What is muscle weakness? Muscle weakness, or myasthenia, is a decrease in strength in one or more muscles. It is a common symptom of muscular, neurological and metabolic disorders. Muscular diseases, such as muscular dystrophy and dermatomyositis (disorder characterized by muscle inflammation), are common causes of muscle weakness. Other common causes include neurological disorders, such as Guillain-Barre syndrome (an autoimmune nerve disorder), amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease), stroke, and even a pinched nerve. The autoimmune neuromuscular disorder known as myasthenia gravis is accompanied by muscle weakness along with drooping eyelids and double vision. Metabolic disorders, such as Addison’s disease and hyperthyroidism, can lead to weakness in one muscle or a group of muscles. In rare cases, muscle weakness may be a symptom of Charcot-Marie-Tooth disease (an inherited disorder affecting the peripheral nerves). Other possible causes of muscle weakness include paralytic shellfish poisoning, botulism, and low levels of potassium in the blood. Depending on the cause, weakness may occur in one muscle, a group of muscles, or all the muscles, and it may be accompanied by pain, atrophy, cramping, or other types of muscular symptoms. In some cases, muscle weakness that happens suddenly, especially on one side of the body, can be a sign of stroke. If it occurs along with severe abdominal pain, it may by a symptom of botulism. Seek immediate medical care (call 911) if you, or someone you are with, have serious symptoms, such as a sudden change in vision, confusion, loss of consciousness for even a brief moment, severe abdominal pain, severe headache, and paralysis or inability to move a body part. Seek prompt medical care if you are being treated for muscle weakness but mild symptoms recur or are persistent.