Evaluation and management of concussion involves many factors, some of which may predict the potential for prolonged or persistent post-concussion symptoms and long-term health effects.
The current international consensus of concussion experts ("Zurich consensus statement")  is that a range of "'modifying' factors may influence the investigation and management of concussion, and, in some cases, may predict the potential for prolonged or persistent symptoms, and in others suggest consideration be given to retirement from contact or collision sports:
Amnesia (Note that amnesia was not found in two 2013 studies [6,7] to be predictive of longer recovery, adding to an already muddled picture as to the role amnesia plays in predicting concussion recovery)
|Sequelae||Concussive convulsions (Note: it is puzzling that impact seizures are still listed as a modifying factor, as a study published simultaneously with the Zurich consensus statement  catagorically states that "the literature does not suppport the inclusion of impact seizures as a concussion modifer."|
Frequency - repeated concussions over time (as discussed in the literature review accompanying the Zurich statement  @ Table 2, citing studies @ notes 31,103, 34, 17, and 12, "in general, athletes with a previous history of concussion are more likely to present a longer duration of symptoms and are withdrawn from competition for longer following their injury." While multiple concussion history was not found to be predictive of concussion symptoms lasting more than 28 days in one 2013 study,  another 2013 study  found that concussion symptoms lasted twice as long for patients with a history of previous concussion as those without such a history (24 versus 12 days); that the median symptom duration for those with a multiple concussion history and who had sustained a concussion within the past year was 2 and 1/2 times longer (28 days) and nearly three times (33 days) longer respectively, compared with patients without such risk factors.
Timing - injuries close together in time 
"Recency" - recent concussion or traumatic brain injury (TBI)
[Note: timing and recency continue to be included in the list of modifying factors despite the finding in the accompanying literature review  that there is "limited evidence to suggest that the timing or 'recency' makes any difference to timeframe of recovery following a concussion." A subsequent 2013 study, however,  suggests that children and adolescents who have sustained a single concussion within the past year are at even greater risk of prolonged symptoms.
|Threshold||Repeated concussions occurring with progressively less impact force or slower recovery after each successive concussion (Note, once again, the literature review  accompanying the Zurich statement concludes that the evidence to suggest longer timeframes of recovery for each subsequent concussion is "limited.")|
|Age||Children and adolescents (less than 18 years old). "Recovery from concussion in children generally takes longer than that in adults. This is evident in time taken for symptom resolution, as well as neurocognitive recovery."  (Note, however, that two 2013 studies [6,7] did not find age to be predictive of symptoms lasting more than a week or more than 28 days, respectively.) Note: delayed onset of symptoms is more common among younger athletes.|
|Co- and pre-morbidities (pre-existing medical conditions)||Migraine, [11,12] depression or other mental health disorders, attention deficit hyperactivity disorder (ADHD)  learning disabilities, sleep disorders
|Medication||Psychoactive drugs, anticoagulants
|Behavior||Dangerous/more aggressive style of play (may be one reason athletes suffer recurrent concussions )|
|Sport||High risk activity, contact and collision sport, high competitive level|
The presence of any of these modifying factors at the time of injury, says the Zurich consensus statement, should alert the treating practitioner -and parents - to the possibility that the recovery period will be longer than the 7 to 10 day period typical of the majority (80-90%) of concussions, or that the concussed athlete may have a poorer long-term prognosis, although, as the discussion above demonstrates, there are no hard-and-fast rules and concussion management continues to be largely a matter of clinical judgment based on individual assessment.
Because of the different physiological response and longer recovery after concussion and specific risks related to head injuries during childhood and adolescence, these modifying factors likely apply even more to kids than adults and may mandate more cautious return to play advice, the Zurich statement says.
Two 2013 studies, [6,7] however, found that neither age nor LOC was a risk factor for concussion symptoms lasting more than a week and 28 days respectively, and reported different findings on amnesia as a risk factor, the first finding amnesia to be associated with concussive symptoms ≥1 week only in males, but not females, and the second finding no link between amnesia and concussive symptoms lasting more than a month.
The two studies likewise reported inconsistent findings regarding multiple concussion history as a risk factor: the first finding that it doubled the risk for concussive symptoms ≥1 week, but only in football players, and the second finding no link between multiple concussion history and persistent symptoms.
Another 2013 study  found an association between post-traumatic migraines (as opposed to simple headaches, the most common concussion symptom), and cognitive impairments and protracted recovery from concussion.
No wonder, then, that the Zurich consensus statement is careful to qualify its findings about modifying factors, correctly acknowledging that "in some cases, the evidence for their efficacy is limited."
Gender: a factor in concussion management?
The Consensus Statement on Concussion in Sport issued by the 4th International Conference on Concussion in Sport1 held in Zurich in November 2012 "accepted that gender may be a risk factor for injury and/or influence injury severity" but did not list female gender as a possible modifier in the management of concussions because there was no "unanimous agreement that the current published research evidence is conclusive enough."
A literature review9 issued with the Zurich consensus statement noted that, while the data suggested that in sports that use the same rules (soccer3 and basketball15), the reported incidence of concussion is greater in females than their male counterparts, concussion risks between genders were not significant in lacrosse, softball/baseball or gymnastics.15 The study cited five studies [3,16,17,18,19,20] to support its finding that "there apppears to be differences in how women experience concussive symptoms as well as how long symptoms persist compared with men."
