Last year professional football players, Len Johnrose and Stephen Darby, announced they’d been diagnosed with motor neurone disease (MND). This follows previous announcements from other prominent footballers in this country and across the world in recent years.
Is it the case that professional football players are more prone to developing MND than the general population? Or is this just the impression created by the high-profile nature of these professionals and the corresponding media coverage these cases bring? What does the science suggest?
Here we look at some of the studies that investigate the incidence (rate of newly diagnosed cases) of MND in professional football players and take a closer look at the suggested causes.
A look at the evidence
Firstly, it should be stated that there hasn’t been a great number of studies on this topic and so only a limited amount of information exists. In 2005 and 2009, Italian researchers Chio and colleagues carried out studies on the same group of 7,325 former Italian male professional footballers (ages range 23-74, median age 41). These are the most well-known studies of football players and have received much attention since they were published.
In 2005, they reported that professional footballers in Italy, who played between 1970 and 2002, were six times more likely to develop MND compared to the general population.
The 2009 study extended the previous work with a prospective follow-up of the 7,325 football players to 2006 and also included investigation of the risk of MND in two other groups of professional athletes; 1,973 professional basketball players (aged 20 to 59, median age 36) and 1,701 professional road cyclists (aged 28-86, median age 62), for comparison. Again, the results suggested an increased risk of developing MND and professional football playing in Italy.
The number of observed cases of MND in all three groups of athletes was compared to the expected number of cases in people of similar ages in the general population in Italy. Because the number of expected MND cases would increase with age, and the numbers of athletes in each group is different, the study performs a calculation to take this into account.
The expected number of MND cases in the general Italian population within the cohort of 7,325 people would be 1.24. The actual number of cases amongst professional footballers in the study was actually 8. This suggests a significant increased risk of MND in professional footballers. Although this is a significant finding, it is important to keep in mind that the number of reported MND cases here is still relatively low, and so concluding that there is a definite increased risk could be misinterpreted if this is simply a cluster due to random chance.
The researchers also found that the longer a person played football (over 5 years), the greater the risk of MND – this is known as the ‘dose-risk effect’. The study concluded that there is a higher risk of Italian football players developing MND compared to the general population, but it is clear that more extensive studies of professional football and other professional athletes are needed to draw a definitive conclusion.
What about the professional basketballers and cyclists in the study? The number of expected cases in the general population within the cohort of the basketball players (1,973 people) would be 0.14 (the basketball players were relatively young which explains the low number of expected cases, as MND is usually seen after the age of 40) and in the professional cyclists 1.82. In fact, no cases of MND were seen in either the basketball or cycling group. Does this mean that taking part in cycling and basketball is protective? Possibly not, but this does reflect the potential danger of misinterpretation from small studies of relatively rare diseases. It is important to remember that the numbers of participants in these groups is much lower than the group of footballers and therefore these results may not be quite as robust.
The headlines surrounding MND and football should be approached with some clear thinking, as it is vital that we truly understand the causes of MND and that any possible underlying factors are uncovered. In short, extensive analyses on large cohorts of footballers and professional sports athletes are urgently required to draw a definitive conclusion. The possibility of a ‘cluster’ in the Italian footballers should also be considered. A cluster is an unusually high incidence of a disease occurring in close proximity in terms of both time and location. This can be caused by one or more environmental risk factors that are common to the group. However, clusters can also happen by chance. So if we take the results at face value, what could it be about football that causes MND?
In short, we simply can’t answer this without more research. Football players are exposed to a number of factors that play a part in the development of MND; repeated injuries (including heading the ball), high levels of physical activity, potential substance use (such as supplements, medicines, performance enhancers) and possible exposure to environmental toxins such as pesticides.
Head injury has been implicated in neural decline in some studies. Perhaps there is a connection here. Let’s look at the evidence with respect to MND.
In 2007 Chen and colleagues combined the results of several studies looking into the connection between head injury and the development of MND. Using this technique, called meta-analysis, they found a moderately elevated risk of MND in people who had a previous head injury. The risk was more than 10 times greater in people with multiple head injuries.
A 2012 case-control study carried out by Pupillo and colleagues reported a traumatic event in nearly 60% of 377 MND cases, which was higher than the control groups. Head injury was the most common trauma seen in people with MND in this study.
