What Is Amyotrophic Lateral Sclerosis (ALS)?
Amyotrophic Lateral Sclerosis (ALS) is also commonly known as Lou Gehrig’s disease and, in Chinese, as “gradually freezing disease.” This vivid term captures the lived experience of many patients: the body slowly becomes immobilized, as if it were being frozen in place.
Symptoms of ALS often begin in the hands, feet, or legs, and then gradually spread to other parts of the body. As motor neurons progressively degenerate and undergo apoptosis, muscle weakness extends throughout the body, eventually affecting chewing, speech, and breathing. The renowned physicist Stephen Hawking lived with ALS, which helps explain why he relied on a wheelchair, lost the ability to speak, and required assistance for movement—his body appearing as though it were slowly encased in ice (Figure 1).
Figure 1. Stephen Hawking
Why Does the Body Gradually Lose the Ability to Move, as If It Were Frozen?
Our bodies are controlled by the brain. However, for the brain to command movement, signals must travel through motor neurons in the brain and spinal cord to reach the muscles. Once a muscle receives these signals, it contracts, allowing movement to occur.
In Amyotrophic Lateral Sclerosis, motor neurons in the brain and spinal cord begin to degenerate, shrink, or die. When these neurons are lost, muscles no longer receive instructions. Without regular neural input, muscles gradually waste away and lose their strength. This is why the body appears to become progressively “frozen.”
How Does the Disease Begin?
Unlike many rare diseases, most cases of ALS are not caused by inherited genetic mutations. Instead, the majority are sporadic, meaning that most patients have no family history of the disease, and the exact cause remains unclear.
ALS does not cause someone to “wake up one day unable to move.” Rather, it advances like a slowly spreading line of ice—quietly beginning in one part of the body and gradually extending outward. The onset of ALS typically starts in a small, localized region, most commonly in one of three ways.
The most frequent onset involves the limbs, including the hands, feet, arms, or legs. Early signs may include difficulty using chopsticks, trouble opening bottle caps, frequent tripping while walking, or weakness when climbing stairs.
In other cases, ALS may begin with bulbar onset, affecting the mouth, tongue, and throat. Speech may become slurred, swallowing liquids may lead to choking, and chewing can feel exhausting.
Less commonly, ALS may begin in the respiratory muscles. Patients may experience shortness of breath during sleep, nighttime breathing difficulties, or a sensation of breathlessness even at rest.
Why Do the Symptoms Gradually Spread?
The core mechanism of ALS is the continuous loss of motor neurons that control muscle movement. As signal transmission is progressively disrupted, muscles lose their neural guidance and eventually undergo atrophy.
Importantly, motor neuron death does not occur simultaneously throughout the body. Instead, it begins in a specific region and then spreads along neural networks, eventually affecting the entire body.
Why Do ALS Patients Remain Mentally Clear?
ALS freezes the body, but not the mind. Cognitive function and sensory awareness are usually preserved. Patients remain fully conscious and aware, as if trapped within their own bodies—one of the most devastating aspects of the disease.