ALS, or amyotrophic lateral sclerosis, progressively destroys the motor neurons that connect the brain and spinal cord to voluntary muscles, gradually stripping the body of its ability to move, speak, swallow, and breathe. The disease typically kills within three to five years of symptom onset, with respiratory failure being the most common cause of death. But the path from first symptom to last breath is neither simple nor uniform, and ALS touches more of the body than many people realize.
Where ALS Starts in the Body
ALS does not always begin the same way. Roughly two-thirds of cases start in the limbs, often with a foot that drags, a hand that fumbles, or an arm that feels oddly weak. This is called limb-onset ALS. The remaining cases begin with the muscles of the mouth, tongue, and throat, a form called bulbar-onset ALS, which typically shows up first as slurred speech or trouble swallowing.
The distinction matters for prognosis. Bulbar-onset ALS tends to carry a worse outlook, with brain imaging showing more widespread degeneration that spreads earlier into frontal and temporal regions beyond the primary motor areas.1PubMed Central. Patterns of grey and white matter changes differ between bulbar and limb onset amyotrophic lateral sclerosis Limb-onset cases, by contrast, tend to show ongoing structural damage concentrated within the motor regions for longer before spreading outward. In both forms, the disease eventually reaches all voluntary muscles, but the starting point shapes the early experience and the speed of decline.
What Goes Wrong Inside Motor Neurons
At the cellular level, the hallmark of ALS in more than 95% of cases is the abnormal behavior of a protein called TDP-43. This protein normally lives in the nucleus of motor neurons and other brain cells, where it helps manage genetic instructions. In ALS, TDP-43 gets pulled out of the nucleus and clumps together in the surrounding cell body.2PubMed Central. TDP-43-The key to understanding amyotrophic lateral sclerosis The result is a double hit: the toxic clumps themselves damage the cell, and the nucleus loses the protein it needs to function normally. This two-pronged assault drives motor neurons toward death.3Aging and Health Research. Advancements in TDP-43 research: Towards biomarkers and therapeutic targets for amyotrophic lateral sclerosis
Recent research has begun identifying what triggers TDP-43 to misbehave. One line of work found that losing a repair enzyme called ASRGL1 causes TDP-43 to misfold, fragment, and accumulate in neurons.4Nature Communications. TDP-43 proteinopathy in ALS is triggered by loss of ASRGL1 and associated with HML-2 expression When researchers restored ASRGL1, neuronal survival improved, which at least hints at future therapeutic targets. But the field still does not know what initiates the entire cascade in most patients. The pathology seems to spread through the nervous system in a way that follows neuroanatomical connections, as if the disease physically propagates from one region to the next.5PubMed Central. Clinical Spectrum of Amyotrophic Lateral Sclerosis (ALS)
How Muscles Lose Their Connection to the Brain
One of the most revealing discoveries about ALS progression is that muscles do not suddenly lose contact with their motor neurons. Instead, the process at the neuromuscular junction, where nerve meets muscle fiber, is surprisingly dynamic. In mouse models, individual branches of a motor neuron are dismantled one at a time over weeks, and the neuron simultaneously sprouts new branches to try to reconnect with nearby muscle fibers.6eLife. Dynamic neuromuscular remodeling precedes motor-unit loss in a mouse model of ALS The body is fighting to compensate even as the disease advances.
This ongoing cycle of denervation and attempted reinnervation means that early in the disease, muscle function can remain surprisingly intact even though damage is already accumulating underneath. Denervation tends to spread outward from the first lost connection, and the farthest branches of a motor neuron seem to be the most vulnerable. Eventually, the compensatory sprouting cannot keep pace with the losses, and whole motor units collapse. That is when weakness becomes obvious.
Muscle Weakness, Spasticity, and Pain
The symptoms people most associate with ALS, progressive weakness and muscle wasting, come from the combined loss of upper and lower motor neurons. Lower motor neuron damage causes muscles to weaken, shrink, and sometimes visibly twitch. Upper motor neuron damage adds spasticity, an abnormal stiffness where muscles resist being moved because they have lost the brain’s normal inhibitory control.
Pain is more common in ALS than many people expect. As muscles weaken and waste, they stop supporting joints properly. Shoulders and hips are the most frequently affected. The resulting joint pain tends to worsen as the disease progresses. Spasticity contributes its own pain through persistent muscle stiffness and spasms. One study found that about a third of ALS patients experience spasticity, and roughly half of those report at least mild pain from it.7Journal of Yeungnam Medical Science. Pain in amyotrophic lateral sclerosis: a narrative review Immobility also causes pressure-related discomfort, though actual pressure sores are less common than you might expect, for reasons that touch on a surprising feature of the disease discussed later.
