Results of this study demonstrate that the K-D test is an accurate and reliable method for identifying athletes with head trauma, and is a strong candidate as a rapid sideline screening test for concussion. High degrees of test-retest reliability were noted for the K-D test in this study. Postfight time scores in this cohort of boxers and MMA fighters were greater (worse) postfight for participants who had head trauma during their match. Changes in scores from prefight to postfight also reflected this pattern and showed more separation between those with and without head trauma than the postfight scores alone. Worsening by 5 seconds or more was a distinguishing characteristic noted only among the participants with head trauma, suggesting that this threshold may be a useful cutoff for exploration in future studies as a criterion for having an athlete stop play pending medical evaluation for concussion.
Closed head injury, even in mild form, is a leading cause of both short-term and long-term cognitive impairment among athletes, particularly for those in contact sports such as football, boxing, soccer, rugby, and hockey.7–30
The annual incidence of sports-related concussion in the United States is 1.6 to 3.8 million, and the likelihood of an athlete in a contact sport experiencing a concussion is as high as 20% per season.21
While the majority of athletes who experience a concussion are expected to recover, the danger is greatly increased by a second concussion.31–33
Having a second concussion before recovering from an initial one can be associated with metabolic brain abnormalities as documented by MRS, electrophysiologic changes, and neuropsychological deficits.21,31–35
In cases of repetitive concussion among boxers, at least 17% develop chronic traumatic encephalopathy (CTE), a condition characterized by long-term cognitive and neurobehavioral difficulties.21,24,26
Symptoms of CTE include chronic headaches, fatigue, sleep difficulties, personality changes, sensitivities to light and noise, dizziness, and short-term memory deficits.7–20
This constellation of symptoms can be disabling for an athlete and can have lifelong implications.9,10,31
The recognition and management of concussion in athletes can be difficult for a number of reasons.21,18–20
1) Athletes who have experienced a concussion present with a wide variety of symptoms.15,17–20
Although the classic symptoms of loss of consciousness, confusion, and memory loss may be present in some athletes with mild concussion, there may or may not be obvious signs that a concussion has occurred. Symptoms of mild concussion are typically quite subtle and may go unnoticed or overlooked by the athlete, team medical staff, or coaches.13,18–20
2) Current management guidelines are not evidence-based and little to no scientific data support the multiple arbitrary systems that are in place to manage concussion.18–20
3) Traditional neurologic and radiologic procedures, such as CT scans, MRI scans, and EEG, although helpful in identifying more serious consequences of head trauma (e.g., skull fracture, hematoma, contusion, seizure), are not useful in identifying many of the effects of concussion.25
Such tests are typically nonspecific or normal, even in athletes sustaining a severe concussion. Thus, while structural neuroimaging techniques are insensitive to the effects of concussion, more advanced fMRI techniques have been able to document damage and to examine the window of metabolic brain vulnerability during which a second concussion could be even more dangerous.7,31–33
4) Most importantly, athletes may not understand the potential consequences of concussion and often minimize or deny symptoms so that they can return to play.13–20
Such underreporting of symptoms is a common practice at all levels of sport participation.
The K-D test is based on measurement of the speed of rapid number naming.36
As such, the K-D test can capture impairment of eye movements, attention, language, and other areas that correlate with suboptimal brain function.21,40
Saccadic and other types of eye movements are frequently abnormal following concussion, and early eye movement function may serve as a predictor of postconcussion syndrome.40
In a recent investigation of 36 patients with postconcussion syndrome vs 36 controls with good recovery following mild closed head injury of similar severity (140–163 days posttrauma),40
patients with postconcussion syndrome had significantly higher numbers of directional errors on antisaccade tasks (p
= 0.006) and memory-guided sequences (p
= 0.002). To the extent that the K-D test captures saccadic eye movements among other important elements of rapid number naming, these data suggest that this quick screening test will be helpful in identifying athletes with signs of concussion. Impaired eye movement function, particularly for those reflecting subcortical pathways, correlated well with severity of postconcussive symptoms and with limitation of activities of daily living. In another study, early eye movement function at 1 week was shown to predict development of postconcussion syndrome.40
Collectively, data on eye movement testing demonstrate a high degree of sensitivity and predictive value as an outcome measure for concussion.40
While the manifestations of postconcussion syndrome are best captured by extensive testing batteries such as ImPACT,37
and may be influenced by intellectual ability or depression, tests of rapid number naming such as K-D represent an objective measure. Because the K-D test captures many aspects of function, including subcortical (subconscious) pathways that extend beyond eye movements, this test may help coaches and trainers with game decisions regarding removal of a player who may have been concussed.
Since our study cohort consisted of boxers and MMA fighters, head trauma that occurred was observable and overt. In other sports such as football, closed head injury and concussion could occur in the context of plays involving many individuals, and head trauma may not be as readily detected. Worsening of K-D scores in this setting might be helpful for determining if a meaningful head blow has taken place.
Another important aspect of evaluating a potential screening test is the correlation of scores with an established battery of tests that measure the construct under study. Ongoing large-scale longitudinal studies of collegiate athletes are examining the relation of K-D test scores and changes in scores over the course of a season with ImPACT and other formal cognitive testing.37,38
In the present study, failing the MACE test (score <25 out of maximum 30 points)38
was associated with worse postfight K-D scores and with greater worsening of K-D scores from prefight to postfight, accounting for age and best prefight K-D scores. While the MACE in our cohort was administered only to participants with head trauma among the boxers, all MMA fighters took the MACE test and none of those without head trauma during their match failed the MACE. Correlations between postfight MACE scores as a continuous variable and K-D scores were also strong, thus providing preliminary evidence that K-D test scores reflect more global aspects of cognitive function in the setting of acute mild closed head injury and concussion.
In order for the K-D test to be used effectively over time and to be administered by different testers, such as athletic trainers, the interrater and test-retest reliability of this measure must be assessed. Results of our study show that K-D test time scores have a high degree of test-retest reliability, with ICCs of 0.97 (95% CI 0.90–1.0) for the 2 prefight measurements. These values indicate that a very high proportion of the variability in the dataset (97%) is due to between-participant differences rather than differences between testing sessions for the same participant. Reliability for K-D measurements prefight vs postfight was also excellent among participants who did not have head trauma during their match (ICC 0.95 [95% CI 0.87–1.0]). This finding suggests that K-D measurements are stable over the short term in the absence of intervening concussion.
The relatively uncommon occurrence of head trauma in our boxing cohort must be viewed in context of the fact that sparring sessions, rather than actual boxing matches, were assessed in this study. Boxers are at high risk for concussion and for more long-term manifestations of CTE, formerly termed dementia pugilistica.21,24–26
Nonetheless, our data indicate that head trauma events can be effectively identified by K-D test time scores, and that analyses of this new measure in future studies can establish its role as a rapid sideline screening test for concussion.
The K-D test, based on measurement of the speed of rapid number naming, can capture impairment of eye movements, attention, language, and other areas that correlate with suboptimal brain function. Data from this investigation demonstrate that the K-D test is an accurate and reliable method for identifying athletes with head trauma. The K-D test represents a strong candidate rapid sideline screening test for concussion, and has particular relevance to contact sports including football, soccer, hockey, MMA, and boxing. Prospective studies of collegiate athletes that are ongoing at the University of Pennsylvania will establish large-scale K-D test norms and expected ranges of precompetition scores for this age group, and will further investigate the capacity for K-D scores to capture closed head injury and concussion.