Lambert-Eaton myasthenic syndrome: the lack of short-term in vitro effects of serum factors on neuromuscular transmission.

Serum was obtained from 7 patients with the Lambert-Eaton myasthenic syndrome (LES), 3 patients with small-cell carcinoma of the lung (SCCL), and 9 healthy control subjects. Serum samples were applied in vitro to the rat neuromuscular junction (for 1-3 h for control LES sera; 4 h for SCCL sera), following which the pre- and postjunctional physiological effects of serum factors were studied in the presence of 10 mM [Mg2+]o. All sera produced a marked reduction in the frequency of spontaneous miniature end-plate potentials (MEPPs), while causing slight to moderate changes in MEPP amplitude. There were no consistent changes in the quantum content of the impulse-evoked end-plate potentials, though the serum from one LES patient significantly and reversibly inhibited the evoked quantal release. No significant effect was found when a human intercostal muscle was exposed to serum from another LES patient for 2 h. Therefore, when applied in vitro on a short-term basis, the putative LES autoantibodies do not consistently react with voltage-dependent calcium channels in the motor nerve terminal and thus fail to reproduce the physiologic abnormality of the syndrome. We suggest that the pathogenic IgG molecules may require more than 3h of incubation in order to gain access to, and inhibit the function of, the prejunctional Ca2+ channels.

Familial autoimmune myasthenia gravis and thymoma: occurrence in two brothers.

At ages 31 and 42 years, two brothers presented with clinical, pharmacologic, electrophysiologic, and immunologic characteristics of autoimmune myasthenia gravis. At thymectomy, both had histologic findings of epithelial thymoma. HLA analysis revealed A2, A3, B7, and B39 antigens in one patient and A3, A24, B7, and B40 antigens in the other. Familial myasthenia gravis with thymoma has not been described previously. Familial thymoma has been rarely reported, but never with myasthenia gravis.

Synergistic interaction of 4-aminopyridine with neostigmine at the neuromuscular junction.

The pre- and postsynaptic events at the neuromuscular junction were studied in vitro in rat skeletal muscle exposed to clinically significant concentrations of 4-aminopyridine (4-AP), neostigmine or combinations of the two drugs. Simultaneous application of 4-AP and neostigmine produced increases in the amplitudes of nerve-evoked end-plate potentials which were significantly greater than the summed effects of the drugs applied individually. Such synergism was present at the junctions where transmission was blocked either postsynaptically by d-tubocurarine or presynaptically by low [Ca2+]0 and high [Mg2+]0. Quantal content analysis in the latter preparation indicated that the evoked release of acetylcholine was potentiated significantly more than the amplitude of spontaneous miniature end-plate potentials, suggesting that the site of synergism is predominantly presynaptic. For symptomatic relief and long-term management of the neuromuscular junction disorders, we propose a combined medication of aminopyridine and anticholinesterase at reduced dosages. Such therapy would minimize adverse effects and be particularly effective in the treatment of such presynaptic disorders as the Lambert-Eaton myasthenic syndrome and botulism.

Long-term corticosteroid treatment of myasthenia gravis: report of 116 patients.

One hundred sixteen patients, aged 8 to 82 years, with myasthenia gravis were treated with prednisone, 60 to 80 mg daily, until the onset of improvement, followed by lower-dose alternate-day therapy of several years' duration. Of all patients, 80.2% achieved either remission (27.6%) or marked improvement (52.6%). Moderate improvement occurred in 14.7%, and 5.2% showed no improvement. Increasing age correlated with a favorable outcome, but sex, duration of illness prior to treatment, severity and distribution of weakness at the time of onset of treatment, and presence of thymoma were not factors in the response to therapy.

Presynaptic and postsynaptic actions of procainamide on neuromuscular transmission.

