New method for the estimation of the number of motor units in a muscle. 2. Duchenne, limb-girdle and facioscapulohumeral, and myotonic muscular dystrophies

Peripheral neuropathy is linked to a severe form of myotonic dystrophy in transgenic mice

Myotonic dystrophy type 1 (DM1) is a multisystem disorder with a variable phenotype. The involvement of peripheral nerves in DM1 disease is controversial. The DM1 animal model DM300 transgenic mice that carry 350 to 500 CTG repeats express a mild DM1 phenotype but do not exhibit motor or sensory pathology. Here, we investigated the presence or absence of peripheral neuropathy in transgenic mice (DMSXL) that carry more than 1,300 CTG repeats and display a severe form of DM1. Electrophysiologic, histologic, and morphometric methods were used to investigate the structure and function of peripheral nerves. We observed lower compound muscle action potentials recorded from hind limb muscles and slowing of sciatic nerve conduction velocity in DMSXL versus control mice. Morphometric analyses showed an axonopathy and neuronopathy in the DMSXL mice characterized by a decrease in numbers of myelinated motor axons in sciatic nerve and in spinal cord motor neurons. Pathologic alterations in the structure of hind limb neuromuscular junctions were also detected in the DMSXL mice. These results suggest that peripheral neuropathy can be linked to a large CTG expansion and a severe form of DM1.

Motor unit number estimate as a predictor of motor dysfunction in an animal model of type 1 diabetes

Peripheral neuropathy is a common complication of diabetes that leads to severe morbidity. In this study, we investigated the sensitivity of motor unit number estimate (MUNE) to detect early motor axon dysfunction in streptozotocin (STZ)-treated mice. We compared the findings with in vitro changes in the morphology and electrophysiology of the neuromuscular junction. Adult Thy1-YFP and Swiss Webster mice were made diabetic following three interdaily intraperitoneal STZ injections. Splay testing and rotarod performance assessed motor activity for 6 wk. Electromyography was carried out in the same time course, and compound muscle action potential (CMAP) amplitude, latency, and MUNE were estimated. Two-electrode voltage clamp was used to calculate quantal content (QC) of evoked transmitter release. We found that an early reduction in MUNE was evident before a detectable decline of motor activity. CMAP amplitude was not altered. MUNE decrease accompanied a drop of end-plate current amplitude and QC. We also observed small axonal loss, sprouting of nerve endings, and fragmentation of acetylcholine receptor clusters at the motor end plate. Our results suggest an early remodeling of motor units through the course of diabetic neuropathy, which can be readily detected by the MUNE technique. The early detection of MUNE anomalies is significant because it suggests that molecular changes associated with pathology and leading to neurodegeneration might already be occurring at this stage. Therefore, trials of interventions to prevent motor axon dysfunction in diabetic neuropathy should be administered at early stages of the disorder.

Reductions in motor unit number estimates (MUNE) precede motor neuron loss in the plasma membrane calcium ATPase 2 (PMCA2)-heterozygous mice

The potential of MUNE as a unique electrophysiological tool to detect early motor unit abnormalities during a clinically silent period was investigated in the plasma membrane calcium ATPase 2 (PMCA2)-heterozygous mice. There was a significant reduction in MUNE in the PMCA2-heterozygous mice as compared to the wild type littermates at two months of age. In contrast, the compound motor action potential (CMAP) was not altered. The conduction velocity (CV) of the sensory nerve and sensory nerve action potentials (SNAP) were not modified indicating lack of major sensory deficits. Interestingly, despite a decline in MUNE at this age, no changes were detected in choline acetyl transferase (ChAT) positive motor neuron number in the ventral horn of the lumbar spinal cord. Hindlimb grip strength, a test that evaluates clinical dysfunction, was also similar to that of the wild type controls. However, motor neuron number significantly decreased by five months suggesting that a drop in MUNE preceded motor neuron loss. In the two-month-old PMCA2-null mice, reduced MUNE measurements coincided with lower motor neuron number and decreased hindlimb grip strength. The fall in motor neuron number was already detectable at three weeks, the earliest time studied, and became more pronounced by five months. Our results show that even partial reductions in PMCA2 levels are sufficient to cause delayed death of motor neurons and that MUNE may be a reliable and sensitive approach to detect pathology prior to cell loss and in the absence of overt clinical signs.

