Lou Gehrig’s disease, formally called amyotrophic lateral sclerosis (ALS), destroys the nerve cells that control voluntary movement. It progressively strips away the ability to walk, talk, eat, and eventually breathe, with a median survival of about two to four years from diagnosis. The disease targets motor neurons in the brain, brainstem, and spinal cord while typically leaving the senses, intellect, and eye movements intact.
How ALS Attacks the Nervous System
Your brain controls muscle movement through two sets of nerve cells. Upper motor neurons send signals down from the brain’s motor cortex, and lower motor neurons in the brainstem and spinal cord relay those signals to the muscles themselves. ALS destroys both. When lower motor neurons die, the muscles they control weaken, shrink, and begin to twitch involuntarily. When upper motor neurons die, muscles become stiff and reflexes become exaggerated. Most people with ALS experience both types of damage simultaneously, which is part of what makes the disease so disabling.
In about 97% of ALS cases, a protein called TDP-43 clumps abnormally inside the affected nerve cells. This protein normally lives in the nucleus of the cell, where it helps manage genetic instructions. In ALS, it migrates out of the nucleus, misfolds, and accumulates as toxic deposits in the surrounding cell fluid. These clumps appear regardless of whether the disease was inherited or arose spontaneously, making TDP-43 one of the most consistent biological signatures of the disease.
Where Symptoms Usually Start
ALS doesn’t begin the same way for everyone. In a study of 500 patients, roughly 67% started with limb symptoms, 29% started with bulbar symptoms (affecting the mouth and throat), and about 4% noticed breathing problems first.
Limb-onset ALS typically shows up as weakness or fatigue in an arm or leg, often on one side. You might notice trouble gripping objects, buttoning a shirt, or a foot that seems to drag. These problems feel out of proportion to anything that would explain them, like an injury or overuse.
Bulbar-onset ALS begins in the muscles of the face, jaw, and throat. Early signs include slurred speech, difficulty swallowing, drooling, or trouble chewing. Because these symptoms can mimic other conditions, bulbar-onset ALS sometimes takes longer to diagnose.
How the Disease Spreads
ALS typically starts in one body region and spreads to others in a roughly predictable pattern. Clinicians track this using a staging system based on how many regions are affected: bulbar (mouth and throat), upper limbs, and lower limbs. Stage 1 means one region is involved. Stage 2 means two, stage 3 means three. The later stages, 4A and 4B, mark the points where a person can no longer maintain adequate nutrition or breathing on their own.
The functional losses follow a logical but relentless sequence. Walking and self-care become difficult, then impossible. Swallowing deteriorates, requiring dietary changes and eventually a feeding tube. Speech fades, often to the point where people rely on eye-tracking devices or other assistive technology to communicate. Throughout this progression, the person remains mentally aware of what is happening, which is one of the most difficult aspects of the disease for patients and families alike.
What ALS Does Not Affect
Despite the widespread destruction of motor neurons, ALS largely spares several systems. The sensory nerves that carry information about touch, temperature, and pain continue to function normally. Hearing and vision remain intact. Eye movements are preserved in most cases, which is why many people with advanced ALS can still communicate through eye-tracking technology. Bladder and bowel control also tend to be maintained far longer than other motor functions, because the nerve pathways controlling those muscles are anatomically distinct from the ones ALS targets.
Why Breathing Fails
The primary cause of death in ALS is respiratory failure. Breathing depends on the diaphragm, a dome-shaped muscle beneath the lungs that does most of the work of pulling air in. The nerves controlling the diaphragm originate from the cervical spinal cord, a region that tends to be affected early in the disease process, even before obvious breathing symptoms appear.
As diaphragm strength declines, the body first compensates by recruiting accessory muscles in the neck and chest. But these muscles lack the capacity to sustain normal breathing on their own. The result is a gradual buildup of carbon dioxide and a drop in oxygen levels, often noticed first during sleep, when the accessory muscles naturally relax. Weakness of the expiratory muscles also impairs coughing, which makes it harder to clear mucus from the lungs and increases the risk of pneumonia. Shortness of breath, morning headaches, and excessive daytime sleepiness are common early signs of respiratory involvement.
Inherited Versus Sporadic Cases
About 5 to 10% of ALS cases are familial, meaning the person carries a genetic mutation passed down from a parent. The remaining 90 to 95% are sporadic, with no known family history. A large meta-analysis across 165 studies estimated the overall familial proportion at about 8%. Despite the different origins, sporadic and familial ALS look and behave similarly at the clinical level, and the same TDP-43 protein deposits appear in virtually all cases.
Diagnosis
There is no single blood test or scan that confirms ALS. Diagnosis relies on clinical examination and a test called electromyography (EMG), which measures the electrical activity in muscles. To meet current diagnostic criteria, a person must show progressive motor impairment with evidence of both upper and lower motor neuron damage in at least one body region. The EMG must reveal two specific patterns: signs that nerve connections to muscle fibers have been lost (ongoing denervation) and signs that surviving neurons are trying to compensate by taking over orphaned muscle fibers (chronic neurogenic change). The process often takes months, partly because doctors need to rule out other conditions and partly because they need to document that the symptoms are indeed progressing.
Available Treatments
ALS has no cure. Three medications are currently approved by the FDA, and all of them aim to slow the disease rather than stop it. Riluzole, the oldest of the three, has been shown to extend survival by a few months. Edaravone, approved in 2017, slowed the rate of functional decline compared to placebo in a clinical trial. Over 24 weeks, patients receiving the drug lost about 5 points on a standard function scale, compared to 7.5 points in the placebo group. The third, tofersen, is approved only for the small subset of patients who carry a specific genetic mutation.
Beyond medication, most of what helps people with ALS is supportive care. Ventilatory support, whether noninvasive masks or, later, a tracheostomy, can significantly extend survival and improve quality of life. Feeding tubes maintain nutrition when swallowing becomes unsafe. Speech-generating devices preserve the ability to communicate. Physical and occupational therapy help maintain function and comfort for as long as possible. These interventions don’t change the underlying disease, but they directly affect how someone lives with it.
Survival and Outlook
Median survival from diagnosis is approximately 2.2 years, though this number represents the midpoint of a wide range. The interquartile range spans from about 1.1 to 4.4 years, meaning outcomes vary considerably from person to person. Between 5 and 15% of people with ALS survive much longer. In one cohort study, the median survival among these long-term survivors was 13.4 years. Factors that tend to predict longer survival include younger age at onset, limb-onset rather than bulbar-onset disease, and slower initial rate of decline. The physicist Stephen Hawking, diagnosed at 21 and surviving for 55 years, remains an extraordinary outlier, but he illustrates that the disease’s trajectory is not identical for everyone.