From MedlinePlus Genetics
Myasthenia gravis is a disorder that causes weakness of the skeletal muscles, which are muscles that the body uses for movement. The weakness most often starts in the muscles around the eyes, causing drooping of the eyelids (ptosis) and difficulty coordinating eye movements, which results in blurred or double vision. In a form of the disorder called ocular myasthenia, the weakness remains confined to the eye muscles. In most people with myasthenia gravis, however, additional muscles in the face and neck are affected. Affected individuals may have unusual facial expressions, difficulty holding up the head, speech impairment (dysarthria), and chewing and swallowing problems (dysphagia) that may lead to choking, gagging, or drooling.

Other muscles in the body are also affected in some people with myasthenia gravis. The muscles of the arms and legs may be involved, causing affected individuals to have changes in their gait or trouble with lifting objects, rising from a seated position, or climbing stairs. The muscle weakness tends to fluctuate over time; it typically worsens with activity and improves with rest.

Weakness of the muscles in the chest wall and the muscle that separates the abdomen from the chest cavity (the diaphragm) can cause breathing problems in some people with myasthenia gravis. About 10 percent of people with this disorder experience a potentially life-threatening complication in which these respiratory muscles weaken to the point that breathing is dangerously impaired, and the affected individual requires ventilation assistance. This respiratory failure, called a myasthenic crisis, may be triggered by stresses such as infections or reactions to medications.

People can develop myasthenia gravis at any age. For reasons that are unknown, it is most commonly diagnosed in women younger than age 40 and men older than age 60. It is uncommon in children, but some infants born to women with myasthenia gravis show signs and symptoms of the disorder for the first few days or weeks of life. This temporary occurrence of symptoms is called transient neonatal myasthenia gravis.  https://medlineplus.gov/genetics/condition/myasthenia-gravis