Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease that affects the motor neurons in the brain and spinal cord that control voluntary muscles. These cells gradually die off, leading to muscle weakness, atrophy, and eventual paralysis throughout the body. ALS typically begins focally, meaning symptoms usually start localized to one specific body region, often presenting unilaterally, or on one side of the body. This initial presentation is a defining characteristic that helps distinguish ALS from other neurological conditions.
The Unilateral Beginning: Focal Onset Patterns
The initial manifestation of ALS is classified into two main patterns based on where the motor neuron degeneration first occurs. The most common form is limb onset, where symptoms first appear in an arm or a leg, accounting for 70% to 75% of all cases. This onset is typically asymmetrical, starting with weakness or stiffness in a single limb. For instance, a person might notice an unexplained tripping or foot drop on only one side, or they may experience difficulty with fine motor tasks, like buttoning a shirt or turning a key, using only one hand.
The initial symptoms in limb-onset ALS can reflect damage to either upper motor neurons, causing stiffness and spasticity, or lower motor neurons, leading to muscle weakness, twitching (fasciculations), and atrophy. In the lower limbs, the first sign is frequently a weakness in the muscles of the foot and ankle. When the disease starts in the upper limbs, patients may first notice a loss of dexterity or grip strength, often starting in the small muscles of the hand. This localized, one-sided presentation is a direct result of the disease process targeting a specific segment of the spinal cord.
The second major presentation is bulbar onset, which accounts for about 25% of cases. This form begins in the muscles controlled by the brainstem, known as the bulbar region, which are responsible for speech, chewing, and swallowing. Initial signs include slurred speech (dysarthria) or difficulty swallowing (dysphagia). While bulbar symptoms affect the midline structures of the face and throat, they represent a highly localized start before the disease spreads to the spinal cord.
Progression Beyond the Initial Site
Following its focal beginning, the disease inevitably progresses as the motor neuron loss extends to other body regions. This spread is most often contiguous, meaning it moves to adjacent muscle groups and nearby segments of the central nervous system before affecting distant areas. For example, if the initial symptoms began in the right hand, the weakness would typically move up the right arm to the shoulder before crossing the midline to the left arm or descending to the trunk or legs.
The degeneration moves from one spinal or brainstem region to the next, often crossing the midline to affect the corresponding limb on the opposite side. This progression from a focal, unilateral presentation to a generalized, bilateral, and systemic disease can vary significantly in its speed among individuals. The predictable, contiguous nature of the spread suggests a mechanism where the pathology may propagate from cell to cell within the nervous system.
Functions That Remain Intact
Despite the widespread loss of voluntary muscle control, ALS typically spares several important neurological functions. The sensory nervous system, which is responsible for the five senses, generally remains unaffected. This means that the ability to see, hear, taste, smell, and feel touch is preserved, even in advanced stages of the disease.
Additionally, the muscles controlling the bladder and bowel are usually spared, as the motor neurons that regulate these involuntary functions are distinct from those destroyed by ALS. For many patients, the muscles that control eye movement, known as the extraocular muscles, also retain their function until very late in the disease. While cognitive changes and behavioral symptoms can occur in a subset of patients, the core defining feature of ALS is the loss of motor function, and many individuals maintain their intellectual capacity throughout the disease.