Concussion

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Concussion, from the Latin concutere (“to shake violently”), is the most common and least serious type of traumatic brain injury. The terms mild brain injury, mild traumatic brain injury (MTBI), mild head injury (MHI), and minor head trauma and concussion may be used interchangeably, although the latter is often treated as a narrower category. The term ‘concussion’, has been used for centuries and is still commonly used in sports medicine, while ‘MTBI’ is a technical term used more commonly nowadays in general medical contexts. Frequently defined as a head injury with a transient loss of brain function, concussion can cause a variety of physical, cognitive, and emotional symptoms.

It is not known whether the concussed brain is structurally damaged the way it is in other types of brain injury (albeit to a lesser extent) or whether concussion mainly entails a loss of function with physiological but not structural changes.^[1] Cellular damage has reportedly been found in concussed brains, but it may have been due to artifacts from the studies. A debate about whether structural damage exists in concussion has raged for centuries and is ongoing.

Common causes include sports injuries, bicycle accidents, auto accidents, and falls; the latter two are the most frequent causes among adults ^[2]. Concussion may be caused by a blow to the head, or by acceleration or deceleration forces without a direct impact. The forces involved disrupt cellular processes in the brain for days or weeks.

Due to factors such as widely varying definitions and possible underreporting of concussion, the rate at which it occurs annually is not known; however it may be more than 6 per 1000 people.

Repeated concussions can cause cumulative brain damage such as dementia pugilistica or severe complications such as second-impact syndrome.

Concussion can be diagnosed and assigned a level of severity based largely on symptoms.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

^[2]

The Hippocratic Corpus, collection of medical works from ancient Greece, mentions concussion, later translated to commotio cerebri, and discusses loss of speech, hearing and sight that can result from “commotion of the brain”.^[1] This idea of disruption of mental function by ‘shaking of the brain’ remained the widely accepted understanding of concussion until the 19th century.^[1] The Persian physician Muhammad ibn Zakarīya Rāzi was the first to write about concussion as distinct from other types of head injury in the 10th century AD.^[2] He may have been the first to use the term “cerebral concussion”, and his definition of the condition, a transient loss of function with no physical damage, set the stage for the medical understanding of the condition for centuries.^[3] In the 13th century, the physician Lanfranc of Milan’s Chiurgia Magna described concussion as brain “commotion”, also recognizing a difference between concussion and other types of traumatic brain injury (though many of his contemporaries did not), and discussing the transience of post-concussion symptoms as a result of temporary loss of function from the injury. In the 14th century, the surgeon Guy de Chauliac pointed out the relatively good prognosis of concussion as compared to more severe types of head trauma such as skull fractures and penetrating head trauma.^[2] In the 16th century, the term “concussion” came into use, and symptoms such as confusion, lethargy, and memory problems were described.^[2] The 16th century physician Ambroise Paré used the term commotio cerebri,^[3] as well as “shaking of the brain”, “commotion”, and “concussion”.

Until the 17th century, concussion was usually described by its clinical features, but after the invention of the microscope, more physicians began exploring underlying physical and structural mechanisms.^[2] However, the prevailing view in the 17th century was that the injury did not result from physical damage, and this view continued to be widely held throughout the 18th century.^[2]

The word “concussion” was used at the time to describe the state of unconsciousness and other functional problems that resulted from the impact, rather than a physiological condition.^[2]

In 1839, Guillaume Dupuytren described brain contusions, which involve many small hemorrhages, as contusio cerebri and showed the difference between unconsciousness associated with damage to the brain parenchyma and that due to concussion, without such injury. In 1941, animal experiments showed that no macroscopic damage occurs in concussion.^[4]

The debate over whether concussion is a functional or structural phenomenon is ongoing.^[2] Structural damage has been found in the mildly traumatically injured brains of animals, but it is not clear whether these changes would be applicable to humans. Such changes in brain structure could be responsible for certain symptoms such as visual disturbances, but other sets of symptoms, especially those of a psychological nature, are more likely to be caused by reversible pathophysiological changes in cellular function that occur after concussion, such as alterations in neurons’ biochemistry.

