Neuropathologic and Clinical Findings in Young Contact Sport Athletes Exposed to Repetitive Head Impacts

Key Points Question What are the neuropathologic and clinical findings in a convenience sample of young, deceased, symptomatic contact sport athletes? Findings In this case series of 152 contact sport athletes younger than 30 years at the time of death, chronic traumatic encephalopathy (CTE) was found in 63 (41.4%), with nearly all having mild CTE (stages I and II). Neuropathologic abnormalities associated with CTE included ventricular enlargement, cavum septum pellucidum, thalamic notching, and perivascular pigment–laden macrophage deposition in the frontal white matter. Meaning These findings confirm that CTE and other brain pathologies can be found in young, symptomatic contact sport athletes, but the clinical correlates of these pathologic conditions are uncertain.

A definitive diagnosis of CTE requires neuropathologic evidence of perivascular hyperphosphorylated tau (p-tau) aggregates in neurons, with or without astrocytes, typically at the depths of the sulci in the cerebral cortex. 31,32The clinical syndrome associated with CTE is known as traumatic encephalopathy syndrome (TES). 8,33On the basis of the National Institute of Neurological Disorders and Stroke (NINDS) consensus diagnostic criteria for TES, 8 the core clinical features of TES include cognitive impairment, especially episodic memory and executive dysfunction, and neurobehavioral dysregulation, such as impulsivity, explosivity, and emotional dysregulation. 8Supportive features include delayed onset (ie, core clinical features starting years after RHI exposure ends), parkinsonism, other motor signs (including amyotrophic lateral sclerosis), depression, anxiety, apathy, and paranoia.
The Understanding Neurologic Injury and Traumatic Encephalopathy (UNITE) Brain Bank 1 has harvested brains from more than 1350 donors exposed to RHIs who are considered at risk for CTE.Brain donors in the UNITE bank vary widely in age at death and include teenagers and young adults.Chronic traumatic encephalopathy has been reported in individuals as young as 17 years, 13 yet, to date, there have been no large-scale neuropathologic and clinical studies of young individuals exposed to RHIs.Attention to this age group has several important implications.The study of young athletes allows insight into the earliest features of RHI-induced neuropathologic injury and CTE.Furthermore, it allows analysis in the absence of common ageassociated comorbidities.Moreover, most of these young contact sport athletes played only at amateur levels, as part of teams affiliated with educational institutions; consequently, the study of young athletes adds to our understanding of the long-term consequences of amateur contact sports participation.In this report, we describe the neuropathologic and clinical features of 152 brain donors from the UNITE brain bank who were younger than 30 years at the time of death.

Brain Donors and Study Design
The initial sample for this case series included 156 deceased individuals with a history of exposure to RHIs from contact sports participation who donated their brain to the UNITE Brain Bank from February 1, 2008, to September 31, 2022, and were younger than 30 years at the time of death.Race was determined by next-of-kin report and was included to understand the representativeness of the sample and the associated generalizability of the results.Procedures of brain donation have been previously described. 14,34The inclusion criterion was based on the presence of a history of exposure to RHIs without regard to symptom status.The restriction to brain donors younger than 30 years was selected to minimize any contribution from age-related conditions.Donors were excluded for poor tissue quality.Four donors were excluded because of incomplete brain fragments or prolonged premortem hypoxia, resulting in a final sample size of 152.Institutional review board approval for brain donation, postmortem clinical record review, interviews with informants, and neuropathologic evaluation was obtained through the Boston University Medical Campus and the Veterans Affairs Bedford Institutional Review Board.The next of kin or legally authorized representative of each brain donor provided written informed consent.The methods for this report followed the appropriate use and reporting of uncontrolled case series in the medical literature reporting guidelines. 35

Key Points
Question What are the neuropathologic and clinical findings in a convenience sample of young, deceased, symptomatic contact sport athletes?Findings In this case series of 152 contact sport athletes younger than 30 years at the time of death, chronic traumatic encephalopathy (CTE) was found in 63 (41.4%), with nearly all having mild CTE (stages I and II).Neuropathologic abnormalities associated with CTE included ventricular enlargement, cavum septum pellucidum, thalamic notching, and perivascular pigment-laden macrophage deposition in the frontal white matter.
Meaning These findings confirm that CTE and other brain pathologies can be found in young, symptomatic contact sport athletes, but the clinical correlates of these pathologic conditions are uncertain.also evaluated for the presence of vascular pathologic findings, including gross infarcts, microinfarcts, atherosclerosis, and arteriolosclerosis, as well as white matter rarefaction and the presence of perivascular pigment-laden macrophages within the deep cerebral white matter.For pathologic findings rated on a none, mild, or moderate-severe scale (cerebral amyloid angiopathy, white matter rarefaction, atherosclerosis, and arteriolosclerosis), scores were dichotomized as present vs absent.Neuropathologic diagnoses were reviewed by the 4 neuropathologists (A.C.M., B.R.H., V.E.A., and T.D.S.); any discrepancies were resolved by discussion and consensus of the group.

