ALS is fatal because it progressively destroys the motor neurons that control voluntary muscles, eventually paralyzing the diaphragm and other muscles needed to breathe. Most people with ALS die of respiratory failure within two to five years of diagnosis, with a median survival of about 2.2 years. The disease is currently irreversible because the underlying nerve damage cannot be stopped or repaired.
How Motor Neurons Break Down
Your body has two types of motor neurons that work together to move muscles. Upper motor neurons originate in the brain’s motor cortex and send signals down to the spinal cord. Lower motor neurons pick up those signals in the spinal cord and carry them the rest of the way to your skeletal muscles. ALS attacks both types simultaneously.
The destruction follows a pattern. Upper motor neurons begin dying first, often before any symptoms appear. Lower motor neurons follow, and as they die, the muscles they serve lose their connection to the nervous system entirely. A muscle that no longer receives nerve signals can’t contract. It wastes away. This process spreads across the body over months, progressively stripping away the ability to walk, use your hands, speak, swallow, and ultimately breathe.
The Protein That Poisons Nerve Cells
In roughly 97% of ALS cases, a protein called TDP-43 is at the center of the damage. TDP-43 normally lives inside the nucleus of nerve cells, where it helps manage genetic information. In ALS, the protein misfolds, leaks out of the nucleus, and clumps together in the surrounding cell fluid. These clumps are toxic in two ways at once: the nucleus loses a protein it needs to function properly, and the clumps themselves actively damage the cell.
The misfolded protein disrupts the cell’s energy supply by interfering with mitochondria, the structures that produce fuel for the cell. With less energy available, the neuron struggles to maintain itself. TDP-43 clumps also block the cell’s ability to absorb materials from its environment and impair its stress-response systems, leaving it unable to cope with further damage.
What makes this especially devastating is that TDP-43 aggregates behave like prion proteins. They can spread from one cell to the next, “seeding” new clumps in healthy neurons. This means the disease doesn’t stay confined to its starting point. It propagates through the nervous system, and the damage it leaves behind is permanent. No current treatment can clear these aggregates or stop their spread.
Why Breathing Fails
The diaphragm is the large dome-shaped muscle beneath your lungs that does most of the work of breathing. It’s controlled by phrenic nerves that originate from motor neurons in the cervical spinal cord, a region that ALS tends to affect early. As those motor neurons die, the diaphragm weakens. Accessory breathing muscles in the chest and neck can compensate for a while, but they simply don’t have the capacity to sustain adequate breathing on their own.
As the diaphragm fails, two things happen. Oxygen levels in the blood drop, and carbon dioxide builds up because the lungs can’t exhale fully. This is called hypoventilation, and it’s the primary mechanism of death in ALS. The decline is often gradual at first, showing up as breathlessness during activity, then at rest, then during sleep. Eventually, without intervention, it progresses to complete respiratory failure.
On death certificates of more than 24,000 Americans who died with ALS, respiratory failure was listed as the most frequent co-occurring cause of death, appearing in over 25% of cases. People with ALS are 37 times more likely to die of respiratory failure than the general population.
Swallowing Problems and Infection Risk
ALS also destroys the motor neurons controlling the tongue, throat, and mouth. Swallowing becomes progressively impaired, starting with difficulty managing food in the mouth and eventually affecting the ability to safely move food past the airway. When food or liquid enters the lungs instead of the stomach, it causes aspiration pneumonia, a serious lung infection.
Pneumonia appeared on 5.2% of ALS death certificates in a large U.S. study, and ALS patients were nearly 17 times more likely to die of pneumonia than the general population. Combined with the weakened breathing muscles that make coughing ineffective, even a minor chest infection can become life-threatening because the body can’t clear mucus or fight the infection with adequate ventilation.
Malnutrition Accelerates the Decline
The same swallowing difficulties that raise infection risk also make it harder to eat and drink enough. At the same time, ALS creates a state of hypermetabolism, meaning the body burns more calories than normal even as muscle mass is being lost. This combination pushes many patients into malnutrition, which is considered a significant prognostic factor. Malnourished patients decline faster, and the loss of muscle mass itself appears to worsen motor neuron death, creating a cycle that accelerates the disease.
Why Some People Decline Faster
ALS onset can begin in two main regions: the limbs (spinal onset) or the muscles controlling speech and swallowing (bulbar onset). Where symptoms start has a major impact on survival. Median survival from diagnosis is about 26 months for spinal-onset ALS, but only 12 months for bulbar onset. Bulbar-onset patients face earlier swallowing and breathing problems because the affected motor neurons are closer to the circuits controlling those critical functions.
A small percentage of people live significantly longer. About 21% survive past five years, 7% past ten years, and roughly 2% live beyond twenty years. The reasons for this variability aren’t fully understood, but younger age at diagnosis, spinal onset, and slower initial progression are associated with longer survival.
How Breathing Support Extends Life
Because respiratory failure is the primary killer, supporting breathing is the most effective way to extend survival. Non-invasive ventilation, typically a mask-based device that helps push air into the lungs, reduces the rate of death by about 26% overall. For people with limb-onset ALS, the benefit is even larger: a 37% reduction in the rate of death.
Consistent use matters. People who used non-invasive ventilation for at least four hours per day had a median survival of 10.7 months from the point of starting it, compared to 5.9 months for those who used it less. This intervention doesn’t stop the disease, but it compensates for the diaphragm’s declining function and buys meaningful time. Invasive ventilation through a tracheostomy can sustain breathing indefinitely, but most patients ultimately decline from the broader effects of total paralysis.
The fundamental reason ALS remains fatal is that no treatment can halt or reverse motor neuron death. Supportive care, including ventilation and feeding tubes, can extend life and improve comfort, but the underlying destruction of nerve cells continues until the body can no longer sustain its most essential functions.