ALS, or amyotrophic lateral sclerosis, is a progressive neurological disease that destroys the nerve cells controlling voluntary movement. It affects both the upper motor neurons in the brain and the lower motor neurons in the spinal cord, gradually stripping away the ability to walk, speak, swallow, and eventually breathe. Median survival is roughly 2 to 3 years from symptom onset, though some people live significantly longer. The disease is sometimes called Lou Gehrig’s disease, after the baseball player diagnosed in 1939.
What Happens Inside the Body
In a healthy nervous system, motor neurons send signals from the brain through the spinal cord to muscles throughout the body. In ALS, these neurons progressively die. As they disappear, muscles no longer receive instructions, so they weaken, twitch, and eventually waste away. The disease targets voluntary muscles, the ones you consciously control, while leaving sensory nerves, bladder function, and cognitive ability largely intact in most cases.
At the cellular level, a key player in most ALS cases is a protein called TDP-43. Normally, TDP-43 sits in the nucleus of a cell and helps manage RNA, the messenger molecules that carry genetic instructions. In ALS, TDP-43 migrates out of the nucleus into the surrounding cell fluid, where it clumps into insoluble masses. These clumps disrupt normal cell operations and damage mitochondria, the structures that generate energy for the cell. Studies in motor neurons expressing abnormal TDP-43 show swollen, dysfunctional mitochondria and elevated levels of damaging molecules called reactive oxygen species. The cell essentially loses both its instruction manual and its power supply.
The body has two cleanup systems for removing damaged or misfolded proteins. Both systems fail to keep up in ALS, allowing toxic protein clumps to accumulate and kill neurons.
Early Symptoms and How They Differ
ALS falls into two main categories based on where symptoms first appear. Limb-onset ALS, the more common form, starts in the arms or legs. You might notice a hand becoming clumsy, difficulty gripping objects, tripping while walking, or muscle twitching in an arm or leg. Weakness typically starts on one side and is painless, which is part of what makes it easy to dismiss early on.
Bulbar-onset ALS begins in the muscles of the head and neck. Early signs include slurred speech, a change in voice quality, or trouble swallowing. This form is less common but tends to progress faster. Regardless of where ALS starts, the weakness eventually spreads to other body regions. Someone with limb-onset ALS will in time develop speech and swallowing problems, and someone with bulbar onset will develop limb weakness.
Muscle twitching (fasciculations) and cramping are common early features in both types. Some people also notice that their muscles feel stiff or tight, a sign of upper motor neuron involvement. The combination of both stiffness and wasting in the same limb is a hallmark of ALS that helps distinguish it from other conditions.
Genetics and Risk Factors
About 5 to 10 percent of ALS cases are familial, meaning they run in families. The rest are classified as sporadic, appearing in people with no family history. More than 40 genes have been linked to ALS so far, and four of them account for up to 70 percent of familial cases in European populations.
The most common genetic cause is a mutation in the C9orf72 gene, responsible for 25 to 40 percent of familial ALS. This mutation involves a short DNA sequence that repeats itself hundreds or thousands of times when it should only repeat a few dozen. The expanded repeats produce toxic proteins called dipeptide repeats, particularly two arginine-rich varieties that are highly damaging to neurons. These toxic proteins interfere with how cells transport materials between the nucleus and cytoplasm, cause DNA damage, and flood synapses with excess glutamate, an excitatory chemical that overstimulates and kills neurons.
The SOD1 gene accounts for 10 to 20 percent of familial cases, TARDBP (the gene encoding TDP-43) about 4 percent, and FUS about 5 percent. Understanding which gene is involved matters increasingly for treatment, since therapies are now being designed to target specific mutations.
How ALS Is Diagnosed
There is no single test for ALS. Diagnosis relies on clinical examination, electrical tests of nerve and muscle function, and ruling out other conditions that can look similar. The process often takes months, which is one of the most frustrating aspects for patients.
Doctors look for simultaneous signs of both upper motor neuron damage (stiffness, brisk reflexes) and lower motor neuron damage (weakness, muscle wasting, twitching) that are spreading over time. The most recent diagnostic framework, the Gold Coast criteria, requires progressive motor dysfunction with evidence of both upper and lower motor neuron involvement in at least one body region, plus tests that exclude other explanations.
Roughly 12 percent of people initially suspected to have ALS turn out to have a different condition. The most common mimics include spinal cord compression from cervical spine problems, peripheral neuropathies, and a muscle disease called inclusion body myositis. MRI scans, blood work, and nerve conduction studies help sort these out. Getting the diagnosis right is critical because several ALS mimics are treatable.
Survival and Prognosis
The median survival from symptom onset is approximately 2.5 years, and from diagnosis about 20 months. The gap between those numbers reflects the diagnostic delay most patients experience. Survival ranges widely: some people live only a year after diagnosis, while others survive a decade or more. Stephen Hawking famously lived more than 50 years after his diagnosis, though his case was exceptionally rare.
Several factors influence prognosis. Bulbar-onset ALS generally carries a shorter survival than limb-onset. Younger age at diagnosis is associated with slower progression. Respiratory function at the time of diagnosis is one of the strongest predictors. Death in ALS is most commonly caused by respiratory failure as the muscles that control breathing weaken.
There is modest evidence that survival has improved slightly over recent decades. Data spanning 1995 to 2018 showed median survival after diagnosis increased by about 10 percent in the most recent period studied, reaching 20 months. This improvement likely reflects better supportive care rather than any breakthrough treatment.
Current Treatments
No treatment cures ALS, but several options can slow progression or manage symptoms. Riluzole, the first drug approved for ALS, modestly extends survival by reducing nerve cell damage from excess glutamate. Edaravone, approved more recently, works as an antioxidant to slow functional decline in some patients.
The most notable recent advance is a drug called tofersen, approved by the FDA specifically for people with ALS caused by SOD1 gene mutations. Tofersen works by intercepting the genetic instructions for the faulty SOD1 protein before it can be made, reducing levels of the toxic protein. In clinical trials, patients receiving tofersen showed significant reductions in a blood marker of nerve damage. This is the first ALS treatment tailored to a specific genetic cause, and a confirmatory trial is ongoing in people who carry the SOD1 mutation but haven’t yet developed symptoms.
For the most common genetic form of ALS, caused by C9orf72 mutations, experimental therapies using a similar approach are in development. Early results from one such drug showed it could reduce levels of a toxic protein in spinal fluid by an average of 37 percent. However, postmortem analysis of treated patients revealed that key disease markers in the brain and spinal cord remained largely unchanged, and patients continued to decline functionally. This suggests the drug reached the nervous system but didn’t achieve the concentration needed to meaningfully alter disease progression, highlighting how difficult treatment development remains.
Living With ALS: The Role of Team-Based Care
Because ALS affects so many body functions, specialized multidisciplinary clinics have become the standard of care. A typical ALS care team includes a neurologist, physical therapist, occupational therapist, respiratory therapist, dietitian, speech therapist, social worker, and neuropsychologist. Depending on your needs, the team may also include home health providers and assistive technology specialists who help with communication devices and mobility equipment.
This team-based approach does more than coordinate logistics. It has been shown to improve quality of life, mental health, social functioning, and survival compared to care from a general neurologist alone. Much of ALS management is about staying ahead of the disease: starting breathing support before respiratory muscles fail significantly, adjusting nutrition before swallowing becomes unsafe, and introducing communication tools while a person can still learn to use them. The goal is to preserve independence and quality of life for as long as possible, even as the disease progresses.