These reversible changes could also explain why dysfunction is frequently temporary.^[2] A task force of head injury experts called the Concussion In Sport Group met in 2001 and decided that “concussion may result in neuropathological changes but the acute clinical symptoms largely reflect a functional disturbance rather than structural injury.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Concussion falls under the classification of mild traumatic brain injury (MTBI).^[1] It is not clear whether concussion is implied in mild brain injury or mild head injury.^[2] “MTBI” and “concussion” are often treated as synonyms in medical literature. However, other injuries such as intracranial hemorrhages (e.g. intra-axial hematoma, epidural hematoma, and subdural hematoma) are not necessarily precluded in MTBI or mild head injury,^[3][4] but they are in concussion.^[5] MTBI associated with abnormal neuroimaging may be considered “complicated MTBI”. “Concussion” can be considered to imply a state in which brain function is temporarily impaired and “MTBI” to imply a pathophysiological state, but in practice few researchers and clinicians distinguish between the terms. Descriptions of the condition, including the severity and the area of the brain affected, are now used more often than “concussion” in clinical neurology.^[6]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

In both animals and humans, MTBI can alter the brain’s physiology for hours to weeks, setting into motion a variety of pathological events.^[1] Though these events are thought to interfere with neuronal and brain function, the metabolic processes that follow concussion are reversed in a large majority of affected brain cells; however a few cells may die after the injury.

Included in the cascade of events unleashed in the brain by concussion is impaired neurotransmission, loss of regulation of ions, deregulation of energy use and cellular metabolism, and a reduction in cerebral blood flow. Excitatory neurotransmitters, chemicals such as glutamate that serve to stimulate nerve cells, are released in excessive amounts as the result of the injury.The resulting cellular excitation causes neurons to fire excessively. Giza CC, Hovda DA (2001). “The neurometabolic cascade of concussion”. Journal of Athletic Training. 36 (3): 228–235. PMID 12937489. </ref> This creates an imbalance of ions such as potassium and calcium across the cell membranes of neurons (a process like excitotoxicity). Since the neuron firing involves a net influx of positively charged ions into the cell, the ionic imbalance causes cells to have a more positive membrane potential (i.e. it leads to neuronal depolarization). This depolarization in turn causes ion pumps that serve to restore resting potential within cells to work more than they normally do. This increased need for energy leads cells to require greater-than-usual amounts of glucose, which is made into ATP, an important source of energy for cells. The brain may stay in this state of hypermetabolism for days or weeks. At the same time, cerebral blood flow is relatively reduced for unknown reasons, though the reduction in blood flow is not as severe as it is in ischemia. Thus cells get less glucose than they normally do, which causes an “energy crisis”.

Concurrently with these processes, the activity of mitochondria may be reduced, which causes cells to rely on anaerobic metabolism to produce energy, which increases levels of the byproduct lactate.

For a period of minutes to days after a concussion, the brain is especially vulnerable to changes in intracranial pressure, blood flow, and anoxia. According to studies performed on animals, large numbers of neurons can die during this period in response to slight, normally innocuous changes in blood flow.

Concussion involves diffuse brain injury (as opposed to focal brain injury), meaning that the dysfunction occurs over a widespread area of the brain rather than in a particular spot. Hardman JM, Manoukian A (2002). “Pathology of head trauma”. Neuroimaging Clinics of North America. 12 (2): 175–187, vii. doi:10.1016/S1052-5149(02)00009-6. PMID 12391630. </ref> Concussion is thought to be a milder type of diffuse axonal injury because axons may be injured to a minor extent due to stretching. Animal studies in which primates were concussed have revealed damage to brain tissues such as small petechial hemorrhages and axonal injury. Hall RC, Hall RC, Chapman MJ (2005). “Definition, diagnosis, and forensic implications of postconcussional syndrome”. Psychosomatics. 46 (3): 195–202. doi:10.1176/appi.psy.46.3.195. PMID 15883140.CS1 maint: Multiple names: authors list (link) </ref> Axonal damage has been found in the brains of concussion sufferers who died from other causes, but inadequate blood flow to the brain due to other injuries may have contributed to the damage.^[2]

