Muscles most Commonly Affected by Myasthenia gravis


Myasthenia gravis (MG) is a long-term neuromuscular disease that leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing. It can result in double vision, drooping eyelids, trouble talking,and trouble walking. Onset can be sudden. Those affected often have a large thymus or develop a thymoma. The initial, main symptom in MG is painless weakness of specific muscles, not fatigue. The muscle weakness becomes progressively worse during periods of physical activity and improves after periods of rest.

In about two-thirds of individuals, the initial symptom of MG is related to the muscles around the eye. There may be eyelid drooping (ptosis due to weakness of  double vision (diplopia, due to weakness of the extraocular muscles). Eye symptoms tend to get worse when watching television, reading, or driving, particularly in bright conditions. Consequently, some affected individuals choose to wear sunglasses. The term "ocular myasthenia gravis" describes a subtype of MG where muscle weakness is confined to the eyes, i.e. extraocular muscles, levator palpebrae superioris, and orbicularis oculi. Typically, this subtype evolves into generalized MG, usually after a few years.

The weakness of the muscles involved in swallowing may lead to swallowing difficulty (dysphagia). Typically, this means that some food may be left in the mouth after an attempt to swallow, or food and liquids may regurgitate into the nose rather than go down the throat. Weakness of the muscles that move the jaw (muscles of mastication) may cause difficulty chewing. In individuals with MG, chewing tends to become more tiring when chewing tough, fibrous foods. Difficulty in swallowing, chewing, and speaking is the first symptom in about one-sixth of individuals.

Due to weakness of the muscles of facial expression and muscles of mastication, facial weakness may manifest as the inability to hold the mouth closed (the "hanging jaw sign") and as a snarling expression when attempting to smile. With drooping eyelids, facial weakness may make the individual appear sleepy or sad. Difficulty in holding the head upright may occur.

The muscles that control breathing  and limb movements can also be affected; rarely do these present as the first symptoms of MG, but develop over months to years. In a myasthenic crisis, a paralysis of the respiratory muscles occurs, necessitating assisted ventilation to sustain life. Crises may be triggered by various biological stressors such as infection, fever, an adverse reaction to medication, or emotional stress.

With Kind Regards,
Mark Orwell
Managing Editor
Journal of Pharmacology and Therapeutic Research.