Myasthenia Gravis : Neuromuscular Disorder : Causes & Diagnosis
Myasthenia gravis is a neuromuscular disorder characterized by variable weakness of voluntary muscles, which often improves with rest and worsens with activity. The condition is caused by an abnormal immune response.
Causes & Risk Factors
In myasthenia gravis, weakness occurs when the nerve impulse to initiate or sustain movement does not adequately reach the muscle cells. This is caused when immune cells target and attack the body’s own cells (an autoimmune response). This immune response produces antibodies that attach to affected areas, preventing muscle cells from receiving chemical messengers (neurotransmitters) from the nerve cell.
The cause of autoimmune disorders such as myasthenia gravis is unknown. In some cases, myasthenia gravis may be associated with tumors of the thymus(an organ of the immune system). Patients with myasthenia gravis have a higher risk of having other autoimmune disorders like thyrotoxicosis,rheumatoid arthritis, and systemic lupus erythematosus.
Myasthenia gravis affects about 3 of every 10,000 people and can affect people at any age. It is most common in young women and older men.
Tests & Diagnostics
Examination may be normal or may show muscle weakness that progressively worsens as the muscle is used. In many patients the eye muscles are affected first. Reflexes and sensation are normal. Weakness may affect the arms, legs, breathing or swallowing muscles, and any other muscle group.
- Repetitive stimulation (type of nerve conduction study) is more sensitive.
- Single-fiber EMG can be very sensitive.
- Acetylcholine receptor antibodies may be present in the blood.
- A Tensilon test is positive in some cases but must be interpreted carefully by an experienced doctor. Baseline muscle strength is evaluated. After Tensilon (edrophonium, a medication that blocks the action of the enzyme that breaks down the transmitter acetylcholine) is given, muscle function may improve.
- The patient may need a CT or MRI scan of the chest to look for a possible tumor in the thyroid.