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The leading causes of MTBI in adults are falls and vehicle accidents, and other causes include sports injuries, assaults, industrial and work-related injuries, and injuries that occur in the home. Among children aged 5 to 14, sports and bicycle accidents cause the greatest number of concussions. Soldiers are at elevated risk for concussion from causes such as bomb blasts, with as many as 15% of U.S. infantry soldiers who return from the Iraq War meeting the criteria for MTBI. The relative contribution of causes of mild head injury differs by region, gender, and age. For example, in Scotland and Sweden, falls account for the greatest percentage of MHIs, while in the U.S. and Australia, transportation is the largest cause.

The brain is surrounded by cerebrospinal fluid, one of the functions of which is to protect it from light trauma, but more severe impacts or the forces associated with rapid acceleration and deceleration may not be absorbed by this cushion. Concussion may be caused by impact forces, in which the head strikes or is struck by something, or impulsive forces, in which the head moves without itself being subject to blunt trauma (for example, when the chest hits something and the head snaps forward).

Forces may cause linear, rotational, or angular movement of the brain, or a combination of these types of motion. In rotational movement, the head turns around its center of gravity, and in angular movement it turns on an axis other than its center of gravity. The amount of rotational force is thought to be the major type of force to cause concussion and the largest component in its severity. Studies with athletes have shown that the amount of force and the location of the impact are not necessarily correlated to the severity of the concussion or its symptoms, and have called into question the threshold for concussion previously thought to exist at around 70-75 g.

The parts of the brain most affected by rotational forces are the midbrain and diencephalon. It is thought that the forces from the injury disrupt the normal cellular activities in the reticular activating system located in these areas, and that this disruption produces the loss of consciousness often seen in concussion. Other areas of the brain that may be affected include the upper part of the brain stem, the fornix, the corpus callosum, the temporal lobe, and the frontal lobe.^[1]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Diagnosis of concussion can be complicated because it shares symptoms with other conditions. For example, post-concussion symptoms such as cognitive problems may be misattributed to brain injury when they are in fact due to post-traumatic stress disorder (PTSD).^[1]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Most cases of traumatic brain injury are concussions. A World Health Organization (WHO) study estimated that between 70 and 90% of head injuries that receive treatment are mild.^[1] However, due to underreporting and to the widely varying definitions of concussion and MTBI, it is difficult to estimate how common the condition is.^[2] Estimates of the incidence of concussion may be artificially low, for example due to underreporting. At least 25% of MTBI sufferers fail to get assessed by a medical professional. The WHO group reviewed studies on the epidemiology of MTBI and found a hospital treatment rate of 1–3 per 1000 people, but since not all concussions are treated in hospitals, they estimated that the rate per year in the general population is over 6 per 1000 people.^[1]

Young children have the highest concussion rate among all age groups. A Canadian study found that the yearly incidence of MTBI is lower in older age groups (graph at right). Studies suggest males suffer MTBI at about twice the rate of their female counterparts.^[1] However, female athletes may be at a higher risk for suffering concussion than their male counterparts.^[3]

Up to five percent of sports injuries are concussions.^[4] The U.S. Centers for Disease Control and Prevention estimates that 300,000 sports-related concussions occur yearly in the U.S., but that number includes only athletes who lost consciousness. Since loss of consciousness is thought to occur in less than 10% of concussions,the CDC estimate is likely lower than the real number.^[5] Sports in which concussion is particularly common include football and boxing (a boxer aims to “knock out”, i.e. give a mild traumatic brain injury to, the opponent). The injury is so common in the latter that several medical groups have called for a ban on the sport, including the American Academy of Neurology, the World Medical Association, and the medical associations of the UK, the U.S., Australia, and Canada.^[6]

