ALS affects the motor neurons, the nerve cells that control voluntary muscle movement throughout the body. This means it progressively impairs the ability to move your limbs, speak, swallow, and eventually breathe. Around 33,000 people in the United States are living with ALS, and the disease touches nearly every muscle system while leaving certain functions, like sensation and bladder control, largely intact.
How Motor Neurons Break Down
Your body has two types of motor neurons that work together to produce movement. Upper motor neurons live in the brain and send signals down the spinal cord. Lower motor neurons sit in the spinal cord and carry those signals out to individual muscles. ALS destroys both types, but they degenerate in different patterns.
When lower motor neurons die, damage tends to spread from one arm to the other, following the anatomy of the spinal cord. Upper motor neuron damage follows a different map: it spreads from an arm down to the leg on the same side, tracing the layout of the brain’s motor cortex. Because these two systems deteriorate on their own timelines and in their own directions, the pattern of weakness becomes increasingly complex as the disease progresses. This is also why no two people experience ALS in exactly the same way.
Limbs: Where It Usually Starts
About two-thirds of people first notice ALS in an arm or leg. It often begins with muscle twitching, cramping, or weakness in the hands or feet. You might drop things more often, trip while walking, or struggle to button a shirt. These early symptoms can be easy to dismiss, which is one reason the average time from first symptom to diagnosis stretches to about a year.
Once weakness appears in one limb, it typically spreads to nearby muscle groups before reaching more distant ones. A study of lower-limb onset ALS found that overall survival from symptom onset roughly equaled two years plus however long it took for weakness to appear beyond the first affected limb. In other words, faster spread generally means a shorter disease course, while slower spread is associated with longer survival. Median survival in that study was 41 months from symptom onset, though the range was enormous: 5 months to over 23 years.
Speech and Swallowing
Around 85% of people with ALS develop noticeable problems with speech and swallowing at some point during the disease. Some people experience these as their very first symptoms, a pattern called bulbar-onset ALS, which accounts for roughly a quarter to a third of cases.
Speech changes often appear before words become hard to understand. Speaking rate slows down first, partly as the brain compensates for weakening tongue muscles by relying more on jaw movement. Over time, speech becomes slurred and increasingly difficult for others to follow. Swallowing problems are particularly dangerous because weakened throat and tongue muscles can allow food or liquid to slip into the airway. Many people with ALS experience “silent” aspiration, meaning material enters the airway without triggering a cough, leaving the person and their caregivers unaware of the problem. This raises the risk of lung infections significantly.
Beyond the physical risks, losing the ability to communicate and eat normally takes a serious toll on mental well-being and social life. Many people eventually use communication devices and modified diets or feeding tubes to maintain nutrition and stay connected.
Breathing and Respiratory Muscles
Respiratory involvement is the most life-threatening aspect of ALS and accounts for the majority of deaths from the disease. ALS weakens the muscles you use to breathe in (especially the diaphragm), the muscles you use to breathe out (abdominal and chest wall muscles), and the muscles of the upper airway.
Early breathing changes are subtle. You might feel unusually tired during everyday activities or wake up frequently during the night. That’s because the diaphragm weakens first while lying flat, so nighttime breathing suffers before daytime breathing does. Mild oxygen drops during sleep can also impair thinking and concentration, compounding the cognitive challenges some people with ALS already face.
Coughing becomes weaker as expiratory muscles fail. A strong cough requires the diaphragm to contract at nearly 50% of its maximum force, a threshold that becomes impossible to reach as motor neurons die. Weak coughs mean the lungs can’t clear mucus effectively, which opens the door to infections. Combined with swallowing problems that allow food and saliva into the airway, this creates a cycle of respiratory decline that requires close monitoring from early in the disease.
Cognitive and Behavioral Changes
ALS was once considered a disease that spared the mind entirely. That view has changed. Between 5% and 22% of people with ALS also develop frontotemporal dementia, a condition that affects personality, decision-making, and language. A larger group, perhaps up to half, experiences milder cognitive or behavioral shifts that don’t meet the threshold for dementia but are still noticeable: difficulty with planning, impulsivity, apathy, or trouble finding words.
These changes reflect the fact that ALS and frontotemporal dementia share overlapping biology. The connection matters practically because cognitive changes can affect a person’s ability to manage their care, use assistive devices, or make medical decisions.
What ALS Does Not Affect
Despite the breadth of damage ALS causes, several functions are typically preserved, at least until very late in the disease. Eye movements remain intact in most cases, which is why many people with advanced ALS communicate using eye-tracking technology. Bladder and bowel control are generally spared because the specific motor neurons controlling those functions (located in a region of the spinal cord called the Onuf nucleus) are resistant to the disease. Sensory nerves, the ones that let you feel touch, temperature, and pain, are not motor neurons and remain functional. You can still feel everything happening to your body, even when you can no longer move.
This preservation of sensation and awareness is one of the most difficult aspects of ALS. The mind and senses remain largely intact inside a body that progressively loses the ability to act on their instructions. In patients who survive for many years, often with ventilator support, even the traditionally spared systems like eye movement can eventually show some involvement, but this is the exception rather than the rule.
How ALS Is Diagnosed
There is no single blood test or scan that confirms ALS. Diagnosis still rests on clinical evaluation: a neurologist looking for signs of both upper and lower motor neuron damage, documenting that symptoms are progressive, and ruling out other conditions that can mimic the disease. The most recent diagnostic framework, known as the Gold Coast criteria, simplifies earlier systems by making the diagnosis a yes-or-no determination rather than assigning levels of certainty. It requires progressive motor impairment after a period of normal function, evidence of both upper and lower motor neuron involvement, and thorough testing to exclude other causes. Nerve conduction studies and electromyography play a supporting role, with electrical signs of lower motor neuron damage considered equivalent to what a doctor can see on a physical exam.