In the USA, a ban on heading a ball has been in place since 2016 for under 11s playing soccer, to mediate any increased risk of neurological injury from head trauma. In February 2018, UEFA announced that they had commissioned two separate academic research studies to ascertain the exact extent and nature of heading in youth football to provide data to determine whether heading in youth football has any effect on players’ brain structure and function. We wait with interest these results which may to help us understand if there is a connection between head injury and MND.
We have covered the possible role of physical activity and the development of MND in a previous blog article. This concluded that, although there was a higher incidence of MND in people with high levels of physical activity, such as professional athletes, it was unlikely that physical activity itself was the only cause. The disease was more likely caused by a combination of a person’s genetic makeup and, as yet unknown, environmental factors. Possibly the same genetic makeup that predisposes a person to be athletic, together with exposure to environmental risk factors such as those discussed in this blog article, may convey a slightly increased risk of developing MND (read Physical activity and MND – part 3).
The use of drugs has also been proposed to explain the observed risk in football players. It includes the use of therapeutic drugs, taken in excessive doses and/or for longer periods than advised, nutritional supplements such as vitamins and minerals, possibly even substances assumed to improve athletic performance.
A study of drug use in English professional football in 2005 highlighted that many players use supplements and one fifth of these do so without seeking qualified advice. The study also reported that 6% of the 706 respondents indicated that they knew of players who used performance-enhancing compounds. However, because this information is self-reported and anecdotal it does not provide firm evidence that a connection to MND exists. In addition, other sports, such as cycling, have evidence of drug use without a higher rate of MND cases.
A 2007 study looked at the use of permitted drugs in Italian professional soccer players. Of the 743 players questioned, nearly 93% reported use of oral anti-inflammatory products in the previous year and most of these were current users. 36% of players reported current use of analgesics and almost 83% reported current use of supplements and vitamins.
Therefore, although medicine use has been speculated as a possible cause of MND in footballers it should be remembered that professional athletes from other sports probably use the same compounds. There is therefore still no clear evidence of a specific link between the use of medicinal drugs in football and MND.
A distinctive feature of football players is the continuous contact with grass treated with organophosphates, a common pesticide. This is a potential environmental exposure that links them to farmers, another population group which has an increased risk for MND (Merwin and colleagues, 2017). Organophosphates work by damaging an enzyme in the body called acetylcholinesterase, which helps to break down the neurotransmitter acetylcholine, , potentially resulting in neurological damage. However, there have been no studies that show a direct link between pesticides and an increased risk of MND among football players, and other professional athletes are equally exposed to grass treated with pesticides.
What can we take from this?
A 2016 review by Bozzoni and colleagues, ‘Amyotrophic lateral sclerosis and environmental factors’, looked at the links between physical activity, football and MND. The authors reviewed several studies, including the two Italian studies mentioned previously, and concluded that the literature would suggest there may be an increased risk of MND in football players, which isn’t seen in other sports professionals. There does not seem to be a single risk factor associated with the disease. However, a number of risk factors acting together cannot be ruled out in football players. The researchers also reviewed several studies in which no association between playing football and developing MND could be identified. More studies are needed that use matched groups of professional sports people from different countries, together with control groups, to confirm this suggested association.
MND has already claimed the lives of Tony Hopper (who died in October last year), Jimmy Johnstone, Don Revie and Willie Maddren. Fernando Ricksen is currently living with the disease. There are also many footballers affected by other neurodegenerative conditions such as Alzheimer’s Disease, or chronic traumatic encephalopathy (brain damage). The apparent high rate of neurological illnesses found in footballers has spurred on the FIELD study (Football’s Influence on Lifelong Health and Dementia) which will compare the physical and mental health, including risk of neurodegenerative disease, of approximately 15,000 former professional footballers with the wider population. It is hoped this, along with the work being carried out by the Drake Foundation (which aims to improve understanding of concussion in sport and its effects on long-term brain health), will help us understand any association between brain injury and neurodegeneration.
It is clear that more work is needed to understand the causes of MND, the biology of the disease and the interaction between genetics, lifestyle and environment. Only then will we be able to tackle these perceived issues and finally defeat MND.