AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Background

Concussion has many different meanings to patients, families, and physicians.^{1, 2, 3, 4} One definition of concussion is a condition in which there is a traumatically induced alteration in mental status, with or without an associated loss of consciousness (LOC).⁴ A broader definition for concussion is a traumatically induced physiologic disruption in brain function that is manifest by LOC, memory loss, alteration of mental state or personality, or focal neurologic deficits.⁴ Concussions usually result in relatively temporary impairment of neurologic function.^{3, 5, 6}

Concussion or mild traumatic brain injury (MTBI) is common among most contact and collision sports participants.^{4, 7, 8, 9, 10, 11, 12} For many physicians, even those who specialize in MTBI, this area is confusing due to the paucity of scientific evidence to support much of the clinical decision making that is faced in the office.^{1, 2, 9, 13, 14} The inconsiderable amount of good scientific research in the area of MTBI is due to problems with ambiguous definitions of concussion, inconsistent criteria when selecting patients to study, variability of injury mechanisms and locations, and differing means of measuring cognitive function.^{15, 16} The purpose of this article is to review the epidemiology and diagnosis (but not necessarily the classification) of MTBI, as well as the role of imaging studies, issues regarding return to play, and complications surrounding MTBI.

(See also the eMedicine articles Traumatic Brain Injury: Definition, Epidemiology, Pathophysiology [in the Physical Medicine and Rehabilitation section], Head Injury [in the Neurology section], and Head Trauma [in the Pediatrics section], as well as Even Mild TBI Significantly Increases Epilepsy Risk in Children and Young Adults, Nonfatal Traumatic Brain Injuries from Sports and Recreation Activities --- United States, 2001--2005, and Previous Head Injury May Increase Risk for Subsequent Head Injury in Children on Medscape.) 

For excellent patient education resources, visit eMedicine's Back, Ribs, Neck, and Head Center, Brain and Nervous System, and Dementia Center. Also, see eMedicine's patient education articles Concussion, Head Injury, and Dementia in Head Injury.

Frequency

United States

The incidence of head injury varies with the sport and the age of the participants; many head injuries are likely unreported due to their supposed mild nature; mild concussions may go unnoticed by teammates, coaches, and even the athletes themselves.⁴ An athlete's fear of medical disqualification may also lead to underreporting. Studies of high school athletes show the rate of concussions per 1000 exposures as follows: 0.59 for football (boys), 0.25 for wrestling (boys), 0.18 for soccer (boys; 0.23 for girls), 0.09 for field hockey (girls), and 0.11 for basketball (boys; 0.16 for girls).

Among National Collegiate Athletic Association (NCAA) soccer players, the rate of injury has been reported as 0.4-0.6 per 1000 athlete exposures¹¹; 72% of these injuries were described as mild and were almost always secondary to direct contact with an opponent. None of the injuries in this group of Atlantic Coast Conference (ACC) soccer players was noted to be a direct result of heading the ball. In contrast, boxing is the sport with the highest rate of head injuries and has more deaths than any other organized athletic activity. At the professional level, many of the boxing bouts end with a technical knockout (ie, brain injury).

Sports activities that place the athlete at high risk for a head injury include boxing, football, ice hockey, wrestling, rugby, and soccer. Physicians and other allied health providers who are responsible for the medical care of such contact or collision sports participants should be adept at evaluating, treating, and making playability decisions related to the short- and long-term consequences of an injury to the brain.

Sport-Specific Biomechanics

The mechanisms of brain injury may differ among sports activities. Possible mechanisms of injury include compressive forces, which may directly injure the brain at the point of contact (coup); tensile forces produce injury at the point opposite the injury (contrecoup) because the axons and nerves are stretched; finally, rotational forces may result in a shearing of axons. Therefore, the direct force at the point of contact may not be solely responsible for the severity of an injury if a high rotational component with a significant shear effect occurs.

