Anti-MuSK (muscle-specific kinase) Antibodies
When you have symptoms that suggest MG, such as a drooping eyelid, double vision, difficulty chewing or swallowing, weak neck muscles, and difficulty walking and the result of your acetylcholine receptor (AChR) antibody test is negative
A blood sample drawn from a vein
Anti-muscle-specific kinase (anti-MuSK) antibodies are autoantibodies, proteins produce by the immune system that mistakenly attack proteins called muscle-specific kinases. This test detects these autoantibodies in the blood of people with myasthenia gravis (MG), a rare but often severe autoimmune disease that causes muscle fatigue and weakness.
Muscle-specific kinases are proteins found where nerve endings and muscle fibers come together (neuromuscular junctions). Although the cause of MG is not fully understood, it is thought that the anti-MuSK antibodies interfere with the transmission of signals between nerves and muscles.
MG is typically first noticed when it causes weakness in eye muscles and symptoms such as drooping eyelid(s) and/or double vision. This is often referred to as ocular MG and about 15% of people with MG have ocular problems only. From the eye muscles, MG can spread over time to facial and neck muscles, causing weakness, slurred speech, difficulty chewing and swallowing, and/or difficulty breathing. When muscle weakness spreads from the eyes to the head and neck and other parts of the body, the condition is called generalized MG. Muscle weakness will vary over time. The weakness tends to worsen during physically activity but improves with rest.
Most people with MG have antibodies directed against the acetylcholine receptor (AChR antibodies). However, about 10% to 15% of people with MG do not have AChR antibodies. In cases where AChR antibodies are not detected, a healthcare practitioner will likely order an anti-MuSK antibody test. Estimates of the proportion of people with MG who are negative for AChR antibodies but positive for antibodies to MuSK range from about 6% to 50%. This condition is sometimes called MuSK MG.
According to the Myasthenia Gravis Foundation of America, people with anti-MuSK antibodies are more likely to have the generalized form of myasthenia gravis, compared to ocular myasthenia gravis, which affects only the eyes and eyelids. Women are more often affected by autoimmune MuSK MG than men, and onset can occur at any age. People with MuSK MG may respond differently to certain treatments compared to those with AChR-positive MG.
How is the test used?
An anti-MuSK antibody test is used to help diagnose myasthenia gravis (MG) in people with signs and symptoms associated with MG but who test negative for AChR antibodies. Testing is used to distinguish MG from other conditions that may cause similar symptoms, such as chronic muscle fatigue and weakness.
When is it ordered?
Some signs and symptoms of MG include:
- Drooping eyelid
- Double vision
- Decreased control of eye movement
- Difficulty swallowing and/or chewing, leading to choking, drooling and gagging
- Slurred speech
- Weak neck muscles
- Trouble holding up your head
- Difficulty breathing
- Difficulty walking and an altered gait
- Specific muscle weakness but normal feelings/sensations
- Muscle weakness that worsens during physical activity and improves with rest
What does the test result mean?
MuSK antibodies are not normally present in the blood. A negative anti-MuSK antibody test means it is less likely that you have MG. However, a negative result does not rule out MG. In about 6% to 10% of MG cases, neither AChR nor MuSK antibodies are present. These individuals are said to have double-seronegative MG. Results from other tests for autoantibodies, such as anti-LRP4 (LDL-receptor-related protein 4) antibody test and anti-striated muscle antibody test, may aid in establishing a diagnosis.
If you have a positive test result for anti-MuSK antibodies and symptoms of MG, then it is likely that you have autoimmune muscle-specific kinase myasthenia gravis.
How long will it take for results?
This test requires specialized instruments and trained lab personnel and is not offered by every laboratory. Your blood sample will likely need to be sent to a reference laboratory for testing, so it may take a few days for results to be available.
How serious is myasthenia gravis (MG)?
Most people who have MG can live a normal or near normal life with treatment and monitoring. One of the most serious complications is a respiratory myasthenic crisis that can occur when muscles that control breathing are weakened. This can be a medical emergency and often requires hospitalization. For additional details, see the article on Myasthenia Gravis.
Does it matter which antibodies I have?
Is there anything I can do to prevent getting MuSK binding antibodies?
Can MG be inherited?
Autoimmune disorders are not caused by changes or variants in a specific gene, so they are not considered inherited conditions. They can run in families, however. Some people may inherit a genetic change that causes congenital myasthenic syndrome, which is an inherited condition with similar symptoms as MG but a different cause. For additional details, see the links in Related Content.
Is there anything else I should know?