Using the WB analysis, we detected significant WM changes in the right corona radiata and right inferior longitudinal fasciculus of a single concussed athlete. Other researchers have also found WM abnormalities on DTI in these areas after mild TBI [29
]. Damage in these areas could underlie this athlete’s poor cognitive performance. The inferior longitudinal fasciculus is involved in semantics of language and verbal memory [30
]. The concussed athlete’s performance on verbal memory was relatively poor. However his visual memory performance was even worse. Although damage to corona radiata fibers has not been implicated in decrements of visual memory, strokes involving the inferior longitudinal fasciculus have [32
We also found significant WM changes among six athletes with multiple SHBs. WB analysis allowed us to estimate for each subject the unique spatial location and magnitude of these DTI changes, which were not detected by a group analysis. In fact, we found no significant DTI changes in the five ROIs commonly analyzed in prior studies. In the current study, most of the changes in FA and MD detected in the whole brain analysis were found to lie outside of these five ROIs, in areas such as the posterior thalamic radiata and the cingulum (). We speculate that the location of impact may explain the unique location of injury in each subject. The observed hemispheric asymmetry in changes in FA and MD among all subjects supports this idea. This bootstrap-based analysis, thus, potentially provides unique information to better characterize the pathological changes that occur after concussion and SHBs.
Although WB is a relatively new analytical technique, we believe it detected subject-specific changes in DTI that were real, not spurious. The proportion of significantly changed voxels increased as a function of injury severity, from controls to the athletes with multiple SHBs to the single concussed athlete. Moreover, this incremental increase was strongly related to number of head hits and worsening of post concussive symptoms. Finally, in the concussed athlete voxels with changed FA and voxels with changed MD were found in similar brain locations, while in control subjects they were not. Although we were not able to correlate DTI findings with actual brain tissue histopathology, these observations support the legitimacy of our main results.
The pattern of DTI changes observed after head injury was that of proportionately more voxels with elevated FA and decreased MD. Several authors have found this same pattern of DTI changes[8
], while others have found the reverse pattern[6
]. As the process of axonal injury is a dynamic one, differences in the time between injury and scanning could be one possible reason for this discrepancy. In animal studies, mean diffusion has been observed to decrease immediately after injury, and then gradually rise to normal levels over two to four days, followed by increased levels that last up to six months.[21
] The exact timing of DTI changes among humans is less clear.
The b value used during the DTI scans could be another confounding factor. The larger the b value, the greater the attenuation of the diffusion-weighted signal, and the more sensitive DTI is to slower water diffusion. At relatively low b values such the ones used in two prior studies[8
] as well as in the current study, the contribution of diffusion attenuation comes mainly from the extra-cellular water diffusion [25
]. Axonal injury events outside the axon involve axonal swelling [5
], which reduces the space between axon fibers, decreasing the magnitude of water diffusion (reducing MD) and increasing its linearity (increasing FA). These events would explain the pattern of MD and FA changes we observed. Conversely, at high b values, diffusion attenuation comes mainly from the intra-cellular space. Axonal injury events inside the axon involve disassembly and misalignments of microtubular structures [5
], which might be expected to also decrease the magnitude of water diffusion but reduce, rather than increase, its linearity. Standardization of injury-scan time intervals as well as b values would help resolve some of this controversy.
The six athletes with multiple SHBs had DTI abnormalities that more closely resembled those of the concussed athlete than of the controls. Among four of these athletes (A1, A2, A5 and A6), the total and directional change ratios mirrored that of the concussed athlete. In addition, these changes appeared to correlate with the number of self-reported head blows. These results suggest that WM alterations occur after multiple SHBs and that the magnitude of these changes approaches that seen with frank concussion. If confirmed in larger cohorts, these results would have broad implications for the many youths and young adults who participate in contact sports such as football and hockey. While much effort has been applied toward the prevention of concussions, the need to prevent—or limit—SHBs may also need to be considered. This might represent a challenge because, unlike concussions that are clinically apparent, SHBs are not. Short of prohibiting all head contact, understanding how impact forces lead to changes in WM may best inform preventive efforts. This might be optimally accomplished through the combined use of DTI and helmet impact sensors. Such helmets have already started to shed light on the nature of impact forces encountered during football and hockey.[26
] Examining the relationship between head impact forces and WM changes on DTI might enable the determination of force thresholds above which brain injury is likely to occur.
Our results raise several fundamental questions concerning the nature of SHBs. How long do the DTI changes are associated with SHBs persist? Do they normalize with rest? Do they lower the threshold for developing a clinically evident concussion? What is the relationship of these DTI changes to the risk of second impact syndrome and chronic traumatic encephalopathy? The answers to these questions will help to determine the role of DTI as a complement or replacement to cognitive testing as a guide to the timing of return to contact sports. The poor cognitive performance of the controls in visual motor speed and reaction time ()—possibly due to pain from orthopedic injuries—underscores the limitations of cognitive testing as a measure of brain injury. Future research efforts directed at validating our results and addressing these issues seem warranted.
Several study limitations should be taken into consideration when interpreting our results. The number of included subjects (15) is relatively small. The sample size was determined by available funds, not by recruitment. Based on prior studies, we anticipated that 2–3 of the 10 athletes would suffer a concussion. [27
] In our cohort, however, only one athlete suffered a concussion. Because of this small sample size our results must be considered preliminary.
Symptom validity measures were not employed as a check against athletes intentionally doing poorly on the baseline cognitive test. In schools were cognitive testing plays a key role deciding when an athlete may return to play, athletes have been known to intentionally perform poorly on baseline testing so that post-concussive decrements would not be revealed. If the athletes in our study intentionally performed poorly on the baseline but not the post season test, this could be a possible reason for improved cognitive scores after SHBs. However, at the time of this study, cognitive testing was not in use to make return to play decisions at the involved high schools, making this possibility less likely.
Image mis-registration may have also contributed to the observation that small but significant changes in FA and MD were observed in the controls. After correction of susceptibility artifacts using the field map-based method [28
], linear registration was applied for spatial co-registration. However, residual spatial mismatching may have nonetheless persisted and contributed to false positive errors in the controls. The use of non-linear registration algorithms to improve spatial alignments warrants future study.
The WB-permutation test frame is sensitive to abovementioned residual spatial misalignments. Moreover, because the statistical comparison is performed at each voxel, the level of multiple comparison correction is high, which reduces the statistical power. Fortunately, this limitation serves to underestimate rather than overestimate the number of significant WM voxels changes.