All of the different mechanisms may result in biochemical changes related to perfusion, energy demand, and utilization at the site of injury that are not well understood. At this time, it is unclear whether any experimental animal model or human studies on more severe brain-injured patients accurately reflect the pathophysiology of the typical mild traumatic alteration in brain function.

CLINICAL

Section 3 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

History

Athletes with an MTBI often appear acutely with a confused or blank expression or blunted affect. Delayed response to simple questioning may be demonstrated, along with emotional lability. The emotional lability may become more evident as the athlete attempts to cope with their confusion. Many athletes report an associated headache and dizziness. Visual complaints may include seeing stars, blurry vision, or double vision.

Both pretraumatic (retrograde) amnesia and posttraumatic (antegrade) amnesia may be present. Usually, the duration of retrograde amnesia is quite brief, with a more variable duration of posttraumatic amnesia (seconds to minutes), depending upon the injury.

(See also the eMedicine articles Delirium, Dementia, and Amnesia [in the Emergency Medicine section] and Dementia Due to Head Trauma [in the Psychiatry section].)

A history of persistent vomiting may suggest a significant brain injury with associated elevated intracranial pressure. Other signs of increased intracranial pressure include worsening headache, increasing disorientation, and changing level of consciousness. Possible causes of increasing intracranial pressure include subdural hematomas, epidural hematomas, or some other type of intracranial hemorrhage.

(See also the eMedicine articles Subdural Hematoma and Closed Head Trauma [in the Neurosurgery section], Subdural Hematoma [in the Emergency Medicine section], and Intracranial Hemorrhage and Epidural Hematoma [in the Neurology section], as well as Intracranial Pressure: More Than a Number on Medscape.)

It is important to document a previous history of concussions. Multiple concussions with prolonged neurologic symptoms (eg, headache, hyperacusis, dizziness) suggest postconcussive syndrome and should influence return-to-play decisions.^{7, 8, 12, 17, 18, 19, 20} (See also the eMedicine article Postconcussive Syndrome.)

• Assessment tools
• • The Glasgow Coma Scale (GCS) is routinely used to assess head injuries in an emergency department.
  • • This 15-point scale is used to assess eye (spontaneous opening = 4 to no response = 1), motor (obeys commands = 6 to no response = 1), and verbal responses (oriented = 5 to no response = 1) in an attempt to quantify the patient's level of consciousness.
    • This tool is not sensitive enough to evaluate more mild injuries and should not be used on the playing field to judge playability.
• • McCrea et al developed a sideline evaluation to help the practitioner evaluate the more subtly injured brain.^{16, 21} A 30-point scale is used to assess an athlete's orientation, concentration, immediate memory, and delayed recall. Preseason testing must be done if a practitioner is hoping to use this tool as a supplement to the neurologic and mental status exam; if the baseline status of an individual is not known, assessment for change after a head injury is useless. McCrea's sideline evaluation uses recitation of the months of the year in reverse order after a study by Young et al showed the lack of reliability of the "serial 7s" test (serial subtraction by 7 from 100) in the baseline evaluation of mental status even in non–head-injured athletes.²²
  • Sport Concussion Assessment Tool (SCAT) is another standardized tool. SCAT combines multiple assessments into a single instrument. This combined tool was produced as a part of the Summary and Agreement Statement of the Second International Symposium on Concussion in Sport.²³
• Classification
• • Many different classification schemes have been proposed over the last 2 decades.
  • No one classification system is necessarily better than another classification system.
  • No scientific basis for any of the classification systems exists.
  • Cantu's guidelines,^{12, 24} Ommaya and Gennarelli's guidelines,²⁵ the Colorado guidelines,²⁶ and the 1997 American Academy of Neurology (AAN) guidelines²⁷ were proposed to aid in the evaluation of a concussion. The free CDC Tool Kit on Concussion for High School Coaches is available online in English and Spanish and uses the 1997 AAN guidelines to support a classification scheme.²⁸ The authors prefer to characterize concussions as follows²⁸:
  • • A simple concussion injury progressively resolves after 7-10 days without complication. The key to return to play is to hold the athlete from practice or competition until all symptoms have resolved.
    • A complex concussion consists of persistent symptoms that may include those that recur with exertion, specific sequelae such as seizure associated with the injury, prolonged LOC (>1 min), or prolonged impairment of cognitive function.
  • Some studies have suggested that LOC may not be a great predictor of short-term or long-term neurologic functioning, which makes the guidelines more controversial.^{29, 30}
  • Regardless of the classification scheme that is used, all concur with the ultimate recommendation: Do not allow the concussed athlete to return to play until the patient is completely asymptomatic. The athlete must be free of headache, dizziness, amnesia, blunted affect, and delayed verbal or ocular responses, and all cognitive functioning must have returned to normal.

