Myasthenia gravis is defined by weakness and rapid fatigue of any of the muscles under voluntary control. It is caused by a breakdown in the normal communication between nerves and muscles.
There is no cure for the disease but treatments can relieve symptoms such as weakness of the limbs, double vision, swallowing and breathing.
Myasthenia gravis can affect people of any age but it more commonly occurs in women younger than 40 and in men older than 60.
Myasthenia gravis causes muscle weakness that worsens with repetitive use of the affected muscles. Symptoms usually improve with rest; however, the disease progresses over time until symptoms reach the worst levels.
Symptoms of myasthenia gravis may include:
- Drooping of one or both eyelids (ptosis)
- Double vision (diplopia), which may be horizontal or vertical
- Altered speech
- Difficulty swallowing
- Problems chewing
- Limited facial expressions
- Muscle weakness in the limbs, neck, eyes or face
The nerves communicate with the muscles by releasing chemicals (neurotransmitters) into receptor sites on the muscle cells at the nerve-muscular junction.
Myasthenia gravis causes a production of antibodies that block or destroy muscle receptor sites for a neurotransmitter called acetylcholine.
Fewer receptor sites mean that the muscles receive fewer nerve signals, which results in weakness. Antibodies may also block the function of a protein called a muscle-specific receptor tyrosine kinase that is involved in forming the nerve-muscular junction.
Experts believe that the thymus gland, a part of the immune system located in the upper chest beneath the breastbone, can trigger or maintain the production of antibodies that block acetylcholine.
In adults with myasthenia gravis, the thymus gland is abnormally large. In some cases, tumors can develop on the thymus (thymomas) but they are usually benign.
Myasthenia gravis that is caused by other factors is called antibody-negative myasthenia gravis. Antibodies against another protein called lipoprotein-related protein 4 can contribute to the development of the condition. Furthermore, genetic factors can be associated with development of neonatal myasthenia gravis and congenital myasthenic syndrome.
Factors that can worsen the condition
- Extreme heat
- Certain medications (beta-blockers, quinidine gluconate, quinidine sulfate, quinine, certain anesthetics and antibiotics)
Myasthenic crisis is a life threatening condition that occurs with weakening of the muscles that control breathing. Emergency treatment can provide mechanical assistance with breathing. Medications and blood-filtering therapies can help in regaining breathing abilities.
Benign thymus tumors (thymomas) are discovered in around 15% of myasthenia gravis cases
- Underactive or overactive thyroid (fluctuations in hormones that regulate the metabolism)
- Autoimmune conditions (rheumatoid arthritis or lupus)