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.