Clinical Evaluation
Retrospective clinical evaluations with next of kin were performed on 143 brain donors using online surveys and/or structured and semistructured postmortem telephone interviews, as described previously. 34Established scales modified for retrospective administration were given to informants of brain donors to assess cognitive symptoms, mood disturbances, and neurobehavioral dysregulation.Scales that assessed cognitive and functional symptoms included BRIEF-A Metacognition Index, Cognitive Difficulties Scale, and the Functional Activities Questionnaire.Scales that assessed neurobehavioral dysregulation included BRIEF-A Behavioral Regulation Index and Barratt Impulsiveness Scale 11.The Apathy Evaluation Scale and Geriatric Depression Scale, 15-item version assess apathy and depression symptoms, respectively.These scales were used as primary outcomes in this study.Further details on clinical protocols are outlined in the eAppendix in Supplement 1. Eighteen brain donors with missing clinical scale data were excluded from clinical analyses.

Statistical Analysis
Statistical analysis was conducted between August 2021 and June 2023.Qualitative assessments of the neuropathologic and clinical features were performed, and descriptive statistics were generated from SPSS software, version 20 (IBM Inc).Brain donors with and without CTE were compared on demographic, athletic, medical, and sport characteristics using independentsample t tests (for continuous outcomes) and the χ 2 test or Fisher exact test (for binary outcomes).Neuropathologic and clinical features were compared between brain donors with and without CTE using analysis of covariance for continuous outcomes and binary logistic regression for binary outcomes.The analyses were controlled for age at death.Data were collected and stored using REDCap, version 8.5.1 (Vanderbilt University).A 2-sided P < .05 was considered statistically significant.
Of the 63 brain donors diagnosed with CTE, 60 (95.2%) were diagnosed with mild (stages I or II) CTE, including 39 (61.9%) with stage I and 21 (33.3%) with stage II.Three (4.8%) were diagnosed with stage III (Table 2, Figure 1, Figure 2, and Figure 3; eTable 1 in Supplement 1).Those with stage III CTE included 1 former National Football League (NFL) player, 1 college football player, and 1 professional rugby player.No brain donors were diagnosed with stage IV CTE.

Football Players
Of the 152 brain donors, 92 (60.5%) played US football as their primary sport, and 48 donors (76.2%) with CTE played football compared with 44 (49.4%) without CTE (P < .001).Although significantly more donors with CTE played football at the professional level (11 [22.9%]) compared with those without CTE (1 [2.3%]) (P < .001),37 (58.7%) of those with CTE played football as their primary sport at the amateur level, with 21 (33.3%)playing in college and 16 (25.4%)never playing after high school.Two individuals (3.2%) who played only youth-level football were diagnosed with CTE; however, both had substantial RHI exposure through nonfootball activities, including military service with blast injury (n = 1) and motocross (n = 1).Eleven of 12 professional football players (91.7%) were diagnosed with CTE, including 11 of 11 NFL players (100%).For those who played football, duration of playing career was significantly longer in those with CTE compared with those without CTE (mean difference, 2.81 years; 95% CI, 1.15-4.48years; P < .001).Playing position did not significantly differ between the groups.

Ice Hockey Players
Of the 16 primary ice hockey players, 6 (37.5%) were diagnosed with CTE.Four amateur ice hockey players were diagnosed with stage I or II CTE; 1 played hockey at the youth level, 2 reached the junior level, and 1 played in college.One of the high school hockey players also played 2 years of college football.One (of 1) National Hockey League (NHL) player had stage II CTE.One (of 1) non-NHL professional hockey player had stage I CTE.sport exposure than the male donors.Their primary sources of exposure to RHIs included college soccer (n = 2), high school soccer (n = 4), high school rugby (n = 1), high school ice hockey (n = 1), professional cycling (n = 1), high school softball (n = 1), and horseback riding (n = 1).The professional cyclist additionally played soccer for 10 years as a winger through high school, and the softball player also played field hockey as a forward through high school.The 1 female player diagnosed with CTE was 28 years old at death and played soccer as a forward for 18 years, beginning at age 3 years and playing through 3 years of Division I collegiate soccer.She was diagnosed with attention-deficit/hyperactivity disorder in college and prescribed stimulants.In addition to 2 concussions without loss of consciousness playing soccer, at age 24 years, she experienced a syncopal episode and a traumatic brain injury with loss of consciousness for 3 minutes.Computed tomographic findings were unremarkable.Four years later, she developed paranoia and suicidal thoughts.At 28 years of age, she died by suicide.Postmortem examination revealed stage I CTE, mild arteriolosclerosis, moderate white matter rarefaction, and marked perivascular pigment-laden macrophages in the frontal white matter (Figure 3).

