The onset of ALS may be so subtle that the symptoms are frequently overlooked. The earliest symptoms are obvious weakness and/or muscle atrophy. This is followed by twitching, cramping, or stiffness of affected muscles; muscle weakness affecting an arm or a leg; and/or slurred and nasal speech. The twitching, cramping, etc. associated with ALS is a result of the dying muscle, therefore these symptoms without clinical weakness or atrophy of affected muscle is likely not ALS.
The parts of the body affected by early symptoms of ALS depend on which motor neurons in the body are damaged first. About 75% of people experience "limb onset" ALS. In some of these cases, symptoms initially affect one of the legs, and patients experience awkwardness when walking or running or they notice that they are tripping or stumbling more often. Other limb onset patients first see the effects of the disease on a hand or arm as they experience difficulty with simple tasks requiring manual dexterity such as buttoning a shirt, writing, or turning a key in a lock. Occasionally the symptoms remain confined to one limb; this is known as monomelic amyotrophy.
About 25% of cases are "bulbar onset" ALS. These patients first notice difficulty speaking clearly. Speech becomes garbled and slurred. Nasality and loss of volume are frequently the first symptoms. Difficulty swallowing, and loss of tongue mobility follow. Eventually total loss of speech and the inability to protect the airway when swallowing are experienced.
Regardless of the part of the body first affected by the disease, muscle weakness and atrophy spread to other parts of the body as the disease progresses. Patients experience increasing difficulty moving, swallowing (dysphagia), and speaking or forming words (dysarthria). Symptoms of upper motor neuron involvement include tight and stiff muscles (spasticity) and exaggerated reflexes (hyperreflexia) including an overactive gag reflex. An abnormal reflex commonly called Babinski's sign (the large toe extends upward as the sole of the foot is stimulated) also indicates upper motor neuron damage. Symptoms of lower motor neuron degeneration include muscle weakness and atrophy, muscle cramps, and fleeting twitches of muscles that can be seen under the skin (fasciculations). Around 15–45% of patients experience pseudobulbar affect, also known as "emotional incontinence", which consists of uncontrollable laughter, crying or smiling, attributable to degeneration of bulbar upper motor neurons resulting in exaggeration of motor expressions of emotion.
