Acetylcholine Receptor (AChR) Antibody

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.

AchR antibodies hinder the action of acetylcholine, a chemical (neurotransmitter) that transmits messages between nerve cells. The antibodies do this in three major ways:

• "Binding" antibodies attach to the acetylcholine receptors on nerve cells and may initiate an inflammatory reaction that destroys them.
• "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 test that measures "binding" antibodies are most commonly used because modulating or blocking antibodies are not found on their own, the binding antibody is always present as well. Sometimes the antibody binds for a very short time and is difficult to detect in normal assays and then a “high avidity” test may be used. This is only done if there is a high clinical suspicion of myasthenia gravis and the first test is negative.

AChR antibody tests may be requested as part of a group of tests that may also to help establish a diagnosis. Depending upon results, an anti-MuSK (muscle-specific kinase) antibody test may also be requested and if they are also negative a striated muscle antibody test. Some laboratories measure specific subsets of these antibodies – Titin and Ryanodine antibodies.

The AChR antibody test may be requested initially as a baseline test and then as indicated to evaluate MG disease activity and/or response to therapy.

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 in a developing baby and early life, but normally becomes largely inactive after puberty. 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.

The AChR antibody test may be requested when a person has symptoms that suggest MG, such as:

• Drooping eyelids – worsening as the day wears on
• Double vision
• Decreased eye movement control
• Difficulty swallowing, chewing, with choking, drooling and gagging
• Slurred speech
• Weak neck muscles
• Trouble holding up 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

When a person has been diagnosed with MG, an AChR antibody test may be requested occasionally to evaluate MG disease activity and/or response to therapy.

An AChR antibody test may sometimes be requested when a thymoma is detected.

AChR antibodies are not normally present in the blood. They are autoantibodies and their presence indicates an autoimmune response.

If a person has AChR antibodies and symptoms of MG, then it is likely that the person has this condition, although very low level antibodies may be seen in individuals who never develop the condition.

AChR antibodies may be seen with some thymomas, in people who are being treated with drugs such as penicillamine, with some small cell lung cancers, with autoimmune liver disease, and with Lambert-Eaton myasthenic syndrome (a condition associated with interference with the release of acetylcholine from the nerve ending). In Lambert-Eaton Calcium Channel antibodies may also be seen.

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 generalised MG will be negative for AChR antibodies.

In general, the greater the quantity of AChR antibody, the more likely a person is to have myasthenia, but the test results cannot be used to evaluate the severity of symptoms in a specific person.

Use of drugs such as succinylcholine can increase AChR antibodies.

People who have MG are more likely to also have other autoimmune disorders, such as rheumatoid arthritis or systemic lupus erythematosus.

It is not intended for general screening purposes and most people will never need to have one done.