Acute steroid-induced tetraplegia following status asthmaticus

A 10-year-old girl was treated for an acute asthmatic attack with ventilation and a high-dose steroids regimen. An areflexic paralysis of the four limbs was noted when artificial ventilatory support was withdrawn. Electromyography and a muscle biopsy revealed marked myopathic features. We discuss the unusual presentation of an acute steroid-induced myopathy in an asthmatic patient.

Simulation and analysis of the electromyographic interference pattern in normal muscle. Part II: Activity, upper centile amplitude, and number of small segments

We have defined three new features of the electromyographic (EMG) interference pattern (IP): activity, upper centile amplitude (UCA), and number of small segments (NSS). These parameters were measured in simulated IPs constructed by adding together motor unit action potentials (MUAPs) recorded with a concentric needle EMG electrode. The activity increases linearly with the number of MUAP discharges to approximately 80% of its theoretical maximum value. The UCA correlates strongly with the peak-to-peak amplitude of the largest MUAP in the IP and the mean segment amplitude and does not depend on the discharge rate of the largest MUAPs. We infer that the UCA defines the upper limit of the peak-to-peak amplitude of the MUAPs contained in the IP. The NSS increases with the number of MUAP discharges, but reaches a constant value at higher MUAP discharge rates, probably because small amplitude MUAPs are masked by the large amplitude MUAPs. The potential value of these parameters in automated IP analysis is discussed.

Automatic analysis of the electromyographic interference pattern. Part II: Findings in control subjects and in some neuromuscular diseases

The electromyographic (EMG) interference pattern (IP) was measured in the biceps muscle of 16 normal male and 17 normal female subjects. The activity, upper centile amplitude (UCA), and the number of small segments (NSS) (defined in a companion paper) were measured from 500-msec epochs of the IP. The normal values of these features were defined separately for men and women by plotting the UCA and NSS values against activity for each epoch and defining an area on these plots, called a "cloud," that contained more than 90% of the datum points from each study. The mean deviation of the individual datum points from the overall mean values was also calculated for each study. A study in one muscle is considered to be normal if more than 90% of the datum points from that muscle are within the normal clouds and the deviation values are within their normal range. In patients with neuropathy, the characteristic pattern was increased UCA with normal or decreased NSS. In patients with myopathy, NSS was increased and the UCA was normal or decreased. In all studies, the interpretations of the IP from the plots agreed with qualitative assessments of the IP made independently by an electromyographer. The use of these features to understand and quantitate the changes in the motor units produced by disease is demonstrated by serial studies performed in a patient with motor neuron disease.

Automatic analysis of the electromyographic interference pattern. Part I: Development of quantitative features

We have developed three new features of the electromyographic interference pattern (IP), based on the turns and amplitude of the signal, to quantitate some of the features of the IP that are usually assessed subjectively by an electromyographer. The activity measures the fullness of the IP. The upper centile amplitude (UCA) defines the upper limit of the maximum peak-to-peak amplitude of the motor unit action potentials (MUAPs) contained in the IP. The number of small segments (NSS) measures the complexity of the IP, which is a reflection of the polyphasicity of the component MUAPs. The activity and the logarithm of the UCA correlate strongly with the force of muscle contraction at which the IP is measured. The NSS initially increases with the force of contraction and becomes relatively constant at higher force levels. The normal values of these features and the interpretation of their relationships are described in companion papers.

Simulation and analysis of the electromyographic interference pattern in normal muscle. Part I: Turns and amplitude measurements

The electromyographic (EMG) interference pattern (IP) was simulated by adding together motor unit action potentials (MUAPs) of different sizes that had been recorded by a concentric needle EMG electrode. The number of turns (NT) of the simulated IP increased with the number of MUAP discharges. The mean amplitude (MA) difference between successive turns in the IP increased when large amplitude MUAPs were added. Our analysis demonstrates that the MA of the IP is determined mainly by the amplitude of large MUAPs in the signal and that large amplitude spikes are more likely to be generated by single large amplitude MUAPs than by summation of several small amplitude MUAPs.

