ALS, or amyotrophic lateral sclerosis, progressively destroys the motor neurons that connect your brain to your muscles. As these nerve cells die, muscles lose their signal to move, leading to weakness, paralysis, and eventually the inability to breathe. The average survival after diagnosis is about three years, though roughly 20% of people live five years or more, 10% live beyond a decade, and about 5% survive 20 years or longer.
What makes ALS so devastating is its scope. It doesn’t attack one muscle group and stop. It spreads through the body’s motor neuron network over months and years, gradually taking away voluntary movement while leaving sensation, vision, and most internal organ function intact.
How Motor Neurons Break Down
Your body has two types of motor neurons that work together to produce movement. Upper motor neurons run from the brain down into the spinal cord, acting as the command center. Lower motor neurons extend from the spinal cord out to the muscles themselves, delivering the final signal that makes a muscle fiber contract. ALS attacks both.
When lower motor neurons die, the muscles they control begin to waste away. You lose bulk and strength in a visible, measurable way. When upper motor neurons die, the brain loses its ability to regulate reflexes and smooth out movement. This creates stiffness and abnormally brisk reflexes, a condition called spasticity. In later stages, the muscle wasting from lower motor neuron loss can become so severe that it masks the exaggerated reflexes, because there’s simply not enough muscle tissue left to respond.
At the cellular level, a key protein called TDP-43 plays a central role. This protein normally sits inside the nucleus of nerve cells and helps regulate gene expression. In ALS, TDP-43 clumps together outside the nucleus, forming toxic aggregates. The protein gets pulled away from where it’s needed, so the cell loses a critical function while simultaneously being poisoned by the buildup. Research from PNAS has also shown that support cells in the brain called astrocytes become toxic to motor neurons in ALS, accelerating the damage beyond what the protein aggregation alone would cause.
Where Symptoms Start
ALS typically begins in one of two ways. In limb-onset ALS, which is more common, the first signs appear in the arms or legs. You might notice weakness in a hand, difficulty with buttons or keys, tripping while walking, or a foot that drops when you step forward. These symptoms usually start on one side of the body before spreading to the other.
In bulbar-onset ALS, the disease first targets the muscles controlling speech and swallowing. Words become slurred, the voice turns nasal or strained, and swallowing solid food gets progressively harder. Bulbar-onset accounts for a smaller share of cases but tends to progress faster, partly because it affects breathing muscles earlier.
Regardless of where it starts, ALS eventually spreads. Limb-onset cases develop speech and swallowing problems over time, and bulbar-onset cases develop limb weakness. The disease moves through the motor neuron network in a pattern that varies from person to person, but the trajectory is always toward more widespread paralysis.
What Happens to Breathing
Respiratory failure is the most serious consequence of ALS and the primary cause of death. The diaphragm, the dome-shaped muscle beneath your lungs that drives each breath, is controlled by motor neurons just like any other muscle. As those neurons die, the diaphragm weakens and eventually becomes paralyzed.
This doesn’t happen all at once. Early on, you might notice shortness of breath when lying flat, because gravity makes it harder for a weakened diaphragm to pull air in. Nighttime breathing often deteriorates first, leading to poor sleep, morning headaches, and daytime fatigue. Doctors track this decline by measuring forced vital capacity, which reflects how much air you can push out of your lungs in a full breath. A healthy person’s lungs perform at or near 100% of the predicted value for their age and size. In one documented case, a patient with early diaphragm involvement already showed a forced vital capacity of just 56% of predicted.
Noninvasive ventilation, a mask that pushes air into the lungs, can extend survival and improve quality of life significantly. Eventually, some patients choose full mechanical ventilation through a tracheostomy, while others opt for comfort-focused care.
Eating, Swallowing, and Weight Loss
Swallowing problems affect virtually all ALS patients at some point. The muscles of the tongue, throat, and jaw gradually weaken, making it harder to move food safely from your mouth to your stomach. Early signs include coughing or choking during meals, food or liquid going down the wrong way, and meals that take so long they become exhausting rather than enjoyable.
Weight loss in ALS is driven by more than just difficulty eating. The body enters a hypermetabolic state, burning roughly 10% more energy at rest than a healthy person of the same size and age. This is paradoxical, because muscle wasting and reduced physical activity would normally lower your metabolic rate. The elevated calorie burn appears to involve problems with energy production inside cells, though the exact mechanism isn’t fully understood. The result is that the body consumes itself from two directions: taking in fewer calories while burning more.
Constipation compounds the problem by suppressing appetite, a consequence of weakened abdominal and pelvic muscles combined with reduced movement. Excess saliva pooling in the mouth, caused by weakened facial muscles that can no longer swallow it effectively, also discourages eating. Psychological distress and depression further reduce appetite. Nutritional decline is a significant independent predictor of survival, meaning that how well someone maintains their weight directly influences how long they live with ALS. When swallowing becomes unsafe or weight loss becomes dangerous, a feeding tube placed through the abdomen allows nutrition to bypass the throat entirely. Guidelines recommend placing the tube before lung capacity drops below 50% of predicted, since the procedure carries more risk once breathing is already compromised.
Cognitive and Behavioral Changes
ALS was long considered a disease that spared the mind. That understanding has changed substantially. At least half of people with ALS develop some degree of cognitive symptoms resembling frontotemporal dementia, and a subset meet full diagnostic criteria for it.
These changes can include personality shifts, impaired judgment, difficulty planning or organizing, and loss of empathy or social awareness. Care partners frequently report personality changes (30% cite this as the first sign something was wrong) and mood changes (14%). Others notice reduced motor planning ability, relationship problems, sleep disruption, or even delusions. These cognitive symptoms can appear before, after, or alongside the physical symptoms, and they complicate caregiving significantly because the person with ALS may not recognize the changes in themselves.
What ALS Typically Spares
Despite its devastating reach through the motor system, ALS leaves several body functions largely intact. The sensory nerves that carry touch, pain, and temperature signals are not primary targets of the disease, so most people retain full sensation even in paralyzed limbs. Eye muscles are among the last to be affected, which is why many people with advanced ALS communicate using eye-tracking devices. Bladder and bowel control, governed partly by the autonomic nervous system, generally remains functional much longer than voluntary movement.
That said, the boundary isn’t as clean as once believed. Emerging evidence shows that some sensory and autonomic changes do occur in ALS, including altered pain perception and mild disruptions in heart rate regulation. These are typically subtle compared to the motor symptoms, but they challenge the older view that ALS affects motor neurons exclusively.
Why Diagnosis Takes Time
There is no single blood test or scan that confirms ALS. Diagnosis relies on clinical criteria that require evidence of both upper and lower motor neuron involvement, along with progressive spread of symptoms and the exclusion of other conditions that can mimic ALS. The newest diagnostic framework, the Gold Coast Criteria introduced in 2020, was designed to catch the disease earlier than previous standards. It achieves a sensitivity of about 94%, meaning it correctly identifies the vast majority of people who have the disease, compared to older criteria that missed roughly 40-45% of cases in their stricter categories. Even so, many people go through months of testing before receiving a definitive answer, because early ALS can look like a pinched nerve, carpal tunnel syndrome, or dozens of other conditions.