Physical

• Perform a thorough, organized assessment to better define the degree of injury when a player is brought to the sidelines or emergency department for evaluation.
• The initial evaluation should focus on airway, breathing, and circulation for any unconscious patient. Assume all unconscious or mentally impaired patients have sustained an injury to their cervical spine until proven otherwise.
• For conscious patients, the remainder of the examination should be performed in a quiet place, on the sidelines or in the locker room away from teammates and coaches, or in a private room in an emergency department in order to get an accurate assessment of the cognitive status of the injured athlete.
• The initial clinical examination should include a careful inspection of the athlete's general appearance.
• Palpating the head and neck is important when looking for an associated skull or cervical injury.
• Palpate the facial bones and the periorbital, mandibular, and maxillary areas after any head trauma.

  (See also the eMedicine articles Facial Trauma, Sports-Related Injuries, Facial Trauma, Maxillary and Le Fort Fractures, and Facial Trauma, Management of Panfacial Fractures [in the Plastic Surgery section].)

• Open and close the mouth to help in the evaluation of possible temporomandibular joint (TMJ) pain, malocclusion, or mandible fracture.

  (See also the eMedicine articles Initial Evaluation and Management of Maxillofacial Injuries [in the Trauma section], Mandible, Fractures [in the Radiology section], and Mandibular Body Fractures [in the Otolaryngology and Facial Plastic Surgery section].)

• Inspect the nose for deformity and tenderness, which may indicate a possible nasal fracture.

  (See also the eMedicine articles Nasal and Septal Fractures [in the Otolaryngology and Facial Plastic Surgery section], Nasal Fracture [in the Sports Medicine section], and Facial Trauma, Nasal Fractures [in the Plastic Surgery section].)

• Persistent rhinorrhea or otorrhea (clear) indicates a possible associated skull fracture. (See also the eMedicine articles Skull, Fractures [in the Radiology section] and Skull Fracture [in the Neurosurgery section].)
• Perform a careful detailed neurologic examination to include examinations of the visual fields, extraocular movements, pupillary reflexes, and level of the eyes.
• Assess upper-extremity and lower-extremity strength and sensation.
• Assess coordination and balance. Concussed patients often have difficulty with the finger-nose-finger test and will use slow, purposeful movements to complete the task.
• Significant sway in Romberg testing may indicate persistent injury.
• When examining an athlete on the sideline, perform repeat examinations every 15 minutes until the symptoms have cleared. Repeat the examinations even if the athlete is allowed to return to play.
• The patient should not be allowed to return to competition if his/her symptoms or physical examination findings do not return to normal after 15 minutes. For a few hours after the initial injury, close observation and monitoring of the athlete for worsening mental status or neurologic status is warranted on the sideline or in the emergency department.

Causes

• A previous concussion is a significant risk factor for sustaining a concussion.^{7, 8, 12, 17, 18, 19, 20}
• One study reported that the risk of sustaining a concussion was 4-5 times higher in patients who had at least 1 concussion in the past. Another study reported that athletes with a history of 3 or more previous concussions were 3-fold more likely to have a concussion than players who had no history of concussion.¹⁹
• Other risk factors for sustaining a concussion that have been suggested but not proven include not wearing mouth guards, poor fitting helmets, and genetic predisposition.^{31, 32} Research in all of these areas continues.