Botulinum toxin for blepharospasm: single-fiber EMG studies

In four patients who received periocular injections of botulinum toxin for blepharospasm, abnormal neuromuscular transmission was demonstrated by single-fiber EMG in arm muscles. The time course with which the abnormalities developed and cleared, as well as the inverse relationship between the neuromuscular jitter and the firing rate in the abnormal muscles, indicated that the toxin caused the abnormalities in arm muscles. No weakness was detected clinically in muscles distant from the face, but the abnormal neuromuscular transmission indicates that the toxin spread remotely from the site of injection.

Late-onset Mcardle's disease with unusual electromyographic findings

Symptoms of McArdle's disease (muscle phosphorylase deficiency) commonly begin in childhood or adolescence. Late onset of the disease is rare. We describe a 76-year-old man whose symptoms began at age 74 years with sudden onset of proximal muscle weakness and fatigability. Electromyography disclosed substantial spontaneous activity and myopathic features as seen in inflammatory muscle disease. The diagnosis of McArdle's disease was made by histochemical studies of muscle, an abnormal ischemic lactate test, and absence of myophosphorylase activity.

Neuromuscular blocking properties of suxamethonium and decamethonium in normal and myasthenic rat muscle

Patients with myasthenia gravis (MG) have increased tolerance to the neuromuscular blocking properties of suxamethonium (SCh) and decamethonium (C10) and exhibit a reversal of the C10-induced block by neostigmine. The effects of these drugs were compared in forelimb flexor digitorum longus muscle from normal rats and from rats with experimental autoimmune myasthenia gravis (EAMG) to investigate the similarity of EAMG to MG. The depolarization induced by 1, 5, 10 and 25 microM SCh or C10 at the motor end-plates was significantly higher in normal than in EAMG muscle. However, both normal and EAMG end-plates responded in a similar qualitative manner to each drug. The depolarization produced by SCh was typically maintained until the drug was washed from the bath. The depolarization produced by C10 tended to decrease after reaching its peak despite continued application of the drug. With both drugs, miniature end-plate potential (MEPP) amplitude reduction is maintained until a saline wash. Neostigmine interaction with SCh and C10 in normal and EAMG muscle was compared by measuring isometric twitch tension in vitro. Neostigmine potentiated the neuromuscular block produced by either SCh or C10 in both normal and EAMG muscle. Thus muscle from rats with EAMG shares with MG an increased tolerance to SCh and C10 when compared to normal muscle but does not exhibit the qualitatively different interaction of C10 affected muscle with neostigmine that is found in MG patients. This and other studies comparing EAMG and MG indicate that EAMG is an appropriate model of MG but differences such as we have noted should be considered when extrapolating data from EAMG to the human disease.

Digital reproduction of biopotential waveforms for neurophysiological studies

A simple biological signal generator capable of reproducing complex biopotential waveforms is described. It is constructed by a combination of digital and analog circuit components and can be used under different experimental conditions, such as in calibration of biomedical instrumentation systems, or simply as a function generator providing voltage outputs of various waveforms. The biopotential waveform to be generated is sampled at a high frequency and the samples are stored sequentially in a programmable read only memory (PROM). The samples are then fed in the same sequence to a digital-to-analog (D/A) converter and the resulting output is amplified and a DC offset is added. External controls are provided to adjust the DC offset, amplitude and repetition rate of the signal generated. The reproduced voltage signals are stable and superior in quality to those produced by conventional biological signal generators.

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.

