Acetylcholine Receptor (Muscle AChR) Antibodies, Ach Receptor Ab, AChR (Acetylcholine Receptor), AChR Receptor Blocking Ab, Anti-Skeletal Muscle Antibodies, Binding, Blocking, and Modulating Abs, Blocking Ab, Muscle End-Plate Antibodies, MUSK, MuSK myasthenia, Myasthenia Gravis Antibodies, Myasthenia Gravis, Myoid Antibody, Thymoma, LEMS, LES, Lambert-Eaton Myasthenic syndrome, Lambert Eaton, Lambert-Eaton syndrome, P/Q, Calcium channel antibody
Confirming the autoimmune basis of a defect in neuromuscular transmission (eg, myasthenia gravis [MG], Lambert-Eaton myasthenic syndrome [LEMS])
Distinguishing LEMS from autoimmune forms of MG
Providing a quantitative autoantibody baseline for future comparisons in monitoring a patient's clinical course and response to immunomodulatory treatment
1. Patient should have no general anesthetic or muscle-relaxant drugs in the preceding 24 hours.
2. For optimal antibody detection, specimen collection is recommended prior to initiation of immunosuppressant medication.
3. This test should not be requested in patients who have recently received radioisotopes, therapeutically or diagnostically, because of potential assay interference. The specific waiting period before specimen collection will depend on the isotope administered, the dose given, and the clearance rate in the individual patient. Specimens will be screened for radioactivity prior to analysis. Radioactive specimens received in the laboratory will be held 1 week and assayed if sufficiently decayed, or canceled if radioactivity remains.
Specimens should be collected prior to administration of immunosuppressant therapy as this may reduce the diagnostic sensitivity of the assay; the neurological diagnosis is further confounded if steroid myopathy develops.
These results should only be interpreted in the appropriate clinical and electrophysiological context and are not diagnostic in isolation.
Positive muscle acetylcholine receptor (AChR) may occur in autoimmune liver disorders and in patients with graft-versus-host disease and recipients of D-penicillamine.
Weakly positive results may occur with hypergammaglobulinemia and should be interpreted with caution in the appropriate clinical context.
AChR modulating antibodies will only be performed if AChR binding antibodies are present or if there is an interfering substance present which precludes testing for AChR binding antibodies.
Seropositive rates and quantitative results differ across laboratories and patient results tested at different laboratories should not be treated equivalently.
The presence of alpha-bungarotoxin antibodies may interfere with the AChR muscle binding antibody assay and therefore if detected, AChR binding results will not be reported.
ARBI, CCPQ: Monday- Sunday
ACMRS: Monday, Wednesday, Saturday
MUSK: Monday- Friday
ARBI, CCPQ, MUSK: Radioimmunoassay (RIA)
ACMFS: Flow Cytometry
Myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) are acquired autoimmune disorders of neuromuscular transmission. MG is caused by pathogenic autoantibodies binding and potentially removing (modulation) the muscle's nicotinic acetylcholine receptor (AChR) from the surface of the neuromuscular junction. Serologically, the detection of AChR binding antibody provides the best diagnostic sensitivity. However, the presence of both AChR binding and modulating activity improves diagnostic accuracy. A subset of patients who are AChR seronegative will have muscle-specific kinase (MuSK) antibodies.
LEMS is caused by autoantibodies binding to motor nerve terminal's voltage-gated P/Q-type calcium channel. Synaptic transmission fails when autoantibodies cause a critical loss of junctional cation channel proteins that activate the muscle action potential.
Both MG and LEMS can affect children as well as adults, although LEMS is very rare in children. In adults MG is 10 times more frequent than LEMS, but it is sometimes difficult to distinguish the two disorders clinically. Electrophysiological testing is extremely helpful in distinguishing these 2 disorders. MG patients have decrements of compound muscle action potential (CMAP) amplitudes on repetitive stimulation whereas LEMS has immediate and dramatic post exercise facilitation (elevation) of CMAP amplitudes. Neoplasms associated with LEMS or MG are an endogenous source of the antigens driving production of the autoantibodies that characterize each disorder. In adults with MG, there is at least a 20% occurrence of thymoma and, very rarely (<1%), extrathymic cancers. LEMS is frequently associated (80%) with small-cell lung carcinoma (SCLC). Thus far, MuSK antibody associated MG has not been associated with any neoplasm.
The diagnostic sensitivity of these tests depends on the disease severity and duration of symptoms. AChR binding antibodies may be undetectable for 6 to 12 months after MG symptom onset and similarly P/Q-type calcium channel antibody may be undetectable for 6 to 12 months after LEMS onset. Only about 5% of adult patients with generalized MG who are not immunosuppressed remain seronegative for muscle AChR beyond 12 months. Although immunotherapy is universally beneficial for MG, in LEMS resection of the identified SCLC and initiation of 3-4 diaminopyridine, which facilitates acetylcholine release by increasing presynaptic calcium concentration, is most beneficial.
Note: Single antibody tests may be requested in the follow-up of patients with positive results previously documented in this laboratory.
If acetylcholine receptor (AChR)-binding antibodies are greater than 0.02 nmol/L, then AChR muscle modulating antibody will be performed at an additional charge.
If AChR-binding antibodies are 0.02 nmol/L or less, then muscle-specific kinase (MuSK) autoantibody will be performed at an additional charge.
If unable to report AChR binding antibody due to interfering substances, then AChR muscle modulating antibody will be performed at an additional charge.
If unable to report AChR binding antibody due to interfering substances and AChR muscle modulating antibody is negative, MuSK autoantibody will be performed at an additional charge.
< or = 0.02 nmol/L
Positive results in this antibody evaluation are indicative of an autoimmune neuromuscular junction disorder. These results should be interpreted in the appropriate clinical and electrophysiological context.
In the presence of either acetylcholine receptor (AChR) antibodies or P/Q antibodies, a paraneoplastic basis should be considered with thymoma being the most commonly associated tumor with myasthenia gravis and small cell lung cancer being the most commonly associated cancer with Lambert-Eaton myasthenic syndrome. Currently, muscle-specific kinase (MuSK) antibody positive myasthenia gravis (MG) is not associated with a paraneoplastic etiology.
Negative results do not exclude the diagnosis of an autoimmune neuromuscular junction disorder. If clinical suspicion remains and symptoms persistent or worsen consider re-testing.