Procainamide (PA), a cardiac anti-arrhythmic agent, was applied in vitro to the rat neuromuscular junction and its effect on neuromuscular transmission was investigated using intracellular microelectrode recording techniques. Clinically relevant doses of PA produced a dose-dependent decrease in the amplitude of spontaneous miniature end-plate potentials (MEPPs) and a marked increase in the half-decay time without altering MEPP frequency or resting membrane potential. The amplitude of impulse-evoked end-plate potentials was also reduced in a similar dose-dependent manner, with a marked prolongation of the decaying phase. Direct quantal analysis using magnesium-blocked preparations showed that the drug also caused a reduction in the mean number of acetylcholine quanta released per nerve impulse. This presynaptic inhibitory effect of the drug, however, contributed less to the overall blocking action than did the postsynaptic effect. The junctional effects of the drug were completely reversible, with all affected parameters returning to the control levels after washout of PA with control solution. These direct inhibitory actions on neuromuscular transmission could account for the clinical exacerbations associated with the administration of PA to patients with myasthenia gravis and other neuromuscular diseases.

Reduction of miniature end-plate potential amplitude in extraocular and limb muscles in an animal model of myasthenia gravis.

Miniature end-plate potential (MEPP) amplitudes of the superior oblique extraocular muscle (EOM) and forelimb flexor digitorum longus muscle (FDL) of rats with experimental autoimmune myasthenia gravis (EAMG) were expressed as reductions from mean MEPP amplitudes of the respective muscles of pair-matched control animals. Postjunctional blockade of neuromuscular transmission at EOM end-plates was no greater than at limb muscle end-plates. In contrast, the amount of reduction measured in EAMG-affected FDLs (52.3%) was marginally, but significantly, greater than that in EOMs (38.3%). A weak correlation was present between the MEPP reduction observed in an animal's FDL and that recorded in its EOM.

Intercostal muscle biopsy studies in myasthenia gravis: clinical correlations and the direct effects of drugs and myasthenic serum.

We have found a wide range of mean MEPP amplitude in intercostal muscle biopsies from 43 patients with MG, including several values in the normal range. There was no correlation between MEPP amplitude and the severity of clinical disease as assessed by manual muscle testing or by single-fiber EMG measurements of jitter in arm muscles. Through most of these patients were in a state of clinical remission or marked improvement after treatment with prednisone, we could not attribute the difference between our results and those of others to this factor alone. The application of morphine, meperidine and aminoglycoside antibiotics to intercostal muscle in vitro confirms effects previously demonstrated in rat muscle: (1) At equal therapeutic concentrations, meperidine has greater neuromuscular blocking effects than does morphine, but neither has significant effects at concentrations achieved in the serum clinically. (2) Tobramycin, netilmicin and neomycin have varying severity and sites of action, but their effects are the same in human myasthenic muscle as in normal rat muscle. Bath application of serum from myasthenic patients produces an acute, reversible worsening of neuromuscular blockade in myasthenic muscle. Electrophysiologic measurements in intercostal biopsies from patients with MG can provide information about the basic abnormality of neuromuscular transmission in this disease and can confirm the relevance of studies made in animal muscle.

Single-fiber electromyography in myasthenia gravis.

One-hundred-sixty single-fiber EMG studies of the extensor digitorum communis muscle were performed on 127 patients with myasthenia gravis; 131 demonstrated defective neuromuscular transmission. Jitter determinations in the biceps, deltoid or frontalis muscles increased the diagnostic yield significantly. Evoked-potential EMG studies were abnormal in less than 50 percent of patients in whom they were performed. The most sensitive criterion of abnormality was the percentage of fibers with increased jitter; the sensitivity of the test was enhanced, however, if the mean jitter of the tested muscle was also used as a criterion of abnormality. Since increased jitter may also be seen in primary muscle and nerve disease, these disorders must be excluded by other means before diagnosing myasthenia gravis on the basis of the single-fiber studies.

Experimental autoimmune myasthenia gravis in rats. Modification by thymectomy and prednisolone.

In rats immunized with purified acetylcholine receptor protein (AChRP) from Torpedo electroplax, a defect of neuromuscular transmission physiologically identical to that seen in myasthenia gravis developed. The most sensitive index of the neuromuscular blockage was miniature end-plate potential (MEPP) amplitude. As early as 24 hours after inoculation with AChRP, the thymus showed reactive changes that are probably nonspecific. Removal of the thymus before or within three days after immunization delayed, but did not prevent, development of reduced MEPP amplitude. Prednisolone given within 35 days after immunization produced reversal of MEPP reduction within 24 hours, but had no such immediate effect when given 15 days later. It is probable that prednisolone acted by reducing the immunologic responsiveness of the animals during the developing phase of the defect of neuromuscular transmission.