Motor unit number estimation

Since its introduction 30 years ago, MUNE techniques have increasingly been refined and applied to a wide variety of neuromuscular disorders. Differences of opinion remain among MUNE investigators as to which method is best; however, statistical and MPS MUNE are currently the most widely used. Numerous methodologic issues remain, including the development of detailed universal standards for each technique and the implementation of modifications for the enhancement of reproducibility. These issues are the subjects of ongoing investigation. Despite technical variability, the MUNE values obtained using different methods show good agreement in studies of normal subjects and in patients with a variety of neurogenic processes. MUNE has been applied most successfully to patients with amyotrophic lateral sclerosis and to animal models of motor neuron disease, providing significant insight into the pathophysiology of these disorders. These techniques are increasingly being incorporated into clinical therapeutic trials. MUNE offers promise in the study of neuromuscular disease, enabling the collection of novel data in the living patient unobtainable by any other method.

Motor Unit Estimate Number in the Anterior Tibial Muscle: Normative Data versus Findings in Critically Ill Patients in Intensive Care Units

OBJECTIVES

To determine the number of motor units (MUNEs) in the anterior tibial muscle of normal subjects for comparison with those of severely paretic or paralytic muscles of critically ill patients in intensive care units.

RESULTS

The mean MUNE for 24 normal subjects (194 +/- 5; mean +/- standard deviation) was similar to that of the 22 patients with critical illness (184 +/- 10). However, both the mean amplitude of the evoked compound muscle action potential (CMAP) and of the single motor unit action potential (S-MUAP) among patients were approximately one third of those in normal subjects.

CONCLUSION

Critically ill patients in this study demonstrated normal MUNEs with reduced CMAP and S-MUAP amplitudes in the setting of severe clinical weakness, suggestive of predominantly myopathic injury. MUNE may provide a valuable tool for distinguishing between neuropathy and myopathy in critically ill patients.

Motor unit number estimation in human neurological diseases and animal models

Motor unit number estimation (MUNE) was introduced in 1971 as a way of providing an objective and meaningful estimate of axon loss in diseases affecting the motor system. Over the last 30 years, different methods of MUNE have been proposed, with each having specific strengths and limitations. The goal of this paper is to review the available methods, and to present data generated using MUNE in a variety of disease entities. The incremental, multiple point stimulation, spike-triggered averaging, F-wave, and statistical methods of MUNE are reviewed, along with data obtained using these methods in patients with neuropathy, motor neuron disorders, and muscle disease. All methods reviewed have theoretical concerns associated with them. However, with the exception of the spike-triggered averaging method, all give results in normal subjects that are quite similar. MUNE has been of great value in assessing progression of motor neuron disease, and has also shown promise in the assessment of generalized neuropathy. Despite the lack of a perfect method for performing MUNE, it has great clinical value in the assessment of progressive motor axon loss. Further refinements in the method will likely increase its utility in the future.

Limb-girdle muscular dystrophy: a follow-up study of 79 patients

The limb-girdle muscular dystrophies (LGMD) are autosomally inherited neuromuscular diseases. Recently six different loci for LGMD have been reported: 5q (LGMD1A), 15q (LGMD2A), 2p (LGMD2B), 13q (LGMD2C), 17q (LGMD2D) and 4p-14-q21.2 (LGMD2E) respectively. We have studied 79 patients affected by LGMD during the period 1976 to 1995. All patients were examined clinically, and various investigations, including genetics were performed. According to their data we divided them as follow: 1) Cases with autosomal recessive inheritance (34.19%) of these two families are linked to chromosome 2p and the others were subdivided according to the age at onset into childhood LGMD and juvenile-adult LGMD; 2) Cases with dominant inheritance (13.92%); 3) Sporadic cases (51.89%). Onset of symptoms occurs from the first to the third decade. The clinical course varies considerably, as does the degree of disability. Our study allowed to identify two different groups of patients who relatively homogeneous with respect to their clinical and laboratory characteristics.