A comparison of stapedial reflex fatigue with repetitive stimulation and single-fiber EMG in myasthenia gravis

The pattern of stapedial reflex fatigue in response to pulsed acoustic stimulation was measured and compared to results of repetitive nerve stimulation and single-fiber electromyography (EMG) in 89 patients with myasthenia gravis. Studies were also made on 22 patients with other neuromuscular disorders and 40 control subjects with no evidence of neuromuscular impairment. Stapedial reflex fatigue exceeded normal control values in 84% of the patients with myasthenia gravis. Repetitive stimulation and single-fiber EMG measurements were abnormal in 56% and 91% of this same population, respectively. Stapedial reflex abnormalities were most prevalent in patients with mild forms of myasthenia (predominantly ocular or oropharyngeal weakness). Of 22 nonmyasthenic patients with neuromuscular disease tested, 6 had abnormal stapedial reflex fatigue according to our normal values, indicating that this form of testing also detects other diseases of the motor unit. The measurement of stapedial reflex fatigue is painless, is easy to perform, and requires minimal patient cooperation. Due to the relatively high occurrence of abnormal stapedial reflex fatigue in patients with myasthenia gravis, this procedure appears to have considerable potential value in screening and monitoring patients for the presence of defects in neuromuscular transmission.

Eaton-Lambert syndrome: a clinical and electrophysiological study of a patient treated with 4-aminopyridine

In a patient with the Eaton-Lambert syndrome, 4-aminopyridine produced temporary improvement of clinical and electromyographic abnormalities. Application of the drug in vitro to intercostal muscle from the patient produced an increase in the evoked release of neurotransmitter from intramuscular nerves.

Passive transfer of human myasthenia gravis to rats: 1. Electrophysiology of the developing neuromuscular block

Adult female Lewis rats were rendered immunologically tolerant to human gamma globulin, and were given a single intravenous injection of human myasthenic or normal control serum containing 7.5 to 12 mg of immunoglobulin G. The mean amplitude of miniature endplate potentials (MEPPs) in the forelimb flexor digitorum longus muscles from treated animals did not differ from control values during the first 24 hours after serum injection. Subsequently, MEPP amplitude was reduced in muscles from animals that had received myasthenic serum; maximum reduction was reached by 6 days after transfer. Mean MEPP amplitude at maximum reduction was 30 to 40% below the amplitude of controls and returned to control values 14 weeks after transfer. Similar reductions in endplate potential amplitudes were found in immunologically tolerant animals receiving myasthenic serum. No significant reduction of MEPP amplitude was seen in recipients that were not immunologically tolerant or that had received cobra venom factor to reduce complement activity. The delayed development of reduced MEPP amplitude indicated that the defect of neuromuscular transmission produced by myasthenic serum was not due entirely to a simple curare-like block of the acetylcholine receptor site by an IgG antibody.

Facilitatory effects of 4-aminopyridine on normal neuromuscular transmission

The effects of 4-aminopyridine (4-AP) on neuromuscular transmission were studied in vitro in the rat flexor digitorum longus muscles. 4-AP produced dose-dependent increases in endplate potential (EPP) amplitude, in rise time to peak, and in the average number of acetylcholine quanta released by presynaptic nerve impulses. The neuromuscular blocking effects of d-tubocurarine or low Ca2+/high Mg2+ concentrations could be completely reversed by 4-AP, and EPPs developed into muscle action potentials (APs). The drug had minimal effects on the amplitude or frequency of spontaneous miniature endplate potentials, but increased the duration of indirectly elicited muscle APs. The action of 4-AP required the presence of extracellular Ca2+; thus, its effect may be to promote Ca2+ entry into the motor nerve terminal, and thereby increase the neurally evoked transmitter release. 4-AP is effective in overcoming both presynaptic and postsynaptic blockade of neuromuscular transmission, suggesting a potential role for this drug in the treatment of neuromuscular diseases.