Speech, Swallowing, and Voice
When ALS reaches the bulbar motor neurons, the muscles that control the tongue, lips, palate, and larynx begin to fail. Speech becomes slurred, then increasingly unintelligible. Swallowing grows difficult and eventually dangerous, as the coordinated muscle contractions that guide food safely past the airway become unreliable. In bulbar-onset cases, voice changes and swallowing trouble are often the very first signs of disease.8PubMed Central. Laryngeal dysfunction in Amyotrophic Lateral Sclerosis: a review and case report
Bulbar-onset ALS also appears to carry a unique burden on the brain itself. Research has identified pathological changes in brain regions associated with speech and language processing, specifically Broca’s and Wernicke’s areas, that were seen in bulbar-onset cases but not in limb-onset cases.9PubMed Central. The neuropathological signature of bulbar-onset ALS: A systematic review Some bulbar-onset patients also showed unusual pathological features such as neurofibrillary tangles that were absent in spinal-onset cases. This suggests that bulbar ALS may not simply be the same disease starting in a different location, but may involve partially distinct patterns of brain degeneration.
How Breathing Deteriorates
Respiratory decline is the single most consequential change in ALS, and it is often insidious. The diaphragm, the dome-shaped muscle beneath the lungs that does most of the work of breathing, loses motor units just like the limb muscles. In ALS patients, both the slow-twitch and fast-twitch fibers of the diaphragm shrink significantly compared to healthy tissue. The normal checkerboard pattern of mixed fiber types breaks down into clusters of same-type fibers, a sign that surviving motor neurons have tried to compensate by reinnervating orphaned muscle fibers nearby.10European Respiratory Journal. Human diaphragm atrophy in amyotrophic lateral sclerosis is not predicted by routine respiratory measures
As the diaphragm weakens, the first problems usually show up during sleep, especially when lying on the back. In that position, gravity pushes the abdominal organs against the weakened diaphragm, making it even harder to draw a full breath. The result is low oxygen and rising carbon dioxide levels that can go unnoticed during the day but fragment sleep profoundly.11PubMed Central. Diaphragmatic Neurophysiology and Respiratory Markers in ALS Nocturnal hypoventilation, the medical term for this insufficient breathing during sleep, tends to appear before full-blown respiratory failure sets in.12PubMed Central. Sleep disturbances in patients with amyotrophic lateral sclerosis: current perspectives
Sleep quality deteriorates across multiple measures. A meta-analysis of sleep studies in ALS patients found significant reductions in sleep efficiency, total sleep time, deep sleep, and blood oxygen levels, along with increases in time spent awake after falling asleep and in time spent with dangerously low oxygen saturation.13PubMed. Sleep in amyotrophic lateral sclerosis: A systematic review and meta-analysis of polysomnographic findings Poor sleep accelerates fatigue, worsens cognitive function, and compounds every other symptom the person is already coping with.
The Body’s Metabolic Shift
ALS does something counterintuitive to metabolism: even as patients lose muscle mass and become less physically active, their bodies burn more energy at rest. A study comparing ALS patients to healthy controls found that resting metabolic rates were significantly higher in the ALS group, about 1,500 calories per day compared to 1,230 calories per day in controls. More than half of the ALS patients met the criteria for hypermetabolism, compared to only about 13% of controls.14Journal of the Neurological Sciences. Hypermetabolism is a reality in amyotrophic lateral sclerosis compared to healthy subjects
This metabolic mismatch creates a dangerous cycle. Patients are burning extra calories while simultaneously losing the ability to eat enough, especially once swallowing becomes difficult. Weight loss accelerates, and nutritional decline has been consistently linked to faster disease progression and shorter survival. Many ALS clinics now monitor weight and caloric intake closely, and feeding tubes are among the most common interventions recommended as swallowing fails.
Cognitive and Behavioral Changes
For decades, ALS was understood as a disease that left the mind untouched while the body failed. That understanding has changed substantially. Current estimates suggest that roughly half of people with ALS remain cognitively normal, but about 35% experience mild to moderate cognitive or behavioral changes, and around 15% develop full frontotemporal dementia.15PubMed. Neuropsychological impairment in amyotrophic lateral sclerosis-frontotemporal spectrum disorder ALS is now understood as a spectrum disorder that can range from pure motor disease at one end to ALS with frontotemporal dementia at the other.
The cognitive changes, when they appear, tend to cluster in specific areas. Difficulty generating words, trouble with planning and mental flexibility, and impaired social reasoning are the most common. On the behavioral side, apathy is the most frequent change, but disinhibition, loss of empathy, repetitive behaviors, and altered eating habits also occur.16PubMed. Cognitive and behavioural impairment in amyotrophic lateral sclerosis These symptoms reflect damage spreading beyond motor neurons into the frontal and temporal lobes, and they can make the disease significantly harder to manage for caregivers. In some cases, cognitive decline actually precedes motor symptoms, with the person developing behavioral changes characteristic of frontotemporal dementia before any limb weakness appears.17PubMed Central. Frontotemporal Dementia and Amyotrophic Lateral Sclerosis: A Case Report and Clinical Insights
What ALS Tends to Spare
Given how relentlessly the disease attacks voluntary movement, it is striking that certain motor neuron pools resist degeneration far longer than others. Eye muscles are among the best-known examples. Even in advanced ALS, when nearly all other voluntary muscles have failed, the ability to look around usually persists. This preserved eye movement is not just a clinical curiosity; it forms the basis for eye-tracking communication devices that become a patient’s primary link to the outside world.