A review of techniques employed to estimate the number of motor units in a muscle

Being the smallest functional units under neural control, motor units play an integral role in muscle physiology. However, at the present time, there does not exist any widely accepted technique for quantifying or estimating the number of motor units in a muscle. Specifically, the existing techniques are the increment-counting technique, a technique based on spike-triggered averaging, and a macro-EMG based technique which vary in invasiveness from noninvasive to highly invasive, respectively. We discuss each of these techniques, along with their associated shortcomings, in detail.

Skeletal muscle changes associated with equine myotonic dystrophy

A progressive neuromuscular disorder in young horses, clinically apparent as early as 1 month of age, is characterized by generalized myotonia, muscle stiffness, muscle weakness and atrophy. Myotonia is identified by percussion dimpling and myotonic EMG discharges. Changes in one case included testicular hypoplasia, cataract formation, and glucose intolerance, indicating a systemic involvement. Pathologic changes in skeletal muscles from three affected foals were examined. Sarcoplasmic masses, ringed fibers, internal positioning of sarcolemmal nuclei, and nuclear rowing were among the primary histologic changes noted. Variation in fiber diameter size, especially atrophy, and type I predominance were also prominent changes. A neurogenic involvement was indicated by type grouping changes in several muscles.

Electromyography in congenital nemaline myopathy

To clarify the discrepancies between earlier reports of electromyography (EMG) in congenital nemaline myopathy (CNM), conventional electromyography was done on 13 patients with CNM, and results were compared with those of 18 earlier EMG examinations of the same patients. Fiber density was measured in 10 patients with a computerized method and neuromuscular jitter in 3 with single-fiber EMG. With age, the EMG abnormality progressed, and "neuropathic" EMG features developed in distal muscles. In 9 of 10 patients fiber density was higher than normal. In two of three patients jitter was abnormal. Motor (13 of 13 patients) and sensory (3 of 3 patients) nerve conduction velocities were normal. Our results seem to explain the conflicting reports of EMG in CNM. We conclude that active degeneration and regeneration of muscle fibers takes place in CNM and suggest that the "neuropathic" motor unit potentials seen in our patients may be secondary to myopathic disease activity.

Number and size of motoneurons in a forelimb motor nucleus of normal and dystrophic (C57BL/6J dy2j/dy2j) mice

The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify the motoneurons that innervate the distal forelimb muscles via the ulnar nerve in normal and dystrophic (C56BL/6J dy2j/dy2j) mice. In both normal and dystrophic mice this motor nucleus was located in spinal segments C6 through T1. No clear division, on the basis of size, into alpha and gamma motoneuron populations was apparent. The motoneurons of dystrophic mice were fewer in number (26.5 vs. 35) but larger in cross-sectional area (780 vs. 674 microns2) than those of age-matched control mice. These results are quantitatively similar to those reported for the hind limb soleus muscle of dystrophic mice and suggest that the motoneuronal changes are a reflection of the dystrophic process rather than the associated spontaneous action potential generation seen in the dystrophic hind limb muscles.

