It’s difficult, if not impossible, to know for certain the exact number of people living with amyotrophic lateral sclerosis (ALS) in the world – and therefore challenging to determine the exact chances of any one person developing the disease. Differences in data reporting from country to country, uncertainties around diagnoses, and even competing definitions of what constitutes a case of ALS rather than other forms of motor neuron disease, are among the many complicating factors in determining these statistics.

However, many researchers have looked at the question of just how prevalent ALS is in the population. Using a variety of data-gathering techniques and statistical analyses, they have published a number of studies that, while displaying some variation, support the conclusion that roughly 1 in every 400 people will develop ALS in their lifetime.

This 1 in 400 figure is, of course, an estimate, and seeks to posit the average chance that any one person, of any sex, ethnic background, or age will develop ALS. Once we begin to break down the population into smaller groups, the numbers get more complicated. Most studies have found that men are more likely to be diagnosed with ALS than women. For example, a study of populations in Ireland by researchers from Trinity College, Dublin found that about 1 in 347 men and 1 in 436 women were affected by ALS at some point in their lives. Other studies have found very similar numbers. An analysis by Dr. Carmel Armon, MD, MSc, MHS of Tufts University found the risk to be 1 in 350 for men and 1 in 450 for women. Another study from researchers at the University of Minnesota, examining records from throughout the UK, put the risk at one in 350 for men and one in 472 for women.

Bringing other factors into consideration can also affect the statistics. For example, a study by Dr. Ammar Al-Chalabi, PhD FRCP et al. from King’s College, London looked at populations that included more advanced ages than other studies. The study found that, by the age of 85, the lifetime risk of ALS is about 1 in 250 for men and 1 in 400 for women. By considering the age of patients in this analysis, the numbers appear higher, likely because evidence shows that ALS is much more common in older people. However, as not every person will live to reach that age, the likelihood of an average currently unaffected individual developing the disease may be somewhat lower.

There are clear limitations to the data we have available about the number of people affected by ALS, or the prevalence of ALS, at any given time – the majority of studies in the literature have been based on populations in Europe, where reliable data sources are more available to researchers. Further studies in other populations would enable us to form a more reliable picture of just how many people throughout the world have ALS – and give us a more nuanced picture of just how likely any one person is to develop it. However, based on the available data, the science team at ALS Therapy Development Institute (ALS TDI) is comfortable with the assertion that 1 in 400 is a reasonably accurate estimate of ALS lifetime risk.

1 in 400 is much smaller than the lifetime risks of developing a disease like cancer, but for ALS, which is considered to be a ‘rare disease’, it may seem surprisingly high. As mentioned above, the fact that ALS is often not diagnosed until later in life, and the short life expectancy following diagnosis, may play a role in making the disease be perceived as rarer that it actually is.

If more people became aware of this 1 in 400 lifetime risk statistic, would it move those, not yet impacted by ALS, to be invested in research to find treatments?

Next Steps:

  • To learn more about ALS, and ALS TDI’s research to end it, click here. 
  • To learn how you can help to spread ALS awareness, click here.