Amateur Athletes
Overall   1 and Table 2).In cases of CTE, cavum septum pellucidum was often accompanied by thinning and atrophy of the fornices; 1 case had large septal fenestrations (Figure 1).

Microscopic Features
In the 63 brain donors diagnosed with CTE, pathognomonic lesions of CTE consisting of p-tau immunoreactive neurofibrillary tangles (NFTs), pretangles, and dotlike neurites surrounding a central blood vessel were found most frequently at the depths of the sulci in the dorsolateral frontal, superior frontal, and superior temporal cortices, followed by the inferior parietal, inferior frontal, and Rolandic cortices (Table 2 and Figures 2 and 3; eTable 1 in Supplement 1).Neurofibrillary tangles were also common in entorhinal cortex, amygdala, and locus coeruleus in those with stage II or III CTE.Multiplex immunofluorescent labeling of the pathognomonic CTE lesions indicated that the NFT-containing cells colocalized with MAP2, a marker of neurons, and that the predominant p-tau isoform was 4 repeat (4R) p-tau (eFigure in Supplement 1).No p-taucontaining astrocytes were found in the pathognomonic CTE lesions or the surrounding neuropil, and no p-tau thornshaped astrocytes were found in the subpial region of any brain donor.In all brain regions sampled except the mammillary bodies and calcarine cortex, there were significantly more p-tau NFTs in brain donors with CTE compared with those without (eTable 1 in Supplement 1).

Non-p-Tau Pathologic Alterations
White matter rarefaction, evident as reduced density of myelinated nerve fibers, conspicuous reactive astrocytes, and macrophages in the white matter, was found more often in those with CTE than those without CTE, although this finding did not reach significance (Figure 3).Perivascular pigmentladen macrophages in the frontal subcortical white matter were found significantly more often in those with CTE (52 [94.5%] vs 59 [70.3%];P < .001)compared with those without CTE (Figure 3 and Table 2).Among the 92 football players in the sample, years of football play was significantly associated with CTE status (odds ratio, 1.20; 95% CI, 1.07-1.34;P = .002)and perivascular macrophages in the frontal white matter (odds ratio, 1.26; 95% CI, 1.08-1.47;P = .003).Years of play was not associated with ventricular enlargement, cavum septum pellucidum, thalamic notch, or white matter rarefaction.One 28-year-old brain donor with stage II CTE showed mild cerebral amyloid angiopathy in the frontal leptomeninges, as reported previously. 42Neither diffuse nor neuritic plaques were found throughout the sample.One 27-year-old brain donor with CTE stage II had sparse Lewy bodies in the medulla.A semiprofessional soccer player was diagnosed with comorbid stage II CTE and amyotrophic lateral sclerosis. 43

Clinical Features
Across the sample of symptomatic brain donors exposed to RHIs, with and without CTE, cognitive, behavioral, and mood symptoms, as reported by informants using standardized scales, were highly frequent (eTable 2 in Supplement 1).There were no statistically significant differences between donors with a CTE diagnosis compared with those without CTE for any clinical symptom (eTable 2 in Supplement 2).Mood and neurobehavioral dysregulation symptoms were particularly frequent.Clinically meaningful symptoms of neurobehavioral dysregulation, based on the BRIEF-A Behavioral Regulation Index, were present in 50 of 88 (56.8%).Mean scores on the Barratt Impulsiveness Scale 11 were also high (mean [SD], 74.25 [16.40]).Clinically meaningful symptoms of apathy (Apathy Clinically meaningful symptoms of executive dysfunction, based on the BRIEF-A Metacognition Index, were present in 48 of 88 donors (54.5%).Attention and memory symptoms were less frequent.Functional difficulties were not common, as evidenced by the low Functional Activities Questionnaire scores.
Thirty-four of 119 donors (28.6%) sought substance use treatment during life.Alcohol abuse was present in 60 of 140 (42.9%),drug abuse was present in 54 of 141 (38.3%), and ste-roid use was present in 7 of 141 (5.0%).Stimulant use was present in 22 of 127 (17.3%).There were no differences in substance, alcohol, steroid, or stimulant use based on CTE status.