Duchenne muscular dystrophy: pathogenetic aspects and genetic prevention

Duchenne muscular dystrophy (DMD) is the most common sex linked lethal disease in man (one case in about 4000 male live births). The patients are wheelchair bound around the age of 8-10 years and usually die before the age of 20 years. The mutation rate, estimated by different methods and from different population studies, is in the order of 7 X 10(-5), which is higher than for any other X-linked genetic disease. Moreover, unlike other X linked diseases such as hemophilia A or Lesh-Nyhan's disease, there seems to be no sex difference for the mutation rates in DMD. Several observations of DMD in girls bearing X-autosomal translocations and linkage studies on two X chromosomal DNA restriction fragment length polymorphisms indicate that the DMD locus is situated on the short arm of the X chromosome, between Xp11 and Xp22. It may be of considerable length, and perhaps consisting of actively coding and non-active intervening DNA sequences. Thus unequal crossing over during meiosis in females could theoretically account for a considerable proportion of new mutations. However, there is no structurally or functionally abnormal protein known that might represent the primary gene product, nor has any pathogenetic mechanism leading to the observed biochemical and histological alterations been elucidated. Among the numerous pathogenetic concepts the hypothesis of a structural or/and functional defect of the muscular plasma membrane is still the most attractive. It would explain both the excess of muscular constituents found in serum of patients and carriers, such as creatine kinase (CK), as well as the excessive calcium uptake by dystrophic muscle fibres, which, prior to necrosis, could lead to hypercontraction, rupture of myofilaments in adjacent sarcomeres and by excessive Ca uptake to mitochondrial damage causing crucial energy loss. The results of studies on structural and functional membrane abnormalities in cells other than muscle tissue, e.g., erythrocytes, lymphocytes and cultured fibroblasts, indicate that the DMD mutation is probably demonstrable in these tissues. However, most of the findings are still difficult to reproduce or even controversial. DMD is an incurable disease; therefore most effort, in research as well as in practical medicine, is concentrated upon its prevention.(ABSTRACT TRUNCATED AT 400 WORDS)

Duchenne dystrophy viewed as a disturbance of nerve-muscle interactions

The possibility that some features of Duchenne muscular dystrophy (DMD) can be explained as a disturbance of nerve-muscle interaction during development is discussed. It is argued that the initial disturbance in DMD is due to a slower rate of maturation of skeletal muscle fibers. Normally, the maturation of the motor reflexes is closely followed by maturation of the muscle fibers. The possibility is considered that if muscle fibers mature more slowly than normal, they are not able to withstand the type of activity imposed upon them by the mature motoneuron. This applies mainly to the high frequency activity imposed on the fibers of motor units that are destined to become "fast." Experimental evidence suggesting such a mechanism is presented.

The motor unit in muscular dystrophy, a single fibre EMG and scanning EMG study

Dystrophic muscle shows increase in fibre density, abnormally low jitter in some recordings and more often increased jitter. The cross section of the motor unit has normal length. There are no signs of abnormal volume conduction characteristics. The increased fibre density is believed to be due to localised increase in the number of muscle action potential generators. The findings are compatible with a remodeling of the motor unit due to fibre loss and a reparative process with fibre regeneration and reinnervation.

Measurement of motor conduction velocity with Hopf's technique in myotonic dystrophy

Hopf's technique was used to measure maximal and minimal motor nerve conduction velocities, and the percentage of fibres with intermediate velocity, in the posterior tibial nerve in patients with myotonic dystrophy. A reduction of maximal and minimal conduction velocities was found. The distribution of fibres with intermediate velocity was nearly identical to that of the control group and the dispersion values were normal. These data do not support the hypothesis that a primary disturbance of the motor neurons is responsible for the muscle changes in myotonic dystrophy. The reduction of the motor nerve conduction velocity, which was an inconstant finding, should not be considered an indication of a neurogenic aetiology of myotonic dystrophy, but only one of the many disorders of a multisystem disease.

Motor unit potential abnormalities in multiple sclerosis: further evidence for a peripheral nervous system defect

We have recently reported abnormalities of single fibre EMG in patients with multiple sclerosis. The present study applies quantitative electrophysiological techniques to the same group of patients. The number of motor units in the extensor digitorum brevis muscle was measured and their characteristics recorded. Also the shortest distal motor latency and fastest motor conduction velocities were estimated. Abnormalities suggesting a patchy denervating/reinnervating process due to pathology in the intramuscular nerve network or at the endplate were found in a number of patients. There was a good correlation between patients with abnormal motor unit potentials and those with abnormal single fibre EMG "jitter".

