An athlete can experience two types of post-traumatic amnesia (PTA) after a concussion: anterograde (reduced ability to form new memories) and/or retrograde (partial or total loss of the ability to recall events before injury).
Anterograde amnesia is characterized by a reduced ability to form new memories after a brain injury, which may lead to decreased attention  and inaccurate perception. Anterograde memory is frequently the last function to return after the recovery from a loss of consciousness (LOC) . Following the recovery of consciousness patients may be unable to recall little or anything that occurred for days, weeks, or even months after their injury.
Retrograde amnesia (RGA) is characterized by the partial or total loss of the ability to recall events that occurred during the period prior to brain injury. RGA may extend backwards for seconds, minutes, hours, days, months or even years depending on the severity of the injury. The symptoms of retrograde amnesia may improve over time (1).
When checking for amnesia on the sports sideline, the Sport Concussion Assessment Tool 3 (SCAT3) (7) - issued in conjunction with the Consensus statement on concussion in sport issued after the 4th International Conference on Concussion in Sport held in Zurich in November 2012 (9) - includes tests for retrograde amnesia (the ability of the athlete to recall past events) by asking an athlete 13-years and older, a series of so-called "Maddocks" questions (At what venue are we today? Which half is it now? Who scored last in this match? What team did you play last week/game? Did your team win the last game?).
When conducting a more thorough examination for anterograde amnesia deficits, the Zurich consensus statement (and the SCAT3 (7) and Child-SCAT3 (8) recommends that the team physician or ATC conduct three cognitive assessment tests:
Whether amnesia predicts a more severe concussion and longer recovery is unclear.
Until the early 2000's, the most widely followed concussion management guidelines  relied heavily on loss of consciousness (LOC) and PTA to determine the severity, or "grade," of a concussion and return to play. Many considered the duration of PTA the best indicator of traumatic brain injury severity and the most dependable factor in predicting outcome, even in mild cases.
Beginning in 2001 with the Vienna international consensus statement on concussion in sport (10), and continuing with the Second Consensus Statement on Concussion In Sport of the 2nd International Conference on Concussion in Sport in Prague in 2004 (11), the trend was away from using PTA in predicting a slower recovery:
By the time the third quadrennial consensus statement on concussion statement was issued in May 2008 (2), however, there was "renewed interest" in the role of anterograde amnesia as a surrogate measure of injury severity. The Zurich statement therefore included anterograde amnesia as a "'modifying' factor " that might predict the potential for prolonged or persistent concussion symptoms and influence concussion investigation and management. Noting that retrograde amnesia "varies with the time of measurement post-injury," the statement continued the expert trend in the 2000s viewing it as "poorly reflective" of concussion severity.
The American Academy of Pediatrics' 2010 clinical report on concussions (5), concurred, stating that, "[a]long with LOC, [anterograde] amnesia may be an important indicator of more serious injury."
A 2011 study (3) by researchers at the University of Pittsburgh, however, found that neither anterograde nor retrograde amnesia were predictive of protracted recovery (21 days or more to return to play), which the authors said may be because they are part of the normal acute response to a concussion and resolve relatively quickly with little lasting effects. Their findings "should be interpreted cautiously", they said, "as previous research had supported retrograde amnesia and PTA as predictors of poor concussion outcomes (although not protracted recovery times per se).
Nevertheless, the consensus statement issued after the fourth quadrennial international conference on concussion in sport (9) issued in March 2013, continues to view amnesia as a "modifying" factor that may predict the potential for prolonged or persistent symptoms, although it noted that, "in some cases, the evidence for their efficacy is limited."
Likewise, the American Academy of Neurology's evidence-based guidelines for the evaluation and management of concussion in sports issued in March 2013 (12) includes "early amnesia" as among the "probable risk factors for persistent neurocognitive problems or prolonged return to play."
Most recently, however, a study  issued in April 2013 (6) found, after analyzing data for numerous variables, including total score of concussed athletes on the Post-Concussion Symptom Scale (PCSS)  at initial visit, age, and amnesia symptoms, that only the total score on the PCSS was independently associated with symptoms lasting longer than 28 days; the higher the score, the greater chance of a prolonged recovery time. Researchers, including lead author and MomsTEAM concussion expert, William P. Meehan, III, M.D. of Children's Hospital Boston, found that, contrary to earlier studies , neither age nor amnesia were risk factors for prolonged concussion recovery.
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12. Giza C, Kutcher J, Ashwal S, et al. Summary of evidence-based guideline update: Evaluation and management of concussion in sports: Report of the Guideline Development Subcommittee of the American Academy of Neurology. Neurology. DOI:10.1212/WNL.0b13e31828d57dd (published online before print March 18, 2013).
Revised and updated April 26, 2013