Most people who develop ALS get it without any known family history of the disease, and researchers still cannot point to a single definitive cause. About 90 to 95 percent of cases are considered “sporadic,” meaning they appear without a clear inherited genetic link. The remaining 5 to 10 percent are “familial,” passed down through gene mutations. For the vast majority of people, ALS likely results from a combination of genetic vulnerability, environmental exposures, and biological processes that together push motor neurons past a breaking point.
Sporadic vs. Familial ALS
The distinction between sporadic and familial ALS is the first thing to understand. Familial ALS means at least one other family member has also been diagnosed, and a specific inherited gene mutation is probably responsible. In sporadic ALS, there is no family history, and the disease seems to arise on its own. This doesn’t mean genetics play no role in sporadic cases. Many people with sporadic ALS carry genetic risk factors that make their motor neurons more vulnerable, even if no relative ever developed the disease.
What Happens Inside the Body
ALS destroys motor neurons, the nerve cells in the brain and spinal cord that control voluntary movement. As these neurons die, muscles lose their signal source and progressively weaken, eventually leading to paralysis. But what actually kills the neurons involves several overlapping processes.
One major factor is a protein called TDP-43. In nearly all ALS cases, this protein misfolds and clumps together inside motor neurons instead of staying where it belongs in the cell nucleus. These abnormal protein deposits are toxic, disrupting the cell’s normal functions and eventually killing it. Preventing this misfolding or helping cells clear out the clumps is considered one of the most important targets for treatment.
Motor neurons are also unusually sensitive to overstimulation. A chemical messenger called glutamate normally carries signals between nerve cells, but when too much of it builds up, it essentially overexcites motor neurons to death. This process, known as excitotoxicity, is especially dangerous for motor neurons because they have a limited ability to buffer the calcium floods that result from excessive glutamate signaling. In some cases, gene mutations worsen this by reducing the ability of surrounding brain cells to clean up excess glutamate.
Genes Linked to ALS
Several gene mutations have been identified in familial ALS. The most common is a mutation in a gene called C9orf72, which causes an abnormal repetition of a short DNA sequence. This mutation is the most frequent genetic cause of ALS and is also linked to a form of dementia called frontotemporal dementia. It appears to damage neurons in multiple ways: both by producing toxic protein products and by reducing the gene’s normal, protective function.
Another well-known mutation involves the SOD1 gene, found in about 1 to 2 percent of all ALS patients. SOD1 mutations can shift motor neurons from normal signaling into a state of toxic overstimulation by interfering with glutamate cleanup and mitochondrial function. Mutations in the gene that produces TDP-43 itself have also been directly linked to the disease.
Even in sporadic ALS, researchers are finding that subtle genetic variations across multiple genes can collectively raise a person’s risk. The picture is less like a single broken switch and more like dozens of small vulnerabilities adding up.
Environmental Risk Factors
Environmental exposures appear to play a meaningful role, particularly for sporadic ALS. A CDC-affiliated study of ALS cases in New England found that people who reported job or hobby-related exposure to chemicals like pesticides, solvents, or heavy metals had roughly 2.5 times the risk of developing ALS compared to unexposed individuals. The exposures most strongly associated with increased risk were solvents, lead, and pesticides.
Lead stands out as the single environmental factor with the most convincing evidence. A large umbrella review of existing research identified it as the one non-genetic risk factor supported by strong statistical evidence. Farming, exposure to other heavy metals, and head injuries were backed by suggestive but less definitive data.
Certain occupations carry higher risk. People who worked in construction, manufacturing, mechanical trades, military service, or painting had nearly four times the odds of developing ALS compared to people in lower-exposure jobs. There is also an intriguing link to water sports, particularly water skiing, which may relate to exposure to cyanobacteria, a type of blue-green algae that produces a neurotoxic compound found in some lakes and waterways.
Specific pesticides have been implicated as well. Workers exposed to a component of the herbicide Agent Orange had a roughly 3.5 times higher mortality rate from ALS. Blood levels of certain organochlorine pesticides have also been correlated with increased odds of the disease.
Head Trauma and Physical Activity
ALS is sometimes called Lou Gehrig’s disease, named after the baseball legend who died from it, and that connection to athletics is more than symbolic. Several studies have found elevated ALS rates among professional athletes, including football players and soccer players. The leading theory is that repetitive head trauma may trigger or accelerate the disease.
Research on the brains of athletes with chronic traumatic brain injuries found that 12 percent were also diagnosed with ALS. Repeated blows to the head appear to promote the accumulation of abnormally clumped proteins, including TDP-43, the same protein central to ALS in the general population. This suggests head trauma may activate the same disease pathway through a different entry point.
Military Service
Military veterans develop ALS at higher rates than the general population, a finding confirmed by studies in the United States, Denmark, and Scotland. The exact reasons remain unclear. Possible explanations include exposure to toxic chemicals, physical trauma, extreme exertion, or some combination unique to military environments. The U.S. Department of Veterans Affairs recognizes ALS as a service-connected disease for benefits purposes, regardless of when or where a veteran served.
Who Is Most Likely to Develop ALS
ALS is more common in men than women. In large clinical studies, roughly 57 percent of patients are male. The disease most often appears in middle to late adulthood, though the exact age of onset varies. It can occur in younger adults, but this is less typical. Late-onset ALS, appearing at age 75 or older, may present differently than earlier-onset cases, particularly in how symptoms first manifest.
There is no single profile that predicts who will get ALS. A person with no family history, no known toxic exposures, and no history of head trauma can still develop the disease. This is part of what makes ALS so difficult to study and so frustrating for patients seeking answers. Current research increasingly points to a “multi-hit” model, where several risk factors must converge in the same person before the disease takes hold. One genetic vulnerability alone may not be enough. But add an environmental exposure, or years of physical stress on the nervous system, and the balance tips.
How ALS Is Diagnosed
There is no single blood test or scan that confirms ALS. Diagnosis relies on clinical evaluation by a neurologist, who looks for a pattern of progressive motor neuron damage in multiple body regions while ruling out other conditions that can mimic ALS. The process often involves nerve conduction studies, imaging, and sometimes spinal fluid analysis.
For decades, the diagnostic criteria were highly specific but often missed early-stage cases, leading to long delays between first symptoms and diagnosis. A newer set of criteria introduced in 2020, called the Gold Coast Criteria, was designed to simplify the process and catch the disease earlier. Recent analysis suggests these newer criteria should be the standard for clinical practice, as they improve early detection without sacrificing accuracy.