There are several findings of interest from this report that will be discussed. First, neither clinical symptoms nor neurological deficits were present in MTBI subjects on day 7 post-injury, regardless of whether they suffered their 1st or 2nd concussive episode. Accordingly, all concussed athletes under study were cleared for sport participation by clinical practitioners based upon neurological assessments as well as clinical symptoms resolution. Second, neuropsychological (NS) deficits including information processing speed, working memory, and scanning ability that were present at day 7 post-injury were resolved by day 14 post-injury. Third, we report a significant reduction of EEG-IQ values in athletes suffering from MTBI. This effect was most pronounced after the second concussion. Moreover, the time between two recurrent concussive episodes appeared to be an important factor influencing the amount of reduction in EEG-IQ values. Fourth, we report a differential rate of recovery- a.k.a. the EEG-IQ changed as a function of testing day and event indicating a better functional outcome after the first compared to the second concussion. Finally, the most pronounced impact of concussion in terms of alterations of EEG-IQ appeared to be at occipital, temporal and parietal ROIs. Overall, the results reported here suggest that EEG-IQ measures may be considered as a possible indicator of residual injury and/or functional brain recovery after MTBI.
There is still ongoing debate in the literature whether MTBI is a temporary functional abnormality in the brain or a long-term structural and functional deficit often overlooked by pure clinical assessment. Conventional wisdom mostly driven by neuropsychological (NS) data seems to suggest that athletes with uncomplicated and single MTBI experience rapid symptoms resolution within 1 to 2 weeks after the incident (Echemendia et al., 2001; Lovell et al., 2003
). The NS data from this study are consistent with this commonly accepted notion demonstrating that neuropsychological signs as well as subjective symptoms were resolved in all subjects within 7 days regardless of the number of brain injuries.
The major results from this report may indicate the utility of the EEG-IQ measure as an indicator of functional brain alteration present beyond 7 days resulting from MTBI. To our knowledge this is the first report in humans demonstrating significant reduction of EEG-IQ measures (e.g., reduced complexity of neurological activity) in subjects suffering from MTBI. It should be noted that this effect was more pronounced when subjects have suffered their second concussion within the same athletic season.
Interestingly, the most significant differences in EEG-IQ prior to and after concussion came from the occipital, temporal and parietal areas. This finding is in agreement with the results of our recent EEG study indicating abnormal features in concussed subjects are concentrated in occipital, temporal and parietal areas (Cao et al., 2008
). Specifically, the non-supervised pattern recognition algorithm, the support vector machine (SVM), has been applied in this study as a tool to identify athletes who suffer from residual functional deficits.
It should be noted that most subjects report concussion injury following impact to the side of their head. A recent report by Delaney et al. (2006)
has also indicated that temporal impact of the head or helmet frequently results in a mechanism producing an MTBI. Biomechanical events set up by the concussive blow (such as amount of head movement about the axis of the neck at the time of impact, the site of impact, etc.) ultimately result in concussion (Shaw, 2002
), and their analysis may contribute to a more accurate assessment of the degree of damage and potential for recovery.
Differential recovery of brain functions after first versus second MTBI as revealed by EEG-IQ values was clearly shown in this report. It is important to note, unlike the cases with a single concussion, that no obvious changes in EEG-IQ were observed within 21 days after second MTBI. These new findings are complementary to our previous observation of subjects with recurrent concussions. Specifically, the rate of recovery of “visual-kinesthetic integration” during dynamic postural tasks was significantly slower after the second concussion episode. Most importantly, unlike the first concussion, the presence of “visual-kinesthetic disintegration” was evident far beyond 10 days post-second concussion (Slobounov et al., 2007
). Collectively, our data support the hypothesis that a history of previous concussions may be associated with slower recovery of neurological function (Guskiewicz et al., 2003
Our current EEG-IQ findings may shed additional light to the ongoing debate in the literature regarding the issue of cumulative effects of concussion. Sporadic evidence and clinical observations suggest that athletes with a history of previous concussions are more likely to have future concussive injuries (Guskiewicz et al., 2003
). Recurrent brain injuries are likely to lead to cumulative neurological and cognitive deficits (Cantu, 2006
). In fact, the cumulative effect in athletes experiencing three or more concussions was documented by the computerized NS test battery ImPact (Iverson et al., (2004
). It should be noted, however, that the cumulative effect of one or two previous concussions was undetected using NS methodology (Iverson et al., 2006
). While direct evidence for the “cumulative effect” hypothesis has not been provided, the patterns of results from our recent studies are consistent with the position that each concussion may potentially cause cumulative brain damage that can be detected using advanced electrophysiological measures of brain function (Gaetz et al., 2000
In conclusion, the major findings from this study provide further evidence that residual brain dysfunction in concussed individuals may be detected in “asymptomatic” subjects via EEG-IQ measures. The current findings further reveal that alteration of brain functions as a result of MTBI may not be detected using conventional assessment tools. Whether this alteration is relatively transient resulting in reallocation of neural processing resources during increased processing load (McAllister et al., 2001
), or a long-term persistent residual brain dysfunction, is yet to be determined.
The clinical implication of our current findings is that the athletes who prematurely return to play based solely on conventional symptoms resolution criteria within 10 days post-injury may be highly susceptible to future and more severe brain injuries (Cantu, 2006
). Therefore, a combination of various assessment methods and techniques should be utilized in a clinical practice in order to make more accurate decision in terms of return-to-play and to indentify athletes at high risk for recurrent concussions (Notebaert and Guskiewicz, 2005
). Symptoms resolution does not equal injury resolution. Additional research is needed to further validate and elaborate on the true clinical meaning of EEG-IQ measures in concussed individuals.