ALS (amyotrophic lateral sclerosis) destroys the nerve cells that control voluntary movement, gradually taking away the ability to walk, talk, eat, and eventually breathe. The disease has a lifetime risk of about 1 in 400, and the number of cases worldwide is projected to increase by 70% by 2040. Understanding how ALS works, from its earliest signs to how it progresses, can help you recognize what’s happening and what to expect.
What Happens Inside the Body
ALS targets two types of nerve cells: upper motor neurons in the brain and lower motor neurons in the spinal cord and brainstem. These neurons are responsible for sending signals from your brain to your muscles. When they degenerate and die, the muscles they control weaken, shrink, and eventually stop working altogether.
The destruction happens through several overlapping processes. One of the most significant is called excitotoxicity. Motor neurons communicate using a chemical messenger called glutamate. In ALS, glutamate builds up excessively at the junctions between neurons because the cleanup system (a transporter on nearby support cells) stops working properly. This floods the receiving neuron with calcium, which damages its internal energy factories and triggers the production of toxic molecules called reactive oxygen species. That oxidative damage can, in turn, further impair the glutamate cleanup system, creating a vicious cycle.
Protein clumping is another hallmark. Certain proteins inside motor neurons misfold and aggregate, disrupting normal cell function. The most commonly implicated protein is TDP-43, found in the vast majority of ALS cases. Beyond these processes, mitochondrial dysfunction, chronic inflammation driven by the brain’s immune cells, and failures in the cell’s recycling machinery all contribute to motor neuron loss. Researchers once believed the neurons died through a single, orderly self-destruct process. It’s now clear that multiple forms of cell death, including inflammatory and iron-dependent pathways, play a role.
Early Signs and How ALS Begins
About two-thirds of people with ALS first notice problems in an arm or leg. This limb-onset form typically starts as asymmetric weakness: difficulty gripping objects, fumbling with buttons, a foot that drags, or unexplained fatigue in one limb. Because it often affects middle-aged people with focal weakness, it’s frequently mistaken at first for a spinal disc problem or pinched nerve.
Roughly 29% of cases begin with bulbar-onset ALS, meaning the first muscles affected are those controlling speech and swallowing. Early signs include slurred speech, difficulty chewing, drooling, or trouble swallowing liquids. A small percentage, about 4%, present first with breathing difficulties, which is the rarest and often most rapidly progressing form.
How Doctors Confirm a Diagnosis
There is no single blood test or scan that confirms ALS. Diagnosis relies on clinical examination and a process of ruling out other conditions. The current Gold Coast diagnostic criteria require evidence that both upper and lower motor neurons are affected, that the disease is spreading to new body regions over time, and that no other explanation fits.
Upper motor neuron damage shows up as abnormal reflexes (like a Babinski sign, where the big toe extends upward when the sole of the foot is stroked), spasticity, and slowed coordination. Lower motor neuron damage appears as visible muscle weakness, wasting, and twitching. An electromyography (EMG) test detects electrical changes in muscles that indicate nerve damage, including signs of ongoing denervation and chronic nerve loss. EMG is particularly useful for revealing damage in muscles that don’t yet show obvious weakness on examination. That said, it’s not perfectly sensitive and depends heavily on the skill of the person performing it, especially in cases where upper motor neuron signs dominate.
To count as meaningful, lower motor neuron abnormalities need to appear in at least two body regions (bulbar, cervical, thoracic, or lumbosacral). Additional tests like MRI and nerve conduction studies help exclude mimics such as multiple sclerosis, spinal cord compression, or inflammatory nerve conditions.
The Role of Genetics
Between 5% and 10% of ALS cases are familial, meaning there’s a known family history. The inheritance pattern is typically autosomal dominant with incomplete penetrance, so carrying a gene variant doesn’t guarantee developing the disease. Four genes account for about 55% of familial cases in people of European ancestry: C9orf72, SOD1, TARDBP, and FUS. Pathogenic variants in over 30 genes have been linked to ALS overall.
Interestingly, the same four major genes also show up in about 7% of apparently sporadic cases, people with no family history. This blurs the line between familial and sporadic ALS and suggests that genetic vulnerability plays a broader role than family history alone would indicate. For the majority of sporadic cases, however, no clear genetic cause has been identified.
Environmental and Occupational Risk Factors
Several environmental exposures have been linked to higher ALS risk, though none are considered definitive causes. Pesticide exposure, including Agent Orange, heavy metals, and chemical fumes have all been associated with increased risk, particularly in military populations. Veterans of the Vietnam War with high herbicide exposure had roughly double the odds of developing ALS compared to those with low exposure. Gulf War veterans who served near the Khamisiyah munitions demolition site in Iraq, where chemical warfare agents were released, showed elevated risk as well.
Lead exposure stands out among heavy metals. A doubling of blood lead levels was associated with a 2.6-fold increase in ALS odds in a study of veterans. Head injuries also appear to matter: veterans over age 30 who had experienced head trauma had nearly twice the risk, with the association strongest for injuries occurring within 15 years of diagnosis. Higher blood levels of selenium and zinc were associated with lower risk, while elevated copper levels were linked to higher risk.
How the Disease Progresses
Progressive weakness is the defining feature. ALS spreads from its starting point to adjacent body regions, though the sequence and speed vary enormously from person to person. Someone who starts with weakness in one hand may next notice problems in the same arm, then the opposite arm, then the legs. Someone with bulbar onset may lose clear speech before limb weakness becomes apparent.
Median survival from symptom onset is roughly 2 to 5 years, with most large studies placing it around 2.5 years from onset (or about 20 months from diagnosis, accounting for the delay in getting diagnosed). Recent registry data from Italy showed a modest improvement over the past decade, with median survival after diagnosis increasing by about 10% between 2013 and 2018. This improvement was most evident in patients with moderate rates of progression.
Respiratory failure is the usual cause of death. As the muscles that control breathing weaken, the body can no longer maintain adequate oxygen levels or clear carbon dioxide. This makes respiratory monitoring one of the most important aspects of ongoing care.
Available Treatments
Six FDA-approved medications currently exist for ALS. Three of them are formulations of riluzole: the original tablet (Rilutek, now generic), a thickened liquid version (Tiglutik) designed for people with swallowing difficulties, and a dissolvable oral film (Exservan). Riluzole works by reducing glutamate signaling at the synapse, directly targeting the excitotoxicity pathway. It modestly extends survival.
Radicava (edaravone) targets oxidative stress and has been shown to slow functional decline in some patients. Qalsody (tofersen) is a more targeted therapy designed specifically for people with SOD1 gene mutations, a small subset of ALS patients. Nuedexta treats a specific symptom called pseudobulbar affect, the involuntary episodes of laughing or crying that some people with ALS experience due to damage in the brain’s emotional control circuits.
None of these medications cure ALS or halt progression. Their effects are incremental, and much of ALS care focuses on managing symptoms and maintaining quality of life.
Supportive Care and Breathing Support
Multidisciplinary care, involving neurologists, respiratory therapists, speech therapists, dietitians, and physical therapists working together, is the backbone of ALS management. Non-invasive ventilation, a mask-based breathing support system used at home, is one of the most impactful interventions available. When started at the right time, it can improve both survival and quality of life by up to 18 months.
Optimizing this breathing support depends on several factors: coordinated team-based care, proper selection of the mask interface, ventilator settings tailored to the individual, and effective management of airway secretions. As swallowing worsens, nutritional support through modified food textures or a feeding tube becomes important for maintaining weight and energy. Assistive communication devices help people who lose the ability to speak remain connected and independent for as long as possible.