Another spared region is a cluster of motor neurons in the lower spinal cord called Onuf’s nucleus, which controls the muscles of the pelvic floor, including those governing bladder and bowel sphincters. While some cellular abnormalities do appear in Onuf’s nucleus in ALS patients, the neurons there maintain their size and resist degeneration even in advanced disease, unlike motor neurons elsewhere in the spinal cord.18Frontiers in Neuroanatomy. The Dual Nature of Onuf’s Nucleus: Neuroanatomical Features and Peculiarities, in Health and Disease The reasons for this selective protection are still not fully understood, though differences in the molecular and physicochemical properties of these neuron populations likely play a role. For patients, the practical result is that bladder and bowel control generally remains intact much longer than limb function.
Autonomic Nervous System Involvement
ALS was long thought to leave the autonomic nervous system, which governs involuntary functions like heart rate, blood pressure, and sweating, entirely alone. That is an oversimplification. A large study testing autonomic function in 132 ALS patients found that three-quarters had at least some measurable abnormality. The impairment was typically mild, with abnormalities in sweating found in about 46% and heart rate control problems in 50%. Only about 14% showed changes in blood pressure regulation.19PubMed Central. Autonomic system and ALS No patients had severe generalized autonomic failure.
Sweat function testing has added further detail. ALS patients show reduced sweating responses in the lower extremities compared to controls, pointing to mild damage in the small nerve fibers that control sweat glands.20PubMed. Sudomotor dysfunction in amyotrophic lateral sclerosis These autonomic changes rarely cause dramatic symptoms on their own, but they tell us that ALS is not as cleanly confined to motor neurons as textbooks once taught. The disease touches small fibers and autonomic pathways too, just not with the devastating force it brings to voluntary motor circuits.
Tracking Progression With Biomarkers
One of the frustrations of managing ALS is that the disease progresses at wildly different rates in different people. Some patients decline rapidly over months; others live more than a decade. Researchers have been searching for blood or spinal fluid markers that could predict the pace of decline. The most promising candidate is a protein called neurofilament light chain, which leaks out of damaged nerve cells into the cerebrospinal fluid. Higher levels of this protein correlate with worse functional scores and faster progression rates.21PubMed Central. CSF Neurofilament Light Chain Elevation Predicts ALS Severity and Progression It is not specific enough to diagnose ALS on its own, since other neurological conditions also elevate it, but it is increasingly used alongside clinical assessments to estimate how quickly a given patient may be losing ground.
Changes Beyond the Nervous System
ALS even leaves traces in the skin. Researchers have documented structural abnormalities in the skin of ALS patients, including reduced density of small nerve fibers, abnormal protein deposits, altered mitochondria, and inflammatory changes.22PubMed Central. Skin pathology in ALS: Diagnostic implications and biomarker potential One particularly unexpected finding is that the collagen in ALS patients’ skin becomes denser and more tightly packed as the disease progresses.23PubMed. Increased dermal collagen density in amyotrophic lateral sclerosis This change has been documented in skin over the lower back, one of the areas most vulnerable to pressure sores in immobilized patients. The thicker, denser collagen may partly explain why ALS patients develop pressure sores less frequently than you would expect given their profound immobility, though this connection remains speculative.
These skin findings also hold practical interest as potential biomarkers. A skin biopsy is far less invasive than a spinal tap, and if skin changes can reliably track disease progression, they could offer clinicians another window into what is happening inside the nervous system.
How the Disease Reaches Its End
Respiratory failure is the leading cause of death in ALS, and it reflects everything the disease has done to the breathing apparatus over months or years. By the time the diaphragm and accessory breathing muscles have wasted beyond the point of compensation, even with non-invasive ventilation support, carbon dioxide levels rise and oxygen levels fall to unsustainable levels. A study of death certificates among ALS patients in the United States confirmed that respiratory failure was the most frequent cause of death aside from ALS itself, followed by cardiovascular disease and cardiac arrest.24PubMed Central. Causes of death among United States decedents with ALS: An eye toward delaying mortality
Pneumonia was the fourth most common cause, a predictable consequence of weakened cough muscles and impaired swallowing that allows food or saliva to enter the lungs. Pneumonitis, an inflammation caused by aspiration of stomach contents, and foreign objects trapped in the airway or esophagus due to swallowing muscle failure also appeared among the leading causes.24PubMed Central. Causes of death among United States decedents with ALS: An eye toward delaying mortality Essentially, the final months of ALS represent the convergence of every system the disease has weakened: breathing muscles that cannot clear secretions, swallowing muscles that cannot protect the airway, and a body that is burning calories it cannot replace. The respiratory system is where these failures become fatal, but the vulnerability is built from the full sweep of damage ALS has done along the way.