A 2010 study in the Journal of Athletic Training,  for instance, contradicted the finding of a 2007 study  that girls take much longer than boys for concussion symptoms to resolve and return to play. It found no gender difference in terms of the time it took for symptoms to clear and for high school girls and boys to return to play.
The 2010 study did show, however, that female high school athletes present with different symptoms from male athletes, and because they are more likely to report drowsiness (a neurobehavioral or "sleep" symptom) and sensitivity to noise (a somatic symptom) than males - symptoms which may be more easily missed on a sideline or initial assessment or attributed to other underlying conditions, such as stress, depression, or anxiety - the authors of the study, including Dawn Comstock, who was one of the authors of the 2007 study, recommend that, where such symptoms are reported, they be viewed as symptoms of concussion until pre-existing neurobehavioral conditions are ruled out on more detailed examination.
A 2012 study  by researchers at Vanderbilt University found no statistically significant differences between tightly matched, homogeneous groups of concussed male and female high school soccer players, either in terms of symptom score reported at baseline or in the first week after concussive injury, or in terms of postconcussion computerized neurocogntive test scores, with the only significant difference between baseline-to-postconcusson being in the total number of symptoms reported.
The only significant gender-related difference identified by the Vanderbilt researchers was that female high school soccer players reported a greater number of post-concussion symptoms. The trend in the research, however, is to focus on total symptom score, not differences in individual symptoms, said Tracey Covassin PhD., ATC, an Associate Professor and Undergraduate Athletic Training Program Director at Michigan State University, and author of four studies on the subject.[15,17,21,22]
The reason, said the Vanderbilt researchers, may have been due to the fact that prior studies, "have not routinely and consistently controlled for the potentially confounding variables of age, years of education, specific sport, number of prior concussions, time of testing postinjury, medical history, and other biopsychological moderating variables (for example, medical history, presence of ADHD and/or learning disability and so on)."
While the authors of Vanderbilt study recognized that the "results of this study could well be interpreted to indicate that there is no gender-based difference in acute response to concussive injury in high school athletes," either in terms of symptoms or neurocognitive scores, they neverless viewed the results as "supportive" of the 2009 Zurich consensus statement's position - carried forward in the 2013 Zurich statement, that gender "may be a risk factor for injury and/or influence injury severity" , at least for athletes in soccer.
A 2012 study  recommends that gender be considered by clinicians when interpreting the results of symptom reports, neurocognitive testing, and postural instability assessments (e.g. BESS) following concussion. Another 2013 study  found that sex was not associated with with prolonged (≥28 days) symptom duration.
Most recently, two studies, published just 10 days apart in November 2013,[22,23] reported gender differences in the way females respond to concussion.
The first , found "partial support" for sex differences in male and female concussed high school soccer players, specifically, that female concussed soccer athletes scored lower on neurocognitive tests of visual memory compared with male concussed soccer athletes at 8 days after concussion, and had a significantly greater number of total postconcussion symptoms than male concussed athletes. The findings were in contrast to those reported in the 2012 Vanderbilt study, which did not report any differences in postconcussion neurocognitive measures between male and female high school soccer players. The only difference was that the new study, authored by Michigan State's Covassin (and bringing to four the number of separate studies she has authored on gender and concussions), controlled for BMI (body mass index).
The second found that women injured during the two weeks leading up to their period (the premenstrual phase) had a slower recovery and poorer health one month after injury compared to women injured during the two weeks directly after their period or women taking birth control pills.
The new studies appear to support gender as a modifying factor in the management of concussion, but it remains to be seen whether the next international consensus statement will classify it as such when it is issued in 2017.
AAN Guidelines differ
Further complicating matters, and illustrating just how little true consensus there is in the evaluation and management of sport-related concussion, the 2013 guidelines issued by the American Academy of Neurology (AAN) depart in a number of respects from the Zurich consensus statement and/or the findings of recent studies. The AAN concludes that:
- there is a "high likelihood" that history of concussion is associated with more severe/longer duration of symptoms and cognitive deficits;
- "probable" risk factors for persistent neurocognitive problems or prolonged return to play include:
- early posttraumatic headache. Headache was not listed as a risk factor in either the Zurich statement  or two 2013 studies [6,7] which is not surprising, since headache is a symptom reported by virtually all (90-95%) of athletes, the vast majority of whom report being symptom free within a month). Interestingly, however, another 2013 study,  which broke down concussed athletes into those experiencing post-traumatic migraine (PTM)(headache, nausea, and sensitivity to light or noise), those experiencing headache without migraine-based symptoms, and those who did not report headache as a symptom at all, found that those experiencing PTM were more than 2 times more likely to experience protracted recovery time (>21 days) than those who experienced headache without migraine-based symptoms and more than 7 times more likely to require 21 days or more recovery compared with those not reporting any headache, findings which the study's authors pointed to PTM as a "strong predictor" of concussion severity.
- early amnesia (a modifying factor under Zurich statement,  but not found to be risk factor for symptoms lasting ≥1 week in one 2013 study  or ≥1 month in another 2013 study;  and
- younger age/level of play (a modifying factor under Zurich (1), but not in either of two 2013 studies. [6,7]
- "possible" risk factors for more prolonged RTP include dizziness (not considered a risk factor under Zurich or in two 2013 studies, [6,7] but viewed as a risk factor in a 2011 study; ) and
- the evidence as to whether female or male sex was a risk factor was (as noted above) "conflicting," so "no conclusion could be drawn."
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Most recently revised and updated November 21, 2013