciousness and fall down. The brain is capable of recovering from a concussion. How much force is necessary to cause permanent brain damage is under study, and hence still unclear. Over the years, professional boxers suffer permanent brain damage. The force of a professional boxer's fist is equivalent to being hit with a 13 pound bowling ball traveling 20 miles per hour, about 52 g's. Plopping down into an easy chair can generate up to 10 g's. So, it seems that somewhere between 10 and 50 g's is the threshold to permanent brain injury. This does not mean that accelerations over 50 g's have to cause permanent brain damage. Football players are subjected to 200 g's, and Indy race car drivers have been subjected to 80 g's without permanent injury, but they were wearing helmets.
Football players and race car drivers also protect their heads from being whiplashed. Whiplash seems to be particularly damaging to the brain. Woodpeckers smack their heads against trees with 1200 g's of force without suffering brain damage. Part of the reason is that they keep their heads in the plane of their body; the head does not rotate in a "yes-no" manner during the pecking. If there were some way to stabilize the head when driving - akin to wearing a mail suit from the Middle Ages - more people would walk away from automobile accidents without serious brain injury.
The brain is vulnerable to traumatic damage in two ways. The cerebral cortex can become bruised - contused - when the head strikes a hard object (or a hard objects strikes the head). Or, the deep white matter can suffer diffuse axonal injury when the head is whiplashed without hitting a hard object (or being hit by one). In serious whiplash injuries, the axons are stretched so much that they are damaged.
Cerebral contusions tend to occur at the tips of the frontal and temporal lobes where they bang up against the interior of the skull. Diffuse axonal injury occurs more toward the center of the brain where axons are subjected to maximal stretching.
Any force that penetrates or fractures the skull may cause severe brain injury as destructive shock waves are sent through the brain matter. Displaced fractures of the skull can also push bone into the brain, causing tissue damage.
Direct trauma to the brain can occur when the skull strikes, for example, the floor in a fall accident or strikes a steering wheel in a car accident. Although the skull may not be penetrated or fractured in these types of accidents, the forces imparted to the brain can cause the brain to collide against the inside of the hard skull. When a moving head comes to a quick stop, the brain continues in its movement, striking the interior of the skull. This can cause bruising of the brain (a contusion) and bleeding (hemorrhage). Injury in these types of accidents occurs in parts of the brain closest to the point of impact, quite often the tips of the frontal and temporal lobes. In cases of blunt head trauma the brain can also be injured directly opposite the site of trauma -- on the other side of the brain, an injury known as contrecoup. This injury typically occurs when a moving head strikes a stationary object like the windshield. At impact the brain opposite the site of impact is pulled away from the skull, injuring the brain there.
Medical research has discovered another mechanism of brain injury besides direct blunt trauma to the skull. The well-known phenomenon of the Shaken Baby Syndrome is an example. Severe shaking greatly stretches and damages delicate nerve cells, at times causing very significant injury or even death. In adults, severe whiplash can involve severe forces that may shake or rotate the brain enough to cause permanent brain damage.
Diffuse Axonal Injury
The brain consists of billions of nerve cells located in the gray matter which communicate with distant nerve cells through long nerve fibers called axons, composing the white matter. Severe sudden twisting or torquing of the brain, as occurs in a sudden acceleration/deceleration - whiplash -- accident, can stretch, twist, and damage these delicate axonal fibers. Under the microscope the axonal damage is called Diffuse Axonal Injury (DAI). Although diffuse axonal injury generally results from a severe whiplash injury that renders a patient comatose, recent studies have shown that diffuse axonal injury can also occur - but to a lesser degree -- when there has been only brief loss of consciousness (LOC). Because Diffuse Axonal Injury causes microscopic damage, it cannot be visualized on CT or MRI scans.
Lateral views showing motion of head and neck during whiplash injury.