Discussion
Among 152 brain donors exposed to RHIs who donated their brain to the UNITE Brain Bank and were younger than 30 years at the time of death, 63 (41.4%) had autopsy-confirmed CTE, including a female collegiate soccer player (stage I CTE).Chronic traumatic encephalopathy has been previously diagnosed in women who experienced interpersonal violence and fre- quent head-banging [44][45][46] and, most recently, in an exprofessional Australian rules football player.This is, to our knowledge, the first report of CTE in a woman who was an amateur soccer player. 47Nearly all (60 [95.2%]) of the 63 young brain donors with CTE were diagnosed with mild CTE (stages I or II), and 39 (61.9%) were diagnosed with stage I CTE.Young athletes with neuropathologically confirmed CTE (n = 63) included American football players, ice hockey players, soccer and rugby players, amateur wrestlers, military veterans, and a professional wrestler.The study highlights that CTE can affect amateur as well as professional contact sports athletes, with 45 (71.4%) of the diagnosed with CTE playing only as high as the high school or college level.Professional players also develop CTE at a young age.In this cohort of players younger than 30 years, 11 of 11 NFL players and 1 of 1 NHL players were diagnosed with CTE.
Not all individuals exposed to RHIs will develop CTE, and 89 donors (58.6%) in this sample did not have CTE.Despite the narrow age range of the sample, brain donors with CTE were older, were more likely to play American football, had longer duration of football play, and were more likely to play at an elite level, in line with previous studies 15,41 of older players.These findings emphasize the dual roles of age and duration of exposure to RHIs in the development of CTE, even among younger individuals.
In young brain donors, CTE was often accompanied by other pathologic abnormalities.Cavum septum pellucidum, often with degeneration and thinning of the fornices, was present significantly more often in donors with CTE.In addition, there was more enlargement of the frontal horns of the lateral ventricles and notching of the medial thalamus.
Chronic traumatic encephalopathy was evident in young athletes as neuronal p-tau aggregates, including NFTs and dotlike neurites, oriented around a small vessel most often in the superior frontal, dorsolateral frontal, and superior temporal cortices.The predominant p-tau isoform in the CTE lesions was 4R p-tau, consistent with a previous study 48 showing early 4R p-tau predominance and increasing 3-repeat (3R):4R p-tau ratio with increasing severity of CTE.No p-tau astrocytes were found in the parenchyma or subpial region.The lack of p-tau astrocytes was surprising because subpial p-tau thornshaped astrocytes are a frequent finding in older individuals with CTE. 31,32,49,50Their absence indicates that p-tau astrocytes are not an early or essential feature of CTE.The lack of p-tau astrocytes in young donors with CTE also supports the second NINDS consensus conference conclusion that the pathognomonic CTE lesion requires p-tau aggregates in neurons. 32erivascular pigment-laden macrophages in the frontal white matter were significantly greater in CTE and associated with duration of exposure to RHIs.This finding suggests that disruption of the blood-brain barrier is increased after RHIs and might play a critical role in CTE pathogenesis.Previous autopsy studies 51,52 in older individuals with CTE also have shown blood-brain barrier dysfunction and microvascular alterations.The vascular injury-associated markers intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and C-reactive protein were increased in individuals with CTE com-  pared with RHI-exposed and RHI-naive controls. 53In addition, intercellular adhesion molecule 1 and C-reactive protein levels increased with RHI exposure duration and were associated with increased microglial inflammation and p-tau pathologic findings. 53Microglial inflammation correlates with RHI duration and CTE severity, 49 and reactive astrocytosis at the gray-white matter interface has been reported after RHI and in CTE. 54,55Moreover, these findings are in line with dynamic contrast-enhanced MRI studies 56 of living American football players showing persistent blood-brain barrier dysfunction and white matter alterations on diffusion tensor imaging.White matter rarefaction was increased in the deceased young athletes with CTE, albeit of marginal significance.White matter changes are also common in older individuals with autopsy-confirmed CTE. 23,26,27,57White matter rarefaction in older brain donors with CTE is directly associated with RHI exposure, CTE status, and dementia. 23In addition, a single nuclear RNA sequencing study 27 found that the number of oligodendrocytes was reduced and altered in relative subtype proportions in the dorsolateral frontal white matter of older athletes with CTE compared with controls.A recent autopsy study 26 of 205 older male brain donors with and without CTE found decreased myelin-associated proteins in frontal white matter that corresponded to years of exposure and age at first exposure to American football.Using high spatial resolution ex vivo diffusion tensor imaging, researchers have reported alterations in fractional anisotropy in white matter underlying sulci with CTE lesions and microscopic evidence of axonal disruption. 57In addition, in vivo MRI studies show alterations of the corpus callosum in former NFL players on diffusion imaging scans 25 and greater volumes of white matter hyperintensities on antemortem fluid-attenuated inversion recovery MRI 24 and T1 scans. 58][5][6]28,30 Clinical symptoms, as reported retrospectively by next of kin, were common in young donors exposed to RHI with and without CTE.Across the sample, based on modified standardized scales, approximately 50% had clinically meaningful symptoms of executive dysfunction.However, difficulties with instrumental activities of daily living were infrequent.Regarding neuropsychiatric symptoms, nearly 60% had symptoms of behavioral dysregulation as measured by the BRIEF-A Behavioral Regulation Index, and impulse control difficulties were also frequently endorsed.Approximately 70% reported meaningful symptoms of depression and apathy.The fact that there were no significant differences in clinical symptoms between those with CTE and those without may be attributable to selection bias (ie, those who donate are more likely to have symptoms).It may also indicate that retrospective review of symptoms with family members might not be granular enough to detect subtle clinical differences.It further implies that the symptoms are not specific to lowstage CTE.Despite all brain donors being symptomatic, 58.6% of the sample did not have CTE, emphasizing that not all contact sport athletes with symptoms have CTE.Nontau patho-logic findings related to RHIs (eg, white matter rarefaction, perivascular macrophage infiltration, neuroinflammation, and astrocytosis) and unrelated to RHIs (eg, environmental stressors, medical history, genetic factors, and mental health issues) likely contribute to the clinical symptoms.Future studies that compare RHI-naive individuals with those with varying severities of RHI exposure, with and without CTE, are required to determine whether p-tau pathologic findings, microvascular alterations, or white matter loss are associated with cognitive, behavioral, or mood alterations that occur after RHIs and in CTE in young people.
There is a clear need for improved clinical characterization of living athletes across the age spectrum who have been exposed to RHIs through prospective objective assessments.The importance of prospective research studies on young athletes is underscored by the difficulty of assessing and managing symptoms after RHI in living athletes and the substantial contribution of uncertainty to their psychological stress.A recent study 59 reported that one-third of former NFL players are "extremely concerned" about cognitive difficulties and "having CTE."There is also a critical need for symptomatic individuals exposed to RHIs to be medically evaluated and followed up, because their symptoms can often be successfully treated in a clinical care setting.

