Head injuries linked to higher odds of ALS in large research review

A history of head injury is associated with 36% higher odds of amyotrophic lateral sclerosis (ALS), according to a systematic review and meta-analysis of 29 studies involving more than 5 million people.

The review included 22 studies that compared people with and without ALS, looking at their histories of head injuries. The combined results linked head injuries of varying severity — mild, moderate, and severe — to higher odds of ALS. In contrast, seven studies looked at whether people with and without previous head injuries later developed ALS. When results from four of these studies were combined, the findings suggested a possible increase in risk, but the association was not statistically significant.

Because the studies were observational, the researchers cautioned that the findings cannot show that head injuries cause ALS. Other factors may have influenced the results, and early, undiagnosed ALS symptoms could make people more likely to suffer head injuries.

“Further prospective studies and mechanistic investigations are required to clarify the potential relationship between head injury and ALS development,” the team wrote.

Recommended Reading

Oct. 21, 2026 News by Steve Bryson, PhD

1 sign of ALS may be detected up to 15 years before diagnosis

Review examines link between head injuries and ALS

The study, “History of Head Injury and Risk of Amyotrophic Lateral Sclerosis: An Updated Comprehensive Systematic Review with Meta-Analysis of 5,033,215 Participants,” was published in Neuroepidemiology.

Head injuries, including concussions and other traumatic brain injuries, have long been investigated as potential ALS risk factors. But findings have been mixed. Some studies have linked head injuries to a higher risk of ALS, while others suggest that injuries may be an early sign of undiagnosed ALS rather than a cause.

To better understand the possible connection, researchers searched six scientific databases for studies published through January 2026 that examined head injuries and ALS. Of 7,375 records they reviewed, 29 studies met the inclusion criteria. Together, these studies involved 5,033,215 people across North America, Europe, Asia, and New Zealand.

In 22 studies, researchers compared the head injury histories of 18,987 people with ALS and more than 1.6 million people without the disease. The remaining seven studies involved more than 3.4 million people with and without a history of head injury and tracked whether they later developed ALS.

When researchers combined adjusted results from studies comparing people with and without ALS, they found that a history of head injury was associated with 36% higher odds of the disease. The analysis accounted for other factors that could have influenced the results.

The results were less clear in studies that followed people to see whether they developed ALS after a head injury. When researchers combined results from four of these studies, they found a trend toward increased risk, but the association was not statistically significant.

The researchers cautioned that these findings should be viewed carefully. The studies differed in the people they included, how long participants were followed, how ALS was diagnosed, and how head injuries were defined.

ALS odds vary by head injury severity and timing

Mild and severe head injuries were each associated with 27% higher odds of ALS, while moderate injuries were associated with 50% higher odds. The findings suggest that head injuries of varying severity may be associated with ALS, but “the mechanisms underlying this relationship remain uncertain,” the researchers wrote.

Even a single head injury was associated with 22% higher odds of ALS. Two or more injuries showed a trend toward higher odds, but the association was not statistically significant. The researchers noted that too few studies and the lack of a clear pattern between the number of injuries and ALS odds made it difficult to draw firm conclusions.

The timing of head injuries also mattered. Injuries occurring less than one year before an ALS diagnosis were associated with more than three times the odds of ALS. The association was weaker but remained statistically significant for injuries that occurred more than three years before diagnosis, with 24% higher odds.

The researchers suggested that the stronger association shortly before diagnosis might be explained by reverse causation. This means early, unrecognized ALS symptoms, such as muscle weakness or balance problems, could make people more likely to fall and suffer head injuries.

By contrast, the researchers proposed that injuries occurring more than three years before diagnosis might play a role in ALS development. Possible processes include inflammation, harmful overstimulation of nerve cells, and genetic vulnerability. However, these mechanisms remain unproven.

“This updated meta-analysis provides evidence that a history of head injury is associated with an increased risk of ALS,” the researchers wrote. However, they cautioned that “these findings should be interpreted cautiously” because the available evidence cannot rule out reverse causation or other factors that may influence the results.

The team called for larger studies that follow people over longer periods. These studies should use consistent definitions of head injury and better account for genetic and environmental factors to help determine whether head trauma contributes to ALS development.