DIFFERENTIALS

Section 4 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Other Problems to be Considered

Epidural Hematoma (in the Emergency Medicine section) (See also the eMedicine article Epidural Hematoma [in the Radiology section].)

Intracranial Hemorrhage

Seizure disorders (See also the eMedicine article Seizures and Epilepsy: Overview and Classification.)

Subarachnoid Hemorrhage (in the Neurosurgery section) (See also the eMedicine article Subarachnoid Hemorrhage [in the Emergency Medicine section].)

Subdural Hematoma (in the Neurology section) (See also the eMedicine article Subdural Hematoma [in the Radiology section].)

Trauma-induced migraine

Trauma-induced headache

WORKUP

Section 5 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Imaging Studies

• Computed tomography (CT) scanning
• • In an emergency department-based study, the percentage of abnormal CT scans in adult patients increased from 13% for patients with a perfect GCS score to 37% for those with a GCS score of 13.
  • In a different study that assessed 712 patients with LOC or amnesia and a perfect GCS, the rate of abnormal CT scans was 9%, with less than 1% requiring surgical intervention.³³
  • Indications for ordering a CT scan include focal neurologic examination findings, signs or symptoms of increased intracranial pressure, GCS score less than 15, and seizures related to trauma. Some authors suggest that any athlete with loss LOC (grade 3 concussion) should have a CT scan obtained.³⁴ This area is controversial. Athletes with a brief LOC are at no higher risk for long-term neurologic sequelae, and indications for imaging should not differ from those listed above.
  • CT scanning continues to be the imaging study of choice in evaluating an acute head injury. Better imaging of an acute hemorrhage, speed of the study, and improved ability to monitor the patient are the reasons for using CT scanning rather than magnetic resonance imaging (MRI).
• MRI
• • MRI is the imaging study of choice for patients who have prolonged symptoms (> 7 d) or for a late change in an individual's neurologic signs or symptoms.
  • MRI offers a more detailed examination and possibly detects more subtle findings.
  • Delayed or slowly developing bleeds may be easier to detect on MRI.
• Neuropsychologic testing
• • Detailed neuropsychologic testing is employed more often at the professional level and in research in athletes with MTBI.
  • When evaluating an athlete's performance on the neuropsychologic tests, it is best to compare results with the athlete's previous tests.
  • Both the National Hockey League (NHL) and National Football League (NFL), along with many college teams, are utilizing limited neuropsychologic testing to document the possible prolonged effects of presumed minor head injuries and to assist the clinician in determining possible retirement issues.
  • Neuropsychologic testing is indicated in cases of complex concussions.²³ 
• Although positron emission tomography (PET) scanning and functional MRIs (fMRIs) may be used, their clinical application in most cases of MTBI is uncertain.^{5, 13, 35}

TREATMENT

Section 6 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Acute Phase

Medical Issues/Complications

Most of the complications listed below probably already existed when the athlete sustained the initial head injury; in other words, they are not caused by an MTBI. These conditions may be associated with what was thought of as an MTBI. Therefore, the reader should not think of these conditions as a complication of an MTBI but must consider these other conditions when evaluating an athlete with a head injury.

