Acetylcholine Receptor (AChR) Antibody
When you have symptoms that suggest MG, such as a drooping eyelid, double vision, difficulty chewing or swallowing, and/or weakness in specific voluntary muscles
A blood sample drawn from a vein in your arm
Acetylcholine receptor (AChR) antibodies are autoantibodies produced by the immune system that mistakenly target proteins called acetylcholine receptors that are located on muscles that you can consciously or voluntarily control (known as skeletal muscle fibers). This test detects and measures AChR antibodies in the blood.
Muscle movement starts when an impulse is sent down a nerve to the nerve ending, where it stimulates the release of acetylcholine, a chemical substance (neurotransmitter) that transmits messages between specific types of cells. Acetylcholine travels across the very small gap between the nerve ending and a muscle fiber (this gap is called the "neuromuscular junction"). When acetylcholine reaches the muscle fiber, it binds to one of many acetylcholine receptors or "docking stations" and activates it, initiating muscle contraction.
AChR antibodies impede communication between nerves and skeletal muscles, inhibit muscle contraction, and cause rapid muscle fatigue by preventing activation of the acetylcholine receptors. They do this in three major ways:
- Binding antibodies attach to the receptors on nerve cells and may initiate an inflammatory reaction that destroys the receptors.
- Blocking antibodies may sit on the receptors, preventing acetylcholine from binding.
- Modulating antibodies may cross-link the receptors, causing them to be taken up into the muscle cell and removed from the neuromuscular junction.
The end result of this interference is the development of myasthenia gravis (MG), a chronic autoimmune disorder associated with the presence of these antibodies and with their effects on muscle control.
AChR antibodies may be detected in different ways to determine which mechanism may be the problem, and the antibodies may be referred to as "binding," "blocking," or "modulating." However, the technique that measures "binding" is the most commonly performed and, generally speaking, it is rare for the other two tests to be positive without the "binding" test being positive as well. These other two tests may be useful when a healthcare practitioner strongly suspects myasthenia gravis and the "binding" test is negative.
How is the test used?
An acetylcholine receptor (AChR) antibody test is used to help diagnose myasthenia gravis (MG) and to distinguish it from other conditions that may cause similar symptoms, such as chronic muscle fatigue and weakness.
Three types of AChR antibodies may be tested:
- AChR binding antibodies
- AChR blocking antibodies
- AChR modulating antibodies
The test that measures binding antibodies is most commonly used because it is generally rare for the other two tests to be positive without the binding antibody test being positive as well. These other two tests may be used when a healthcare practitioner strongly suspects myasthenia gravis and the binding antibody test is negative.
One or more of these AChR antibody tests may be ordered as part of a panel of tests that may also include a striated muscle antibody test to help establish a diagnosis. If AChR antibody test results are normal but a healthcare practitioner strongly suspects myasthenia gravis, an anti-MuSK (muscle-specific tyrosine kinase) antibody test may also be ordered.
People with MG often have an enlarged thymus gland and may have thymomas (typically benign tumors of the thymus). Located under the breastbone, the thymus is an active part of the immune system during childhood but normally becomes less active during the teen years. If a thymoma is detected, such as during a chest computed tomography (CT) scan done for a different reason, then an AChR antibody test may sometimes be used to determine whether the person has developed these antibodies.
When is it ordered?
- Drooping eyelid
- Double vision
- Decreased eye movement control
- Difficulty swallowing, chewing, with 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 with sustained effort and improves with rest
In patients with known myasthenia gravis, repeat AChR antibody tests may be done to monitor response to therapy, to guide disease management, or to assess the risk of AChR antibody transfer from a mother to her unborn child.
An AChR antibody test may sometimes be ordered when a thymoma is detected during an imaging scan.
What does the test result mean?
If you have AChR antibodies and symptoms of myasthenia gravis (MG), then it is likely that you have this condition. The extent to which AChR antibodies are elevated does not predict the severity of the disease at the time of diagnosis. However, changes in AChR antibody concentrations over time may help inform the effectiveness of treatment or predict the recurrence of disease. Therefore, repeat testing of AChR antibodies may be requested in some cases.
AChR antibodies may also be positive with some thymomas, in people who are being treated with drugs such as penicillamine, with some small cell lung cancers, with autoimmune liver disease, with Guillain-Barre syndrome, and with Lambert-Eaton myasthenic syndrome (a condition associated with interference with the release of acetylcholine from the nerve ending).
A negative test result does not rule out MG. Up to 50% of those with ocular MG (affecting only eye-related muscles) and about 10-15% of those with generalized MG will be negative for AChR antibodies. Repeat testing of AChR antibodies in those with initially negative results may be useful as the concentration of antibodies may increase as the disease progresses, resulting in a subsequent positive AChR antibody test result.
Results from other tests for autoantibodies, such as anti-MuSK (muscle-specific kinase) antibody test, anti-LRP4 (LDL-receptor-related protein 4) antibody test, and anti-striated muscle antibody test, may also aid in establishing a diagnosis.
Can this test be performed in my healthcare provider's office?
How serious is myasthenia gravis (MG)?
Most people who have it 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.
Can myasthenia gravis affect my heart?
Is there anything I can do to prevent getting AChR binding antibodies?
Can MG be passed from one person to another?
Can MG be inherited?
Is there anything else I should know?