A significant risk of neurodegenerative disease was found in an analysis of male former professional soccer players, with defensive outfield players and those with careers longer than 15 years particularly at risk.
Male former professional soccer players may be at significant risk of developing neurodegenerative diseases, particularly those who played defense and/or had careers longer than 15 years, according to study findings published yesterday in JAMA Neurology.
Compared with the general population, prior research suggests that former professional contact sports athletes are at greater mortality risk from dementia and other neurodegenerative diseases, such as motor neuron disease and Parkinson disease (PD).
As the world’s most popular participation sport with more than 250 million active participants, soccer is not typically associated with traumatic brain injury (TBI), a known risk factor of neurodegenerative disease pathogenesis. Although, the sport has been indicated to increase risk through training and gameplay, in which constant exposure to repetitive head impacts through soccer ball heading may lead to TBI and potentially specific changes in the neurodegenerative pathology known as chronic traumatic encephalopathy (CTE).
“To date, virtually all individuals in whom CTE neuropathologic change is described have prior history of TBI or repetitive head impact exposure,” noted the study authors. “While data are limited, evidence suggests risk of TBI and participation in heading are in part dependent on field position, with risk of head injury in goalkeepers around one-third that of players in other positions and heading by goalkeepers being exceptionally rare.”
Seeking to further assess risk of neurodegenerative disease in these populations by factors including field position (goalkeeper, defender, midfield, forward), career length, and playing era, the researchers conducted a population-based cohort study of Scottish male former professional soccer players born between January 1, 1900, and January 1, 1977, (n = 7676) and controls from the general population matched by year of birth, sex, and area socioeconomic status (n = 23,028).
Participants provided data amounting to 1,812,722 person-years of follow-up, with incidence of neurodegenerative disease defined as a diagnosis of dementia not otherwise specified, Alzheimer disease, non-Alzheimer dementia, motor neuron disease, and PD.
“Outcomes were obtained by individual-level record linkage to national electronic records of mental health and general hospital inpatient and day-case admissions, as well as prescribing information and death certification,” they noted.
Among the study cohort, 386 former soccer players (5%) and 366 matched controls (1.6%) were identified with a neurodegenerative disease diagnosis, indicating a 3.6-fold greater risk in soccer players (HR, 3.66; 95% CI, 2.88-4.65; P < .001).
Moreover, risk of neurodegenerative disease varied by field position, with defenders found to be at greatest risk of neurodegenerative disease (HR, 4.98; 95% CI, 3.18-7.79; P < .001) compared with the general population. The lowest risk was found in forwards among outfield players (HR, 2.79; 95% CI, 2.06-3.78; P < .001) and in goalkeepers overall (HR, 1.83; 95% CI, 0.93-3.60; P = .08).
By career length, former soccer players with professional career lasting longer than 15 years were at a 5.2-fold greater risk of receiving a neurodegenerative disease diagnosis than the general population (HR, 5.20; 95% CI, 3.17-8.51; P < .001), with the lowest risk found in those with careers spanning less than 5 years (HR, 2.26; 95% CI, 1.51-3.37; P < .001). No difference in risk was found by playing era for all players born between 1910 and 1969.
"With the current data, we're now at the point to suggest that [soccer] should be sold with a health warning saying repeated heading in [soccer] may lead to an increased risk of dementia," said study author William Stewart, MBChB, PhD, adjunct professor of neurosurgery, University of Pennsylvania Perelman School of Medicine, in a statement.
Notably, several governing bodies of professional soccer associations announced last week updated guidelines that would limit headers classified as “high-impact” to 10 per week in training for the upcoming 2021-2022 season. Addressing these guidelines, Stewart was critical of the lack of scientific evidence provided.
"There is no basis on which to say 10 headers of a certain level will somehow produce no risk or even make a great difference to risk. It is a best guesstimate and we would have to wait 30 or 40 years to see the impact," he noted.
Russell ER, Mackay DF, Stewart K, MacLean JA, Pell JPP, Stewart W. Association of field position and career length with risk of neurodegenerative disease in male former professional soccer players. JAMA Neurol. Published online August 2, 2021. doi:10.1001/jamaneurol.2021.2403