A second method of how the brain can be injured in high speed velocity change scenarios (a fall from a great height, high speed car accident) is called "Isotropic Stress." Whereas diffuse axonal injury involves the deforming or stretching of the brain tissue, resulting in tearing, isotropic stress causes damage through a "pulse" or "pressure wave" that moves through the brain at extraordinarily high speeds. The damage is caused by a sudden change in the density of the inside of an individual brain cell. The instant compression causes damage to the internal structures of the brain cells.
Many of these same types of injuries have been discovered and treated in veterans returning from the Iraq war. They have often been exposed to the proximity blast of explosive charges. The pressure or pulse from the explosion moves through their body and as it move through the brain it causes damage to the cells. Although many of these soldiers look ?fine? and have no bleeding, they can and will suffer serious brain injury as a result.
Because of the large number of veterans injured in this way, lots of research is being done on this type of brain injury at the present time and there should be studies available for an update on this new insight into TBI in mid-2008.
Hypoxia or Toxic Substance ExposureThe brain is more susceptible to injury through lack of oxygen (hypoxia) than any other part of the body. Hypoxia can occur in conjunction with other injuries (heart attack) or from any other situation where breathing or oxygen intake is impaired. Damage from hypoxia is often seen in the hippocampus, an area of the brain necessary for laying down new memories. Exposure to toxic chemicals (lead, toluene, carbon monoxide, among many others) can also cause brain damage, depending on the level of exposure and the duration of exposure, the combination of which is called the "dose."
Secondary Types of Brain InjuryIn addition to direct neural damage discussed above, injury to the brain can also result as a secondary phenomenon following injury to nonneurologic structures.
Edema - is a swelling of the brain. Swelling of the brain becomes dangerous when the swelling causes a rise in intracranial pressure which prevents blood from entering the skull to deliver glucose and oxygen to the brain. Sustained high intracranial pressure can be relieved through medication, or in more severe cases, by placing a hole in the skull to drain off some of the high-pressure cerebrospinal fluid.
Hematoma - is a collection of blood due to tissue injury or the tearing of a blood vessel. CT scans done at the hospital are particularly effective in detecting brain bleeds. Bleeding into the brain after trauma can occur days after the patient is released from the emergency room. The dura is a tough membrane that covers the entire brain and spinal cord. A blood clot that develops outside the dura, between the skull and dura, is known as an epidural hematoma. A blood clot that develops between the dura and the brain is called a subdural hematoma. Gently resting against the brain itself is a thin, delicate membrane called the arachnoid. Underneath the arachnoid, between the arachnoid and the brain itself, is cerebrospinal fluid bathing and circulating around the brain. Blood leaking into the cerebrospinal fluid is known as a sub-arachnoid hemorrhage.
Hydrocephalus and Hygroma - are collections of fluid in and around the brain. The brain is hollow; the interior cavities, called ventricles, contain cerebrospinal fluid circulating from the ventricles up over the surface of the brain where the cerebrospinal fluid is absorbed. If blood somehow gets into the cerebrospinal fluid and blocks the spinal fluid absorption sites, spinal fluid will back up into the ventricles, enlarging them - a condition called hydrocephalus. If the pressure inside the ventricles becomes excessive (risking damage to the brain), a tube may need to be inserted into the ventricles to relieve the pressure. A hygroma is a localized fluid buildup usually in the subdural space. Again, if pressure in the hygroma presses against the brain, surgery may be necessary to relieve the pressure.
Coronal section through the skull reveals a intracerebral hemorrhage. The central large dark area represents the hemorrhage. Note the midline shift.
Coronal section through the skull and brain reveals a epidural hematoma. The dark area in the lower left area is the hematoma. Note the broken blood vessel and the shift of midline structures.
Coronal section through the skull and brain
reveals a subdural hematoma. The dark area in
the upper left area is the hematoma.
DID BRAIN INJURY OCCUR?
