Myasthenia Gravis

Myasthenia gravis is a disease that causes weakness, but unlike other neurological conditions which cause weakness such as myopathy or ALS, the weakness of myasthenia gravis is variable (can wax and wane). The weakness of myasthenia gravis (trouble with the strength of a muscle) is different than fatigue (general lack of energy) or sleepiness. Typically the weakness is worse later in the day or after a prolonged use of the muscle. Some forms of myasthenia affect only the eyes and eyelids (causing drooping eyelids and double vision); some affect only the bulbar (speech and swallowing) muscles; while some cases are associated with weakness throughout the body. Myasthenia is usually controllable immune suppressing medications or medications to increase the amount of a certain neurotransmitter (acetylcholine) in the neuromuscular junction.

More information at:
Myasthenia Gravis Foundation of America
National Institute of Neurological Disorders and Stroke (NINDS)