• A subdural hematoma is a rare injury in the athlete who presents with a presumed concussion. The classic presentation of a subdural hematoma is an acute and persistent LOC associated with the initial injury.
• No association between epidural hematoma and brain injury exists. This condition classically presents with a brief period of unconsciousness, followed by a lucid period, and then a subsequent deterioration over 15-30 minutes. Tearing of the middle meningeal artery secondary to an associated temporal skull fracture is the usual cause of an epidural hematoma.
• Subarachnoid bleeding may also occur with a head injury of any type. Worsening headache and other signs of increasing intracranial pressure will gradually grow after the initial event.
• Second impact syndrome has been described in many review articles. In this condition, fatal brain swelling occurs after minor head trauma in individuals who still have symptoms from a previous minor head trauma. Thus far, all cases of second impact syndrome have been described in relatively young patients (age < 20 y). Significant controversy exists over the etiology of this condition, although it is thought to be secondary to loss of autoregulation of cerebral blood flow in an already injured brain.
• • Authors have questioned the validity of second impact syndrome due to problems with the documentation of the (1) initial event, (2) persistent symptoms, and (3) severity of the second impact. Despite these problems, practitioners should be aware of this possible complication, especially when treating the relatively immature brain of a young athlete. Treatment of second impact syndrome requires immediate recognition and immediate treatment with hyperventilation and osmotic agents. Surgical treatment for this condition is ineffective. The overall prognosis is usually grim.
• Postconcussive syndrome consists of prolonged symptoms that are related to the initial head injury. Unfortunately, the severity of the concussion does not necessarily predict who will experience prolonged symptoms. Similarly, the number of concussions is not necessarily predictive of future problems. Symptoms usually consist of persistent recurrent headaches, dizziness, memory impairment, loss of libido, ataxia, sensitivity to light and noise, concentration and attention problems, depression, and anxiety.
• Most patients with MTBI recover in 48-72 hours, even with detailed neuropsychologic testing, and are headache free within 2-4 weeks of the injury. Obtain a more detailed history of emotional, concentration, and associated symptoms for patients who have persistent symptoms that last longer than 1 week. 
• • A study of retired professional football players (average age 53.8 +/– 13.4 y) by Guskiewicz et al reported significant memory changes in those players with a history of recurrent concussions.⁸ Another report by the same authors of these retired football players suggested a link between recurrent sports-related concussions and an increased risk of clinical depression.¹⁸

Consultations

Consultation with a neurologist or primary care sports medicine physician is indicated for patients who have prolonged symptoms. Neuropsychologic consultation may also be considered to document any deficits that may interfere with the athlete's return to sport, school, or work.

MEDICATION

Section 7 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Overall, no medical therapy is usually prescribed for patients after an acute brain injury. Pain control is usually achieved with over-the-counter medications, such as acetaminophen. Avoid narcotics so that clouding of the patient's mental status or neurologic examination does not occur.

Drug Category: Analgesics

Pain control is essential to quality patient care. Analgesics ensure patient comfort, promote pulmonary toilet, and have sedating properties, which are beneficial for patients who have sustained trauma or those who have sustained injuries.

FOLLOW-UP

Section 8 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Return to Play

Return-to-play criteria are controversial. Similar to classification guidelines, several different guidelines regarding return to play have been established. No scientific evidence exists to justify one criterion versus another criterion. The main criteria for an athlete's return to play include complete clearing of all symptoms, complete return of all memory and concentration, and no symptoms after provocative testing. Provocative testing includes jogging, sprinting, sit-ups, or push-ups—in other words, some type of exercise that raises the athlete's blood pressure and heart rate.
 
The rules are the same for athletes who have a concussion that prohibits return to play during competition. Only after all symptoms have cleared both at rest and with exertion should an athlete even consider returning to practice or competition. In addition, the athlete has to show complete resolution of any emotional lability, mood disturbance, attention, or concentration difficulty. Relatively minor concussions may have more prolonged neurologic deficits. Therefore, the most important aspect of all published guidelines is the concept of an athlete not being allowed to return to play until he/she is completely asymptomatic.

Complications

See Postconcussive Syndrome, above (in the Treatment, Acute Phase, Medical Issues/Complications section).

Prevention

Injury prevention methods are currently being studied. In the past, rule changes that barred spearing in football and teaching football players not to lead with their head have significantly reduced the frequency of severe head injuries in American football.

Equipment and environmental changes can also prevent injury. Soccer goals must be anchored to the ground because many deaths secondary to head injury in soccer have been the direct result of a goal tipping over onto a player.

There is controversy regarding possible helmet wearing in soccer. Although helmets have been shown to clearly reduce the risk of head injury in recreational bicycle riding, no clear evidence exists that the type of headgear proposed for youth soccer will prevent acute or chronic head injury among soccer players.³¹ Long-term studies that examine soccer players over time and that compare the players to themselves in a longitudinal fashion have not been completed. Thus far, studies that suggest long-term damage from heading have been methodologically flawed by comparing soccer players to other athletes, and these studies have not been able to distinguish heading from previous concussions. Most concussions in soccer are the result of direct contact rather than heading of the ball.