Long-term administration of corticosteroids in myasthenia gravis.

Favorable results were obtained in 30 patients with myasthenia gravis treated initially with high daily doses of prednisone and subsequently maintained on lower doses for a protracted period. In 45 incidents of treatment, complete remission occurred in 69 percent, marked improvement in 20 percent, and moderate improvement in 17 percent. Nineteen patients proceeded to thymectomy, with negligible morbidity and sustained improvement. In four patients, it was possible to discontinue prednisone 1 year or more after thymectomy.

Long-term prednisone followed by thymectomy in myasthenia gravis.

The present series of thirty patients has led us to certain conclusions concerning the management and treatment of patients with myasthenia gravis. The use of cholinesterase inhibitors alone is reserved for those patients with purely ocular myasthenia whose deficits can be satisfactorily corrected with those agents. Some of those with ocular involvement may be disabled; and in light of our excellent results with that small group, as well as similar findings presented by Fischer et al., patients with disabling or refractory ocular myasthenia should be considered for treatment with prednisone. All other patients with myasthenia are given a course of oral corticosteroids (prednisone) initially at high doses, with subsequent tapering to maintenance, alternate-day low-dose therapy. Cholinesterase inhibitors are used as needed while the patient is receiving corticosteroids. We now anticipate that patients will exhibit sustained improvement within the first two weeks, reaching maximal improvement at about three months. Exacerbations of myasthenic weakness may occur in the early phases of treatment. Such exacerbations have been commonly mild and occur with a mean onset at 5 days, and have a mean duration of 6 days. Most patients have been able to tolerate an alternate-day schedule of prednisone therapy when maintenance levels were achieved. The effective maintenance dose has been determined as the smallest dose of prednisone which allows the patient to maintain maximal improvement. Following the establishment of maximal improvement, patients have been considered for thymectomy. In our experience, the sternum-splitting procedure has been tolerated extremely well by patients exhibiting marked imporvement or remission while on corticosteroids. In those patients where thymectomy is contraindicated, irradiation of the thymus might be considered. Patients are continued on maintenance steroid therapy following surgery for a period of time that has been arbitrary. Currently, we consider an attempt to discontinue steroids at approximately one year reasonable. Should the patient relapse after discontinuation of the medication, oral corticosteroid treatment is reinitiated. Consideration is given to the possibility of recurrent thymus in patients who repeatedly fail to maintain a remission when steroids have been stopped. Our experience has not permitted us to draw firm conclusions concerning how long a time high-dose daily steroid treatment should be continued in patients who show no favorable response to that therapy. Other modes of treatment, such as courses of parenteral ACTH, methyl prednisolone, dexamethazone, or antimetabolites might be considered if there is no response after 12 weeks of high-dose, daily corticosteroid therapy.

Neostigmine methylsulfate. Does it have a chronic effect as well as a transient one?

Three groups of 200-gm rats were injected subcutaneously with neostigmine methylsulfate (1 mg/kg/day) for 7, 30, and 100 days. Electrophysiological changes were assessed in vitro, using microelectrode techniques to examine diaphragm muscles of treated and untreated animals. Miniature end-plate potential (MEPP) amplitude decreased in neostigmine-treated preparations. Guanidine hydrochloride enhances transmitter release and increases MEPP frequency in control preparations. Neostigmine-treated animals examined between 6 to 72 hours after discontinuation of neostigmine therapy showed impaired response to the facilitating influence of guanidine. Recovery of response to guanidine was inversely related to length of treatment with neostigmine. Results of electron-microscopic examination of motor end-plates in treated animals revealed ultrastructural changes, including simplified end-plates, and, occasionally, multiple, separate, junctional regions. Therefore, the chronic administration of cholinesterase inhibitors in man may have a deleterious effect, as well as a transient beneficial one.