Analysis of false negative results in the immunoassay for anti-acetylcholine receptor antibodies in myasthenia gravis

Seronegative generalised myasthenia gravis: clinical features, antibodies, and their targets

Myasthenia gravis (MG) is a well-recognised disorder of neuromuscular transmission that can be diagnosed by the presence of antibodies to the acetylcholine receptor (AChR). However, some patients (about 15%) with generalised MG do not have detectable AChR antibodies. There is some evidence, however, that this "seronegative" MG is an antibody-mediated disorder. Plasma from patients with the disorder seems to contain various distinct humoral factors: IgG antibodies that reversibly inhibit AChR function; a non-IgG (possibly IgM) factor that indirectly inhibits AChR function; and an IgG antibody against the muscle-specific kinase (MuSK). The presence of antibodies against MuSK appears to define a subgroup of patients with seronegative MG who have predominantly localised, in many cases bulbar, muscle weaknesses (face, tongue, pharynx, etc) and reduced response to conventional immunosuppressive treatments. Moreover, muscle wasting may be present, which prevents complete response to these therapies.

Tensilon and the diagnosis of myasthenia gravis: are we using the Tensilon test too much?

BACKGROUND

Tensilon (edrophonium chloride) is a reversible acetylcholinesterase inhibitor used in the diagnosis of myasthenia gravis, diagnosis and treatment of arrhythmias, detection of early digitalis toxicity, reversal of neuromuscular blockade, and other medical conditions. Toxicity associated with Tensilon use has appeared in the literature for decades.

REVIEW SUMMARY

This review discusses the risks of Tensilon and the information practitioners should know before administering the drug. We review the literature regarding serious toxicity of this drug and offer recommendations for its safe use.

CONCLUSIONS

A careful medication history should be taken before the administration of Tensilon. Additionally, physicians should be aware of appropriate alternative methods of diagnosis before choosing to administer Tensilon. Physicians should be aware of the clinical situations where the Tensilon test no longer is indicated.

Clinical neuroimmunology

Clinical research has focused on autoimmune disease (AID) for a couple of decades. More sensitive and specific methods have been developed for neuroimmunological research. Gamma fraction bands (bands separated by electrophoresis and visualized by amino black staining) and IgG fraction bands (bands separated by iso-electric focusing and visualized by immunostaining) are used instead of oligoclonal bands. Myasthenia gravis (MG) mainly involves acetylcholine receptors of the postsynaptic membrane at the neuromuscular junction. Myasthenia gravis has been considered to be a generalized AID, because 7% of patients with myasthenia gravis associate with other AIDs and more than one autoimmune antibody is detected in 52.5% patients with myasthenia gravis. Pyramidal signs in myasthenia gravis patients are described; the possible mechanism may at least be partly due to the acetylcholine receptor antibody. P2 protein and its antibody are studied in patients with acute and chronic inflammatory demyelinating polyneuropathy.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Anti-neuroblastoma antibodies in myasthenia gravis: clinical and immunological correlations

Forty-four of 109 myasthenia gravis (MG) patients (40%) had serum antibodies against human neuroblastoma cells (NBL). Anti-NBL antibodies were most frequent in the sera of MG patients who had either a hyperplastic thymus or a thymoma, clinically mild to moderately severe generalized MG, and a long disease duration (greater than or equal to 11 years). No correlation between individual anti-NBL antibody and anti-acetylcholine receptor (AChR) antibody titers was observed. Seven of the 19 patients negative for anti-AChR antibodies (37%) had anti-NBL antibodies in their sera. These findings provide further evidence for immunological heterogeneity in MG. In addition to the typical autoantibodies to the AChR, autoimmunization against neural antigens can frequently be detected in these patients.

Clinical correlations of antibodies that bind, block, or modulate human acetylcholine receptors in myasthenia gravis

Acetylcholine receptor (AChR) binding and AChR modulating antibodies were found with approximately the same frequency (86%) in 349 patients with myasthenia gravis (MG). However, the total yield of positive serological results was significantly improved (90%) by assaying AChR modulating antibodies when AChR binding antibodies were not detected, because in 27 patients (8%) only one of the two tests was positive. The immunoprecipitation test for AChR blocking antibodies yielded fewer positive results (52%), but there was a significant correlation between the degree of AChR blockade and generalization of muscle weakness. In no patient was this the only positive test, because the test for AChR modulating antibodies in this study detected both AChR blocking and modulating antibodies. Human muscle AChR was used as antigen in all tests. False positive results were rare and were attributed to unexplained antibodies to 125I-alpha-Bgt (AChR binding antibody assay) and recent general anesthesia using muscle relaxants (AChR blocking and AChR modulating antibody assays). Unexplained positive results, documented previously in 5% of patients with the Lambert-Eaton myasthenic syndrome and amyotrophic lateral sclerosis (ALS), were found in this study in two of 22 patients with ALS, but in none of 427 patients with miscellaneous neurological diseases. Patients with severe generalized MG and/or thymoma tended to have higher titers of AChR binding antibodies and greater AChR modulating antibody activity. However, some patients with severe muscle weakness had low titers of antibodies, and some patients in remission or with only ocular manifestations had high titers. These seemingly paradoxical results reflect heterogeneity in the specificities, affinities, and isotypes of anti-AChR antibodies. To effect pathogenicity, antibodies must have access in vivo to extracellular antigenic sites on the AChR. One would anticipate that antibodies with greatest pathophysiological potential would be of an IgG with greatest pathophysiological potential would be of an IgG subclass that activates complement, or of a specificity that competes for acetylcholine's binding site on the receptor or readily cross-links two AChR molecules to trigger receptor modulation (e.g., by binding to sites on the duplicated alpha-subunit). In patients with suspected MG who lack serological evidence of anti-AChR antibodies, motor endplate biopsy is required for microelectrophysiological, immunochemical, and ultrastructural studies to establish with certainty whether or not the condition is acquired MG.

Anti-acetylcholine antibodies and the pathogenesis of myasthenia gravis

Using an ELISA system, antibodies recognizing conjugated acetylcholine (ACh) were detected in sera of patients suffering from myasthenia gravis. The mean antibody level was three times higher in sera from myasthenic than from control patients. No correlation was found between anti-ACh antibody levels and anti-ACh receptor (AChR) titer. Also, the anti-ACh antibody titers were independent of sex and age of patients. Competition experiments demonstrated that the most immunoreactive compounds were choline-glutaryl-bovine serum albumin (BSA) and choline-succinyl-BSA. Antibodies present in the sera of myasthenic patients recognized an antigenic determinant mimicking conjugated ACh. The antibody affinity and specificity were sufficiently high for the detection of ACh in locust brain.

An evaluation of signs in ocular myasthenia gravis and correlation with acetylcholine receptor antibodies

Fifteen patients with ocular myasthenia gravis were examined in detail for 21 different signs, and tested for acetylcholine receptor antibodies. The major signs of ocular myasthenia gravis included ptosis, disorders of ocular rotations, weakness of eyelid closure, "pseudosupranuclear" signs and the lid twitch sign. Acetylcholine receptor antibodies were found in eight of the 15 patients. One hundred and four normal, non-myasthenic patients were also examined for the lid twitch response, and the relationship between the lid twitch of ocular myasthenia gravis and that found in normal subjects is discussed.