Due to the lack of a consistent definition, the economic costs of MTBI are not known, but they are estimated to be very high.^[7] These high costs are due in part to the large percentage of hospital admissions for head injury that are due to mild head trauma, but indirect costs such as lost work time and early retirement account for the bulk of the costs.^[7] These direct and indirect costs cause the expense of mild brain trauma to rival that of moderate and severe head injuries.^[8]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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MTBI has a mortality rate of almost zero. The symptoms of most concussions resolve within weeks, but problems may persist. It is not common for problems to be permanent, and outcome is usually excellent.^[1] People over age 55 may take longer to heal from MTBI or may heal incompletely.^[2] Similarly, factors such as a previous head injury or a coexisting medical condition have been found to predict longer-lasting post-concussion symptoms. Other factors that may lengthen recovery time after MTBI include psychological problems such as substance abuse or clinical depression, poor health before the injury or additional injuries sustained during it, and life stress.^[1] Longer periods of amnesia or loss of consciousness immediately after the injury may indicate longer recovery times from residual symptoms. For unknown reasons, having had one concussion significantly increases a person’s risk of having another. The prognosis may differ between concussed adults and children. Little research has been done on concussion in the pediatric population, but concern exists that severe concussions could interfere with brain development in children.

In post-concussion syndrome, symptoms do not resolve for weeks, months, or years after a concussion, and may occasionally be permanent.^[3] Symptoms may include headaches, dizziness, fatigue, anxiety, memory and attention problems, sleep problems, and irritability.^[4] There is no scientifically established treatment and rest, a recommended recovery technique, has limited effectiveness. Symptoms usually go away on their own within months. The question of whether the syndrome is the result of structural damage or other factors such as psychological ones, or a combination of these, has long been the subject of debate.^[5]

Cumulative effects of concussions are poorly understood. The severity of concussions and their symptoms may worsen with successive injuries, even if a subsequent injury occurs months or years after an initial one.^[6] Symptoms may be more severe and changes in neurophysiology can occur with the third and subsequent concussions. Studies have had conflicting findings on whether athletes have longer recovery times after repeat concussions and whether cumulative effects such as impairment in cognition and memory occur.^[7]

Cumulative effects may include psychiatric disorders and loss of long-term memory. For example, the risk of developing clinical depression has been found to be significantly greater for retired football players with a history of three or more concussions than for those with no concussion history.^[8] Three or more concussions is also associated with a five-fold greater chance of developing Alzheimer’s disease earlier and a three-fold greater chance of developing memory deficits.^[8]

Second-impact syndrome, in which the brain swells dangerously after a minor blow, may occur in very rare cases. The condition may develop in people who receive a second blow days or weeks after an initial concussion, before its symptoms have gone away. No one is certain of the cause of this often fatal complication, but it is commonly thought that the swelling occurs because the brain’s arterioles lose the ability to regulate their diameter, causing a loss of control over cerebral blood flow. As the brain swells, intracranial pressure rapidly rises. The brain can herniate, and the brain stem can fail within five minutes. Except in boxing, all cases have occurred in athletes under age 20. Due to the very small number of documented cases, the diagnosis is controversial, and doubt exists about its validity.^[9]

Chronic encephalopathy is an example of the cumulative damage that can occur as the result of multiple concussions or less severe blows to the head. The condition called dementia pugilistica, or “punch drunk” syndrome, which is associated with boxers, can result in cognitive and physical deficits such as parkinsonism, speech and memory problems, slowed mental processing, tremor, and inappropriate behavior.^[10] It shares features with Alzheimer’s disease.^[11]