Even if helmets are used, no guarantee exists that they will necessarily fit. Studies of football helmet use in high school have demonstrated that only 15% of the helmets fit properly.³² Further documentation of the possible increase in the risk of head injury associated with poor helmet fit has not been completed.

Although mouth guards have been advocated for injury prevention purposes, no controlled study has proven their usefulness in concussion prevention.

Prognosis

Most patients with an MTBI are able to return to full competition without complication. Because many patients may not report minor head injuries to the athletic trainer, emergency department, or a primary care physician, the overall prognosis of many head injuries is unclear.

Chronic postconcussive syndrome can be quite severe, with the most dramatic presentation including dementia pugilistica, which is associated with boxing. This Alzheimer-like condition has a reported incidence of 15% among professional boxers. Fortunately, this condition is rare in most other sports. Hopefully, more frequent, detailed neuropsychologic testing will decrease the frequency of postconcussive syndrome among elite and professional athletes by detecting more subtle injuries earlier.

(See also the eMedicine articles Alzheimer Disease [in the Neurology section] and Alzheimer Disease [in the Radiology section], as well as Hypertension Increases Risk for Mild Cognitive  Impairment and Highlights of the 132nd Annual Meeting of the American Neurological Association on Medscape.)

Education

It is important to educate allied health professionals, coaches, families, and athletes about the recognition and acute management of a concussion, the difficulties involved with a concussion, the difficulty in managing and treating concussions, and the subtle problems with long-term complications. Understanding and recognition of these issues by all of the above may help to prevent recurrent concussion problems. Inexperienced healthcare providers may want to use some type of published guideline when initially managing these injuries.

MISCELLANEOUS

Section 9 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Medical/Legal Pitfalls

• Although many classification systems and guidelines exist for the diagnosis, management, and treatment of sports-related concussions, many physicians do not necessarily follow the guidelines. Whether this lack of compliance is because physicians and caretakers are not aware of the guidelines or because they do not think the guidelines are valid is unclear. The reasons may differ among different types of primary caregivers and specialists. Although the physician who does not follow the stated guidelines may be sued for malpractice, no clear standard of care for concussions exists because of the many different classification systems and guidelines.

REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

1. Slobounov S, Cao C, Sebastianelli W, Slobounov E, Newell K. Residual deficits from concussion as revealed by virtual time-to-contact measures of postural stability. Clin Neurophysiol. Dec 6 2007;epub ahead of print. [Medline].
2. Pearce JM. Observations on concussion: a review. Eur Neurol. Nov 30 2007;59(3-4):113-9. [Medline].
3. Lee LK. Controversies in the sequelae of pediatric mild traumatic brain injury. Pediatr Emerg Care. Aug 2007;23(8):580-3; quiz 584-6. [Medline].
4. Kelly JP, Nichols JS, Filley CM, et al. Concussion in sports. Guidelines for the prevention of catastrophic outcome. JAMA. Nov 27 1991;266(20):2867-9. [Medline].
5. Ptito A, Chen JK, Johnston KM. Contributions of functional magnetic resonance imaging (fMRI) to sport concussion evaluation. NeuroRehabilitation. 2007;22(3):217-27. [Medline].
6. Capruso DX, Levin HS. Cognitive impairment following closed head injury. Neurol Clin. Nov 1992;10(4):879-93. [Medline].
7. Moser RS, Iverson GL, Echemendia RJ, et al. Neuropsychological evaluation in the diagnosis and management of sports-related concussion. Arch Clin Neuropsychol. Dec 2007;22(8):909-16. [Medline].
8. Guskiewicz KM, Marshall SW, Bailes J, et al. Association between recurrent concussion and late-life cognitive impairment in retired professional football players. Neurosurgery. Oct 2005;57(4):719-26; discussion 719-26. [Medline].
9. Powell JW, Barber-Foss KD. Traumatic brain injury in high school athletes. JAMA. Sep 8 1999;282(10):958-63. [Medline]. [Full Text].
10. Pieter W, Zemper ED. Head and neck injuries in young taekwondo athletes. J Sports Med Phys Fitness. Jun 1999;39(2):147-53. [Medline].
11. Boden BP, Kirkendall DT, Garrett WE Jr. Concussion incidence in elite college soccer players. Am J Sports Med. Mar-Apr 1998;26(2):238-41. [Medline].
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13. Henninger N, Sicard KM, Li Z, et al. Differential recovery of behavioral status and brain function assessed with functional magnetic resonance imaging after mild traumatic brain injury in the rat. Crit Care Med. Nov 2007;35(11):2607-14. [Medline].
14. McCrea M, Guskiewicz KM, Marshall SW, et al. Acute effects and recovery time following concussion in collegiate football players: the NCAA Concussion Study. JAMA. Nov 19 2003;290(19):2556-63. [Medline]. [Full Text].
15. Bigler ED. Neuropsychology and clinical neuroscience of persistent post-concussive syndrome. J Int Neuropsychol Soc. Jan 2008;14(1):1-22. [Medline].
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17. De Beaumont L, Lassonde M, Leclerc S, Théoret H. Long-term and cumulative effects of sports concussion on motor cortex inhibition. Neurosurgery. Aug 2007;61(2):329-36; discussion 336-7. [Medline].
18. Guskiewicz KM, Marshall SW, Bailes J, et al. Recurrent concussion and risk of depression in retired professional football players. Med Sci Sports Exerc. Jun 2007;39(6):903-9. [Medline].
19. Guskiewicz KM, McCrea M, Marshall SW, et al. Cumulative effects associated with recurrent concussion in collegiate football players: the NCAA Concussion Study. JAMA. Nov 19 2003;290(19):2549-55. [Medline]. [Full Text].
20. McCrory PR, Berkovic SF. Second impact syndrome. Neurology. Mar 1998;50(3):677-83. [Medline].
21. McCrea M, Kelly JP, Randolph C, et al. Standardized assessment of concussion (SAC): on-site mental status evaluation of the athlete. J Head Trauma Rehabil. Apr 1998;13(2):27-35. [Medline].
22. Young CC, Jacobs BA, Clavette K, Mark DH, Guse CE. Serial sevens: not the most effective test of mental status in high school athletes. Clin J Sport Med. Jul 1997;7(3):196-8. [Medline].
23. McCrory P, Johnston K, Meeuwisse W, et al. Summary and agreement statement of the 2nd International Conference on Concussion in Sport, Prague 2004. Br J Sports Med. Apr 2005;39(4):196-204. [Medline].
24. Cantu RC. Second impact syndrome. Phys Sports Med. 1992;20(9):55-66.
25. Ommaya AK, Gennarelli TA. Cerebral concussion and traumatic unconsciousness. Correlation of experimental and clinical observations of blunt head injuries. Brain. Dec 1974;97(4):633-54. [Medline].
26. Colorado Medical Society School and Sports Medicine Committee. Guidelines for the management of concussion in sports. Colo Med. 1990;87:4.
27. American Academy of Neurology. Practice parameter: the management of concussion in sports (summary statement). Report of the Quality Standards Subcommittee. Neurology. Mar 1997;48(3):581-5. [Medline].
28. National Center for Injury Prevention and Control, Centers for Disease Control and Prevention. CDC tool kit on concussion for high school coaches. February 2005;Accessed December 18, 2007. Available at http://www.cdc.gov/ncipc/tbi/Coaches_Tool_Kit.htm.
29. Sullivan SJ, Schneiders AG, McCrory P, Gray AR. Physiotherapists' use of information in identifying a sports concussion: an extended Delphi approach. Br J Sports Med. Nov 29 2007;epub ahead of print. [Medline].
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Article Last Updated: Dec 18, 2007