ALS is 100% fatal, though there are extremely rare cases in which the patient lives many years after diagnosis.

But ALS will kill them, too, assuming that by sheer chance, another illness or accident doesn’t intervene.

In the final stages of any terminal illness, the heart stops beating. There is no longer blood to the brain. Brain cells begin dying.

Whether you get run over by a train, fall off a cliff, drown, ingest poison or have cancer, one thing will certainly happen if any of these conditions cause your death: Your heart will stop.

So when a person dies from ALS, their heart stops. But it would be inaccurate to say that cardiac arrest killed this person.

In ALS, motor neurons begin dying. A motor neuron is responsible for conducting an electro-chemical impulse to muscle fibers to cause those fibers to contract (move).

ALS affects voluntary muscles. Eventually, the patient loses control of all voluntary muscles and is completely paralyzed.

Breathing
In order to breathe, we must use voluntary muscles, even though we don’t consciously plan each breath, and even though we keep breathing when knocked unconscious.

But voluntary muscles are involved in the respiratory process; these muscles must contract in order for you to breathe.

Specifically, the muscle involved in breathing is primarily the diaphragm. But the muscles between the ribs are also involved.

The diaphragm muscles and rib muscles contract to move air into the lungs.

If electro-chemical impulses fail to reach these muscles—because the motor neurons that are supposed to send the signals are dead—then the patient cannot move these muscles—and hence cannot breathe without mechanical support.

This is what will happen eventually to all ALS patients if they don’t die from a non-ALS cause.

What kills the ALS patient is the inability to take breaths. Mechanical ventilation can extend survival, but even that will eventually fail.

According to one study, the extension averaged 14.9 months. Mechanical ventilation, however, is not tolerated by every patient.

The progression of respiratory failure takes months; it’s not a sudden inability to breathe (sudden paralysis of the diaphragm) when in the days immediately prior, the patient was breathing just fine.

In a small number of cases, death from ALS is caused by:
• Swallowing problems causing malnutrition
• Heart arrhythmia
• Pneumonia from food or fluid getting into the lungs
• Pulmonary embolism

Sources:
ncbi.nlm.nih.gov/pmc/articles/PMC2077959/
alsa.org/als-care/resources/publications-videos/factsheets/breathing-difficulties.html
mda.org/disease/amyotrophic-lateral-sclerosis/signs-and-symptoms/stages-of-als

Are All Pulmonary Embolisms the Same?