Facilitatory effects of 4-aminopyridine on neuromuscular transmission in disease states

The in vitro effects of 4-aminopyridine (4-AP) on neuromuscular transmission were determined by microelectrode techniques in intercostal muscles from patients with myasthenia gravis (MG) and the Eaton-Lambert syndrome (ELS), and in forelimb muscles from rats with experimental autoimmune myasthenia gravis (EAMG). In MG and EAMG, the amplitudes of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were reduced, and there was increased sensitivity to the blocking action of d-tubocurarine (dTc). In ELS, MEPP amplitude was normal but the average number of acetylcholine quanta released by nerve impulses was reduced, causing subthreshold EPPs. In EAMG muscle, 4-AP produced dose-dependent increases in EPP amplitude and in the duration of indirectly elicited muscle action potentials but no changes in MEPP amplitude and resting membrane potential. 4-AP completely reversed the postsynaptic blockade produced by dTc and EAMG. 4-AP appears to facilitate neuromuscular transmission in EAMG, MG, and ELS by increasing the neurally evoked transmitter release, thus overcoming either the pre- or the postsynaptic neuromuscular blockade.

Effects of D-penicillamine on neuromuscular transmission in rats

Treatment of patients with D-penicillamine (D-P) has been associated with a syndrome similar to myasthenia gravis (MG). To explore this association, we examined the effects of D-P on neuromuscular transmission in rat muscle. In the first experiment, bath-applied D-P had no significant effect on either miniature endplate potential (MEPP) amplitude or action potential (AP) amplitude. Endplate potential (EPP) amplitude and spontaneous MEPP frequency decreased significantly at concentrations approximately 40 times the maximum human therapeutic level. In the second experiment, rats receiving D-P by daily injections for 33 to 37 days did not differ from controls in any of the measured electrophysiologic characteristics. Electron microscopy of muscle endplates from rats treated with D-P showed no evidence of degeneration or simplification. In all cases, thymus histology by light microscopy was normal, and no antireceptor antibodies were found. Thus, D-P has a mild direct presynaptic effect on neuromuscular transmission at high concentrations, but this effect is too small to account for the weakness seen in the myasthenia-like syndrome in humans.

Short-term effects of prednisolone on neuromuscular transmission in normal rats and those with experimental autoimmune myasthenia gravis

Electrophysiological investigations of the effects of bath-applied prednisolone at the neuromuscular junction were performed in muscles from normal rats and rats with experimental autoimmune myasthenia gravis (EAMG). In muscles from both groups, prednisolone reversible and significantly depressed the amplitudes of minature end-plate potentials (MEPPs), end-plate potentials (EPPs) and indirectly elicited action potentials (APs) without affecting resting membrane potentials. Prednisolone also caused a significant reduction in EPP rise time to peak and half-decay time while markedly increasing MEPP frequency and AP rise time to peak and duration. These effects were shown to be dose-dependent. The percentage decrease in amplitude after prednisolone perfusion was similar for EPPs and MEPPs, indicating that the depressive effect of prednisolone at the junction is postsynaptic. In all of the parameters studied, the percentage effect of prednisolone was the same in EAMG and normal preparations. No stimulus-linked repetitive EPPs or APs were observed after prednisolone. It is concluded that prednisolone has a depressive effect on neuromuscular transmission, but that this occurs only at high concentrations of the drug which are not achieved during the treatment of myasthenia gravis.

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.

Neonatal experimental autoimmune myasthenia gravis

Neonatal rats born of and nursed by mothers immunized with Torpedo acetylcholine receptor protein developed a defect of neuromuscular transmission as indicated by reduced miniature endplate potential amplitudes. It is likely that antibodies to the Torpedo receptor protein were passively transferred to the neonates in the milk. With the exception of the route of transfer, this neonatal form of experimental autoimmune myasthenia gravis appears to be similar to its human counterpart, and thus can serve as an experimental model.

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.

Myotonia congenita with painful muscle contractions

We present a family with dominantly inherited myotonia congenita and painful, electrically silent muscle contractions after exertion. In two family members, painful muscle contractions occurred coincident with hypothyroidism, and improved after thyroid replacement therapy. It is probable that this family represents a disease that is similar to but distinct from the dominant form of myotonia congenita.