Limitations
This study has some limitations.We did not evaluate the incidence or prevalence of CTE in the general population or in young contact sport athletes and other individuals exposed to RHIs, and no estimates of incidence or prevalence can be implied or concluded from this study.This study is constrained to brain donors whose families desired a neuropathologic examination after their loved one's death, primarily White male football players.There are limitations of ascertainment bias in studies associated with participation in a brain donation program, and those with symptoms during life, regardless of RHI or CTE status, are more likely to donate, potentially explaining the high rates of symptoms in the overall sample.The study is also limited by the lack of a comparison group that is representative of all young individuals exposed to RHIs and a control group of age-and sex-matched individuals not exposed to RHIs.Notably, such resources (ie, autopsies of young age individuals unexposed to RHIs) are extremely limited, underscoring the novelty and importance of this sample.Future studies comparing RHI-naive with RHI-exposed young brain donors will help isolate the clinical and neuropathologic effects of RHIs independent of CTE.

Conclusion
In a convenience brain bank sample of 152 young athletes exposed to RHI who were younger than 30 years at the time of death, 63 (41.4%) had neuropathologic evidence of CTE, including 1 female athlete.Most young athletes with CTE played at the high school and college levels, and sports included amateur football, ice hockey, rugby and soccer, and wrestling.Young athletes with CTE had significantly more ventricular

Figure 1 .
Figure 1.Gross Neuropathologic Features Associated With Chronic Traumatic Encephalopathy (CTE) in Young Athletes Control aged 27 y A

Figure 2 .
Figure 2. Stages of Chronic Traumatic Encephalopathy (CTE) Severity in Contact Sport Athletes Younger Than 30 Years Stage I CTE with isolated lesions in the depths of the superior frontal sulciA

Figure 3 .F
Figure 3. Microscopic Pathologic Features Associated With Chronic Traumatic Encephalopathy (CTE) in Young Athletes Pathognomonic CTE lesions A

Table 1 .
Demographic and Athletic Characteristics of the Brain Donors a Female Brain DonorsThere were 11 female brain donors (7.2%).Overall, the female donors were younger (mean [SD] age at death, 20.82 [4.49] years; age range, 16-28 years) and had fewer years of contact