Anti-MuSK (muscle-specific kinase) Antibodies

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.

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