Unfortunately, there are no medically accepted criteria that can predict permanent brain damage from a particular trauma. The factors doctors take into consideration, however, include the following. Keep in mind that normal findings on the tests below do not necessarily mean that no brain injury occurred.
Loss of Consciousness (LOC) - loss of consciousness means loss of conscious awareness. Hence, loss of consciousness can range from being briefly dazed to several days of coma. Focal head trauma - a bullet - can permanently damage an entire cerebral hemisphere without loss of consciousness, but in blunt head trauma loss of consciousness is usually, but not always, necessary to permanently damage the brain. Generally speaking, the longer a period of unconsciousness, the more severe the injury. Medical providers at the scene of an accident tally up a "Glasgow Coma Scale" of the patients neurologic status, which can range from a low of 3 (deeply comatose) to a normal value of 15. The Glasgow Coma Scale is helpful in predicting a patient's ultimate outcome -- the lower the score the worse the outlook.
Post Traumatic Amnesia (PTA) - loss of memory for events prior to the injury (retrograde amnesia) and events following the injury (anterograde amnesia) frequently occur after head injury. In general, a patient with longer periods of post traumatic amnesia tends to have more of a severe injury. Studies have shown that individuals are not good at estimating their own length of amnesia (Gronwall 1980). Therefore, family members should make note of any anterograde or retrograde amnesia and track its improvement.
Concussion - a concussion is an alteration of conscious awareness after head trauma. The collection of symptoms following a concussion is called the postconcussion syndrome (PCS), and include dizziness, nausea, vomiting, headache, disorientation, forgetfulness, irritability, depression, mood swings, insomnia, and loss of libido. Most cases of PCS resolve after a few months, but approximately 20% of cases can involve longer term problems.
Encephalopathy - a disturbance of brain function indicating something is wrong with both sides of the brain's gray matter. Signs of encephalopathy include stupor, confusion, memory loss, inattention, agitation, and inappropriate aggression. An encephalopathy after head trauma only means the brain is not functioning properly. It does not necessarily mean the dysfunction is permanent.
Focal Neurologic Signs - signs that allow a doctor to conclude that a specific part of the brain is not functioning.
Seizure - nerve cells communicate with one another electrically and chemically. One nerve sends an electrical discharge along its axon to stimulate another distant nerve. The actually stimulation is done chemically. When the electrical discharge reaches the end of the axon, the electricity causes the axonal tip to spit a chemical "neurotransmitter" at receptor sites on the next nerve cell. All this takes place in a nice orderly fashion. A "grand mal" seizure occurs when every nerve cell in the brain rapidly fires electrical discharges at one another. The resulting chaos causes the patient to lose consciousness, fall down, and convulse. The same uncontrolled discharges in a focal area of the brain may cause the patient to experience or do what function that focal area normally controls. Such "focal" or "partial" seizures may manifest as recurrent bouts of numbness, fear, anxiety, a forced memory, jerking of a limb or face, lip smacking, sudden staring spells, or inability to speak.
PERL - medical personnel commonly test an individual with a head injury to see if the Pupils are Equal and Reactive to Light (PERL). Unequal pupils or unreactive pupils in a comatose patient after a head injury can signify a dangerous rise in intracranial pressure due to swelling, hematoma, hydrocephalus, etc. Urgent lifesaving surgery is often necessary to relieve the elevated pressure.
There has been a hypothesis that a person struck in the head who suffers facial fractures may have decreased injury to the brain because of the fracture being a "shock absorber" to the brain. However, a recent study (Martin R.C. 2002) showed that the outcome of those with facial fractures verses non-fractures was the same. The presence of fractures of the face does not favor a better outcome.
A study from 2002 (Mosenthal, A.C. 2002) confirmed what was previously believed in regard to the outcome of the elderly with traumatic brain injury. The mortality rate from TBI is higher in the geriatric population at all levels of head injury. There outcome at the time of hospital discharge is worse. This outcome is independent of any other co-factor such as age or other disease.