ALS (amyotrophic lateral sclerosis) is a progressive disease that destroys the nerve cells controlling voluntary movement, leading to increasing muscle weakness, loss of function, and eventually respiratory failure. Most people survive 20 to 48 months after symptoms first appear, though 10 to 20% live longer than five years and 5 to 10% survive beyond a decade.
Sometimes called Lou Gehrig’s disease, ALS affects both the upper motor neurons in the brain and the lower motor neurons in the spinal cord and brainstem. As these cells die, they stop sending signals to muscles throughout the body. The muscles weaken, shrink, and eventually stop working altogether.
How ALS Damages the Nervous System
Your brain controls voluntary movement through a two-step relay system. Upper motor neurons in the brain send signals down the spinal cord, where lower motor neurons pick them up and deliver them to individual muscles. In ALS, both sets of neurons progressively degenerate, which is what separates it from related conditions. Primary lateral sclerosis (PLS) affects only the upper motor neurons, causing stiffness and slowness. Progressive muscular atrophy (PMA) affects only the lower motor neurons, causing weakness and muscle wasting. ALS hits both, which is why it progresses faster and produces a wider range of symptoms.
The degeneration tends to spread outward from wherever it starts. If it begins in the arms, it typically moves downward through the trunk and legs. If it begins in the muscles controlling speech and swallowing (called bulbar onset), it tends to spread faster to the rest of the body. Researchers still debate whether the process starts in the brain and works its way down, starts at the muscle connections and works backward, or begins independently in multiple locations at once.
Early Symptoms of Limb-Onset ALS
About two-thirds of people with ALS first notice symptoms in an arm or leg. The hallmark early signs are focal, asymmetric weakness, meaning they show up in one specific area and on one side of the body. You might drop things more often, struggle to turn a key, or find that one hand has become clumsy. In the legs, it can start as tripping, foot drop, or a feeling of heaviness when walking.
Because the weakness tends to be subtle and one-sided at first, limb-onset ALS is frequently mistaken for a pinched nerve, carpal tunnel syndrome, or degenerative spinal disease. Muscle twitching (fasciculations) and cramping are also common early on. These twitches happen because dying lower motor neurons fire erratically before they go silent. Fatigue in the affected limb, out of proportion to the activity, is another early clue.
Early Symptoms of Bulbar-Onset ALS
Roughly one-third of cases begin with the muscles of the mouth and throat. The first signs are usually slurred or thickened speech, difficulty swallowing, or excessive drooling. Chewing can become effortful. Some people notice their voice sounds nasal or strained before any limb weakness appears.
Bulbar-onset ALS tends to be diagnosed faster than the limb-onset form, partly because speech and swallowing problems are harder to attribute to something routine. It also carries a less favorable prognosis on average, because the muscles controlling breathing are anatomically close to the bulbar region and tend to become involved sooner.
Upper vs. Lower Motor Neuron Signs
ALS produces two distinct patterns of symptoms depending on which motor neurons are affected in a given area. Damage to upper motor neurons causes stiffness, tightness in the muscles (spasticity), and exaggerated reflexes. You might notice that your limbs feel rigid or that movements feel slow and effortful. Damage to lower motor neurons causes the opposite: muscles become weak, floppy, and visibly smaller as they waste away. Twitching and cramping come from lower motor neuron involvement.
Most people with ALS experience a mix of both patterns, and the balance between them varies widely from person to person. Some people deal primarily with stiffness and spasticity early on, while others notice weakness and wasting first. This variability is one reason the disease can be difficult to diagnose.
Cognitive and Behavioral Changes
ALS was long considered a disease that spared thinking and personality. That view has changed substantially. More than 50% of people with ALS develop some degree of change in language, behavior, memory, or social cognition. These changes can appear early, sometimes before motor symptoms become severe.
The most common cognitive effects involve executive functions: planning, decision-making, mental flexibility, and attention. Some people have trouble with verbal fluency, finding it harder to produce words even beyond what speech muscle weakness would explain. Behavioral changes can include impulsivity, poor judgment, loss of empathy, or apathy. These symptoms overlap with frontotemporal dementia (FTD), and researchers now view ALS and FTD as sitting on a shared spectrum. Up to 50% of ALS patients show cognitive or behavioral impairment on formal testing, though many cases are mild enough to go unrecognized without specific screening.
A smaller subset of patients, roughly 10 to 15%, develop full frontotemporal dementia alongside their motor symptoms. This overlap is especially common in people who carry a specific genetic mutation called C9orf72, which is also the most common inherited cause of ALS.
How ALS Progresses Over Time
ALS follows a pattern of gradual, relentless functional decline, though the speed varies enormously between individuals. Clinicians track progression across four domains: bulbar function (speech and swallowing), fine motor skills (writing, buttoning clothes, handling utensils), gross motor function (walking, climbing stairs, turning in bed), and breathing.
In the middle stages, weakness that started in one area spreads to others. Someone who initially had trouble with one hand may find the other hand weakening, then the legs. Walking becomes difficult, and falls become more common. Fine motor tasks that require coordination, like zipping a jacket or using a phone, grow increasingly frustrating. Speech may become harder to understand, and meals take longer as swallowing slows.
In the advanced stages, most people need a wheelchair and assistance with daily activities like dressing and bathing. Swallowing becomes unsafe enough that a feeding tube is often placed to prevent choking and malnutrition. Breathing muscles weaken progressively, which causes fatigue, morning headaches, poor sleep, and difficulty concentrating due to low oxygen levels. Respiratory failure is the most common cause of death in ALS, and most people eventually need some form of ventilatory support.
Who Gets ALS
ALS strikes roughly 2 per 100,000 people each year. About 90% of cases are sporadic, meaning they appear without any family history. The remaining 10% are familial, caused by inherited gene mutations. The most significant of these is a repeat expansion in the C9orf72 gene, which accounts for 30 to 60% of familial cases and 5 to 10% of sporadic ones. Other important genes include SOD1, TARDBP, and FUS.
The average age of onset is in the mid-50s to early 60s, with limb-onset ALS most commonly appearing in middle-aged adults. Men are slightly more likely to develop ALS than women, though this gap narrows with age. Beyond genetics, no definitive environmental cause has been established, though researchers continue to investigate links to physical trauma, military service, and certain occupational exposures.
How ALS Is Diagnosed
There is no single blood test or scan that confirms ALS. Diagnosis rests on clinical evaluation: a neurologist looks for signs of both upper and lower motor neuron damage, confirms that symptoms are progressive, and systematically rules out other conditions that can mimic ALS, such as cervical spine compression, myasthenia gravis, or multifocal motor neuropathy.
The most current diagnostic framework, called the Gold Coast criteria, simplifies earlier systems into a yes-or-no determination. It requires three things: documented progressive motor decline after a period of normal function, signs of both upper and lower motor neuron involvement in at least one body region (with EMG data counting as evidence of lower motor neuron damage), and thorough testing to exclude other diagnoses. Nerve conduction studies and electromyography (EMG), which measures electrical activity in muscles, play a central role. MRI of the brain and spine, blood work, and sometimes spinal fluid analysis help rule out mimics. The process from first symptom to confirmed diagnosis typically takes 9 to 12 months, partly because doctors need to observe progression over time.