ALS has no single cause. The disease results from a combination of genetic vulnerability, cellular dysfunction, and environmental exposures that together destroy motor neurons, the nerve cells controlling voluntary movement. About 5 to 12% of cases run in families, while the rest appear without any family history. Even in those “sporadic” cases, the same biological breakdowns are at work inside affected neurons.
Genetics and Inherited Risk
Roughly 10 to 12% of ALS cases are classified as familial, meaning the person carries a gene mutation passed down from a parent. The most common genetic culprit is a repeating error in a gene called C9orf72, which accounts for about 37.5% of familial cases and around 4.6% of sporadic ones. A second gene, SOD1, was the first ALS gene discovered but turns out to be relatively rare, appearing in under 1% of all patients. Mutations in other genes, including FUS and TARDBP, round out a growing list of over 30 genes linked to the disease.
Having a mutation doesn’t guarantee someone will develop ALS. Researchers believe these genes create a predisposition, and additional factors, whether cellular stress, toxic exposures, or aging, tip the balance toward disease. This “multiple hits” model helps explain why two people with the same mutation can have very different outcomes.
TDP-43 Protein Buildup
One discovery has reshaped how scientists understand ALS at the cellular level: a protein called TDP-43 clumps together abnormally in the motor neurons of roughly 97% of all ALS patients. TDP-43 normally lives inside the cell’s nucleus, where it helps manage gene activity. In ALS, the protein migrates out of the nucleus and accumulates in the surrounding cell fluid, forming sticky deposits.
These deposits cause a double problem. The nucleus loses a protein it needs, disrupting normal gene regulation. At the same time, the clumps themselves become toxic, interfering with the cell’s ability to handle stress and process nutrients. When neurons encounter everyday stressors like heat, infection, or low oxygen, they form temporary protective clusters called stress granules. TDP-43 is involved in building those granules, and in ALS, defective stress responses may cause these temporary structures to harden into the permanent toxic deposits found in affected brains and spinal cords.
Glutamate Overload and Calcium Flooding
Motor neurons communicate using a chemical messenger called glutamate. After a signal fires, surrounding support cells (astrocytes) normally vacuum up the leftover glutamate. In ALS, this cleanup system fails. Up to 60 to 70% of sporadic ALS patients show a dramatic loss of the transporter protein that astrocytes use to absorb glutamate from the space between neurons. The result is a buildup of glutamate that keeps the receiving neuron firing long after it should have stopped.
This overstimulation floods the neuron with calcium. Excess calcium poisons the cell’s mitochondria, its tiny power plants, reducing energy production and generating harmful molecules called reactive oxygen species. Those molecules damage the very transporter that astrocytes need to clear glutamate, creating a vicious cycle: more glutamate accumulates, more calcium floods in, and more damage follows. The calcium surge also triggers enzymes that chop TDP-43 into fragments prone to clumping, linking glutamate toxicity directly to the protein buildup described above. It’s a chain reaction where each broken link makes the next failure worse.
Mitochondrial Damage and Oxidative Stress
Mitochondria do more than generate energy. They buffer calcium levels, regulate cell death signals, and recycle damaged components. In ALS, mitochondrial dysfunction shows up early and at multiple levels. Immune and support cells in the nervous system become major producers of reactive oxygen species, molecules that damage DNA, proteins, and cell membranes. While these molecules probably don’t start ALS on their own, they accelerate its progression once the disease is underway. Lab studies on motor neurons carrying the FUS gene mutation, for example, found they produced roughly 2.5 times more hydrogen peroxide and twice the level of another damaging molecule compared to healthy neurons.
Oxidative stress also depletes a key cellular fuel called NAD+, which in turn shuts down maintenance processes that keep mitochondria healthy. As mitochondria deteriorate, they produce even more damaging molecules, feeding yet another self-reinforcing loop of cellular destruction. This is why oxidative stress is considered both a consequence and an amplifier of ALS pathology.
Environmental and Occupational Exposures
For the roughly 90% of ALS cases without a clear inherited mutation, environmental factors likely play a significant role, though pinning down exactly which exposures matter has proven difficult. The chemicals most consistently linked to higher ALS risk include lead, pesticides, and certain industrial solvents. A doubling of blood lead levels has been associated with a 1.9-fold increase in ALS risk. Elevated copper levels show an even stronger association, with doubled copper linked to 3.4 times the odds of developing the disease.
Military veterans face a notably higher risk. A 2017 meta-analysis covering nine studies found that military service overall raises ALS risk by about 29%. Gulf War veterans faced roughly double the risk of the general population. Vietnam War veterans exposed to Agent Orange had 2.8 times the odds. Specific military exposures linked to increased risk include exhaust from heaters or generators, nearby explosions, burning agents used in the field, and a nerve agent pre-treatment drug called pyridostigmine bromide. The combination of chemical exposures, intense physical exertion, and head trauma during service may help explain why ALS appears disproportionately in this population.
Cyanobacteria, blue-green algae found in lakes and waterways, produce a neurotoxin called BMAA that has also been flagged as a potential environmental trigger. Researchers have speculated that dietary or waterborne exposure to BMAA could contribute to clusters of ALS-like disease observed in certain geographic regions.
Head Injuries and Physical Trauma
Repeated head trauma is an emerging risk factor. Data from over 3,400 people in the National ALS Registry showed that sustaining head injuries between ages 18 and 30 increased the risk of being diagnosed with ALS before age 60. Five or more head injuries over a lifetime, or injuries sustained during childhood, further elevated that risk. Among military personnel specifically, a head injury within 15 years of diagnosis was associated with 2.3 times the odds of ALS.
The connection between head injury and ALS may involve some of the same cellular mechanisms at play in the disease itself. Traumatic brain injury triggers inflammation, disrupts mitochondrial function, and can accelerate protein misfolding, potentially setting the stage for neurodegeneration years or even decades later.
Age, Sex, and the Hormonal Connection
ALS is not a disease of one age group, but risk rises sharply with age. Population-based registries place the typical onset around age 70, though clinic-based data often skew younger because specialized centers attract patients diagnosed earlier. The disease is more common in men, but the gap narrows dramatically as women age. Before menopause, the male-to-female ratio is nearly 4 to 1. After menopause, it drops to about 1.6 to 1, and the proportion of women with ALS jumps from 30% to nearly 44%.
This shift strongly suggests that estrogen or other reproductive hormones offer some protective effect against motor neuron degeneration. Once that hormonal protection declines, women’s risk climbs much closer to men’s. The finding has prompted interest in whether hormonal therapies might one day play a role in prevention, though that remains speculative for now.
Why No Single Cause Explains ALS
What makes ALS so difficult to prevent or treat is that it doesn’t follow a simple “one cause, one disease” pattern. A person might carry a genetic vulnerability that weakens their motor neurons’ ability to clear glutamate or manage protein folding. Environmental exposures, whether lead, pesticides, or head trauma, could add stress to already vulnerable cells. Aging further reduces the body’s capacity to repair damage and clear toxic proteins. At some point, these overlapping insults overwhelm the motor neuron’s defenses, and the self-reinforcing cycles of calcium overload, mitochondrial failure, protein clumping, and oxidative stress take over.
This “threshold” model explains several puzzling features of ALS: why identical twins don’t always both develop it, why some gene carriers live into old age unaffected, and why certain occupations and exposures raise risk without guaranteeing disease. ALS appears to be the final common pathway of multiple converging failures, each one increasing the odds that motor neurons will cross a point of no return.