Lou Gehrig’s disease, or ALS, typically starts with subtle, easy-to-miss muscle weakness in one part of the body. About 80% of cases begin in the arms or legs (called spinal onset), while the remaining 20% start with changes in speech or swallowing (called bulbar onset). The disease can be underway for months before anyone suspects ALS, with a median time of 11.5 months from the first symptom to a confirmed diagnosis.
What Happens Inside the Body First
Long before you notice any weakness, the disease is already damaging motor neurons, the nerve cells that carry signals from your brain to your muscles. Growing evidence suggests ALS may actually originate in the brain’s motor cortex rather than in the muscles or spinal cord. One of the earliest detectable changes is cortical hyperexcitability: the brain’s motor region becomes overactive because the chemical signals that normally keep nerve firing in check start to break down.
In a healthy brain, there’s a careful balance between signals that excite neurons and signals that calm them down. In ALS, both sides of this balance go wrong at once. The calming signals weaken because the brain loses inhibitory neurons and produces less of the chemical that dampens nerve activity. At the same time, a signaling chemical called glutamate begins to build up around nerve cells. Glutamate is the brain’s primary “go” signal, and when it accumulates in excess, it forces motor neurons into a state of constant overactivation. This floods the cells with calcium, exhausts their energy supply, and eventually kills them. This process, called excitotoxicity, is one of the central engines driving ALS from its earliest stages.
The Role of Protein Buildup
In about 97% of ALS cases, a protein called TDP-43 behaves abnormally. This protein normally lives in the nucleus of a cell, where it helps manage genetic instructions. In ALS, TDP-43 gets displaced from the nucleus into the surrounding cell fluid, where it misfolds and clumps together. This creates a two-hit problem: the protein can no longer do its normal job of transporting genetic material (a loss of function), and the misfolded clumps actively recruit healthy copies of the protein, pulling them out of commission too (a gain of toxic function).
Importantly, even before visible clumps form, elevated levels of displaced TDP-43 floating in the cell’s fluid are toxic on their own. Researchers consider this early misfolding one of the most promising points for future treatment because it happens so early in the disease process.
The First Symptoms You’d Notice
The most common early sign of ALS is gradual, painless muscle weakness. What makes it tricky is that it usually starts in just one area, which is easy to write off as a minor injury or aging. Depending on where the disease begins, early symptoms look quite different.
In spinal-onset ALS, the first signs tend to be weakness or fatigue in an arm or leg. You might start tripping while walking, dropping objects, or struggling with tasks that require fine hand movements like buttoning a shirt or turning a key. Muscle cramps and twitching (fasciculations) are also common early on, though these symptoms alone are extremely common in healthy people and rarely indicate ALS.
In bulbar-onset ALS, the earliest change is usually in the voice. Speech may become slurred, slow, or take on an unusual nasal quality. Swallowing can feel effortful. Speech impairment can actually begin up to three years before a formal ALS diagnosis, though on average, speech remains functional for about 18 months after the first bulbar symptoms appear. Bulbar-onset cases tend to progress faster because the muscles involved control critical functions like breathing and swallowing.
Some people also experience episodes of uncontrollable laughing or crying that feel out of proportion to the situation. This emotional symptom, while less talked about, can be an early feature of the disease.
Why Diagnosis Takes So Long
The median time from first symptom to confirmed diagnosis is 11.5 months, with some patients waiting 20 months or more. Part of the delay is that early ALS mimics many other, more common conditions. The typical path involves about four months before someone sees a doctor, then roughly three more months before ALS is even suspected, and another month before it’s confirmed.
There is no single blood test or scan that confirms ALS. Diagnosis still relies on clinical evaluation: a neurologist looks for signs of damage to both upper motor neurons (in the brain) and lower motor neurons (in the spinal cord and peripheral nerves) in at least one body region, confirms that symptoms are progressing over time, and rules out other conditions through testing. The current Gold Coast criteria simplified earlier diagnostic frameworks but still require this combination of upper and lower motor neuron involvement along with documented progression.
Sporadic vs. Inherited ALS
About 90% of ALS cases are sporadic, meaning they occur without any family history or identifiable genetic cause. The remaining 10% are familial, linked to inherited gene mutations. The two most common genetic culprits are mutations in the C9orf72 gene and the SOD1 gene. C9orf72 mutations account for the larger share, with an estimated 4,545 prevalent cases worldwide compared to about 2,876 for SOD1.
One surprising finding is that even these “familial” mutations show up frequently in people with no known family history of ALS. About 74% of people carrying the C9orf72 mutation and 53% of those with SOD1 mutations are classified as sporadic cases. This means a genetic contribution may be more common than the 10% familial figure suggests, but in many cases the family history simply isn’t apparent.
Environmental Factors Linked to Onset
For the majority of sporadic cases, the trigger remains unknown. Several environmental exposures have been studied, though none have been proven to directly cause ALS. Pesticide exposure is one of the more consistently supported risk factors, with meta-analyses showing roughly double the risk among people with significant exposure. Certain heavy metals, particularly selenium, have also been linked to elevated risk in some studies. Researchers have additionally found a neurotoxic amino acid called BMAA, produced by certain algae, in the brain tissue of indigenous Pacific Islanders who had unusually high rates of ALS-like disease, though this connection remains an area of active investigation.
The current scientific view is that sporadic ALS likely results from a combination of genetic susceptibility and environmental exposures accumulating over a lifetime, rather than any single trigger. This “multiple hit” model helps explain why the disease typically appears between ages 55 and 75 and why pinpointing a single cause has proven so difficult.