Myasthenia gravis (MG) is an autoimmune disorder that impairs communication between nerve and muscle cells, resulting in muscle weakness.
The clinical hallmark of MG is muscle weakness usually involving specific susceptible muscle groups, which worsens with repetitive movement. In some individuals, the disease process may stay confined to the muscles around the eyes, described as ocular myasthenia. In those with more generalized MG, affected muscles may include those of the eyes, face, jaw, throat region, arms and legs, and even muscles involved in breathing (respiratory muscles).
Although the disorder usually becomes apparent during adulthood, the onset of the symptoms may occur at any age, be sudden, and often not be readily diagnosed as MG. The age of onset tends to be different among men and women with men usually developing the disease later when they are in their 60s or 80s, and women developing it as early as their 20s and 30s.
Eyes and eyelids
In its initial presentation, MG tends to affect the muscles that control the movement of the eyes and eyelids, causing ocular weakness. This is seen in approximately 85 percent of all MG patients. Consequently, a partial paralysis of eye movements, double vision (diplopia), and drooping of the upper eyelids (ptosis) usually are among the first symptoms of MG.
In about 10 percent of MG patients, symptoms are limited to eye muscles, a situation called ocular myasthenia. But for most people, the weakness spreads to other parts of the body within two years of disease onset.
Face and throat muscles
In about 15 percent of patients with MG, early symptoms also involve weakness of the face and throat muscles. In such cases, associated symptoms may include:
- Altered speaking or dysarthria, causing the voice to sound weak, hoarse, or nasal.
- Difficulty swallowing (dysphagia), leading to choking easily, which makes it difficult to eat, drink or take pills. In some cases, liquids may come out from the nose.
- Problems chewing due to the muscles used for chewing wearing out halfway through a meal, particularly when eating something hard to chew.
- Limited facial expressions such as “snarling” smile. This may occur if the muscles that control the facial expressions have been affected.
Limbs and other parts of the body
In generalized MG, weakness tends to spread sequentially from the face and neck to the upper limbs, the hands, and then to the lower limbs. If this happens it may become difficult to:
- Lift the arms over the head
- Perform physical tasks, such as lifting, rising from a sitting position, walking long distances, climbing stairs or gripping heavy objects.
Constant tiredness and aching muscles after physical activity also can be a sign of MG. The weakness tends to be worse in the upper parts of the body than in the legs and feet.
When MG affects the muscles that control breathing, it can cause myasthenic crisis, a medical emergency that often involves hospitalization and may require the temporary use of a respirator to assist with breathing.
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