Neuromuscular safety margin: genetical, immunological and electrophysiological determinants in relatives of myasthenic patients: a preliminary report

Familial autoimmune myastenia gravis: four patients involving three generations

BACKGROUND

Familial autoimmune myasthenia gravis (MG) is rare, although a genetic role for the development of autoimmune MG is suggested by concordance in monozygotic twins and the increased frequency of other autoimmune diseases in family members of myasthenics.

METHODS

A patient with a family history of MG was evaluated in hospital. Relatives were interviewed and medical records examined for details regarding the diagnosis of MG in three other family members.

RESULTS

The index case first experienced symptoms of MG at age 75 years. She developed generalized MG and required corticosteroids and immunosuppressive therapy to control her disease. Her father developed predominantly bulbar symptoms of MG at age 75 years. He died of complications experienced following a gastrostomy placed for continued difficulty swallowing. His brother developed similar symptoms of MG in his early 60s and died shortly after thymectomy. A 46-year-old nephew of the index case is also beginning to exhibit signs of generalized MG. Acetylcholine receptor antibodies were strongly positive in the index case and her nephew. (The assay was not available for her father and uncle).

CONCLUSIONS

Four individuals in three successive generations had diagnoses of autoimmune MG. Study of familial cases such as these may clarify the contribution of genetic factors to the development of this disease.

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Acetylcholine receptor antibody as a diagnostic test for myasthenia gravis: results in 153 validated cases and 2967 diagnostic assays

Anti-acetylcholine receptor (AChR) antibody was undetectable in 26/153 (17%) sera from myasthenia gravis patients assayed by standard RIA using human acetylcholine receptor. Eight of these were found to be positive with a modified protocol using a mixture of normal and denervated AChR, reducing the proportion of "negative" sera to 12%. Many of these were from patients with a short history; two such patients later developed low positive values. Anti-AChR without clinical evidence of myasthenia was found in one of three monozygotic twins of myasthenia gravis patients, and in one of thirty other first degree relatives of a further 17 patients. Anti-AChR is a valuable and highly specific diagnostic test which, with the assay used here, is positive in about 88% of patients with clinical features of myasthenia gravis.

Acetylcholine receptor antibodies in the elderly and in Down's syndrome

Serum antibodies to the acetylcholine receptor (anti-AChR) have been reported in Japanese individuals who were elderly or had Down's syndrome at frequencies of 18% and 24%, respectively. We have measured serum anti-AChR in 3 Caucasoid groups: 53 elderly patients (aged 65-92 years) with miscellaneous (non-myasthenic) disorders, 30 individuals with Down's syndrome, and 40 elderly patients (aged 71-93 years) known to have strongly positive thyroid autoantibodies. A raised titre (greater than 0.2 nmol/l) was confined to 3 patients in the third group (7.5%). We conclude that an increased frequency of anti-AChR antibodies is not a feature of Caucasians who are elderly or have Down's syndrome, and that, even in an elderly group with a high titre of another autoantibody, the frequency of anti-AChR is lower than in elderly Japanese individuals.

Anti-idiotypic antibodies, acetylcholine receptor antibodies and disturbed neuromuscular function in healthy relatives to patients with myasthenia gravis

Fifty-eight first-degree relatives to 40 patients with myasthenia gravis were investigated regarding presence of acetylcholine receptor antibodies, anti-idiotypic antibodies against the receptor antibodies and clinical and electrophysiological signs of disturbed neuromuscular function. No relative had clinical signs of muscle weakness. The prevalence of low concentrations of receptor antibodies was 54%, of anti-idiotypic antibodies 37% and of pathological and borderline single fibre EMG 45%. No sibling, only 2/11 children and 3/14 parents were normal in all three tests. A combination of receptor antibodies and anti-idiotypic antibodies was the most common finding and was especially frequent in children. In female siblings and children there was a positive correlation between the presence of HLA-antigen A1 and/or B8 and that of receptor antibodies and anti-idiotypic antibodies. Male siblings and children showed no such correlation but had a higher frequency of pathological single fibre EMG than females.

Myasthenia gravis and myasthenic syndromes

More than a decade ago myasthenic symptoms were observed in rabbits immunized with acetylcholine receptor (AChR) [119] and AChR deficiency was found at the neuromuscular junction in human myasthenia gravis (MG) [36]. By 1977 the autoimmune character of MG and the pathogenic role of AChR antibodies had been established by several measures. These included the demonstration of circulating AChR antibodies in nearly 90% of patients with MG [87], passive transfer with IgG of several features of the disease from human to mouse [149], localization of immune complexes (IgG and complement) on the postsynaptic membrane [30], and the beneficial effects of plasmapheresis [20, 123]. Substantial subsequent progress has occurred in understanding the structure and function of AChR and its interaction with AChR antibodies. The relationships of the concentration, specificities, and functional properties of the antibodies to the clinical state in MG have been carefully analyzed, and the mechanisms by which AChR antibodies impair neuromuscular transmission have been further investigated. The clinical classification of MG has been refined, the role of the thymus gland in the disease has been further clarified, and new information has become available on transient neonatal MG. The prognosis for generalized MG is improving, but there is still no consensus on its optimal management. Novel therapeutic approaches to MG are now being explored in animal models. Recognition of the autoimmune origin of acquired MG also implied that myasthenic disorders occurring in a genetic or congenital setting had a different cause. As a result, a number of congenital myasthenic syndromes have come to be recognized and investigated. Finally, an acquired disorder of neuromuscular transmission different from MG, the Lambert-Eaton myasthenic syndrome, has also been shown to have an autoimmune basis. In this syndrome, active zone particles of the presynaptic membrane are direct or indirect targets of the pathogenic autoantibodies.

Acetylcholine receptor antibody in the diagnosis of myasthenia gravis

The acetylcholine receptor (AchR) antibody assay has a key role in the diagnosis of myasthenia gravis. In this article, the role of AchR antibody assay in the diagnosis of ocular and generalized myasthenia gravis is reviewed, and compared to standard means of diagnosing the disease by clinical and electrophysiological methods.

Anti-acetylcholine receptor antibodies in myasthenia gravis. Part 2. Clinical and serological follow-up of individual patients

Circulating antibodies to acetylcholine receptor protein (anti-AChR) were measured in the sera collected from 75 patients (53 women, 22 men, ages 9-83 year, 20 with a thymoma) with myasthenia gravis (MG) during 5-44 (mean 25) months. The clinical state of each patient was graded on a 6-point scale. Anti-AChR concentrations were measured by a radioimmunoassay with human antigen. We analysed the relation between the change in clinical state and the change in anti-AChR concentration in 155 periods (1-7, mean 2.1 per patient). The change in clinical state is given as the difference in score at the onset and at the end of this period. The change in anti-AChR concentration is expressed as the percentage of the original concentration at the onset of the period. The results were analysed in relation to the therapy and to the severity of the MG at the onset of each period. A strong correlation between a change in anti-AChR concentration and a change in clinical condition existed during treatment with prednisone or immunosuppression and in the period after thymectomy, while a weaker correlation was present in periods without immunosuppression. In only 3 patients did the changes in anti-AChR concentration precede the clinical change. No changes in anti-AChR concentrations were found if improvement was due to the effect of anticholinesterases or if deterioration was caused by infection or emotion. The serial measurement of anti-AChR may be a valuable method of following the basic trend of the MG in severely affected patients.