Quantitative electrophysiological study of alcoholic neuropathy

Thirty-one chronic alcoholic patients were investigated using quantitative electrophysiological techniques. Estimates of the numbers of functioning motor units in the extensor digitorum brevis muscles and measurements of the parameters of the potentials of these units are presented along with the values for motor nerve conduction velocities in the innervating lateral popliteal nerves. Motor conduction velocities and sensory nerve action potential amplitudes were also measured in the ulnar nerves. The results and their inter-relationships lead us to conclude that the slowing of motor nerve conduction and reduction in sensory nerve action potential amplitudes in alcoholic neuropathy are a consequence of axon loss. We found no evidence of pathological slowing of conduction in surviving axons. Reinnervation by functioning motor axons is poor compared to a number of other neuropathic conditions. In our patients there was no evidence of preferential involvement of sensory axons. The results support a predominant axonal dysfunction in alcoholic neuropathy.

Incidence, severity, and time-course of motoneurone dysfunction in myotonic dystrophy: their significance for an understanding of anticipation

The numbers of functioning motor units and the amplitudes of the maximum evoked muscle responses have been measured in 198 muscles of 102 patients with myotonic dystrophy. Losses of units could be demonstrated in most of the extensor digitorum brevis and thenar muscles but less commonly in the hypothenar groups. A more proximal limb muscle, the soleus, was also shown to be involved frequently. Investigation of two premature infants withmyotonic dystrophy also revealed reductions of functiong units; in one infant clinical improvement was associated with increased muscle innervation. Repeated examinations of 10 adult patients disclosed an abnormal decline in neuromuscular function below the age of 60 years. The reduction in functioning units amounted to approximately 3% of the mean control value per annum. Analysis of 19 families showed that the severity of neuromuscular involvement was nearly always greater in members of later generations. If the predicted deterioration was also taken into account, the results strongly suggested that anticipation was a true genetic phenomenon rather than an artefact of selection. The combined results are considered to strengthen the concept of motoneurone dysfunction as the major pathogenetic factor in this form of dystrophy.

A quantitative electrophysiological study of motor neurone disease

Thirty-two patients with motor neurone disease were investigated using quantitative electrophysiological techniques. Estimates of the number of surviving motor units in the extensor digitorum brevis muscle and measurements of the electrophysiological parameters of these units are present along with the values for motor nerve conduction velocities. The results indicate that reinnervation in motor neurone disease is sufficient to compensate completely for the loss of up to 50% of the motor neurone pool supplying the muscle. The capacity for reinnervation is greater than we have found in a number of neuropathies but the efficiency of reinnervation decreases as the number of surviving motor units falls. Reinnervation appears to cease when 5% or less of the motor units remain viable. There is no electrophysiological evidence of a preferential loss of fast conducting axons, of pathological slowing of conduction nor of a dying-back process affecting the motor axon. Comparison of the electrophysiological parameters in progressive muscular atrophy and amyotrophic lateral sclerosis shows no significant differences. The underlying pathophysiological mechanisms are discussed in terms of the results.

The neural hypothesis of muscular dystrophy. A review of recent experimental evidence with particular reference to the Duchenne form

Recent observations are considered to provide further evidence for an abnormality involving motoneurones in DMD. The dystrophic process appears to take place in two stages of which the first occurs during early embryonic life. This stage is thought to involve faulty inductive actions of the neural tube upon mesoderm and upon itself. The neural consequences vary among individuals and are manifested as mental retardation; EEG abnormalities and losses of functioning motor units. While the first two abnormalities are non-progressive, a further loss of motor units, associated with striking reductions in the numbers of excitable muscle fibers, takes place in trunk and large limb muscles at 9--12 years. The latter process, the cause of which is uncertain, constitutes the second stage of DMD.

The development of the extensor digitorium brevis muscle in progressive proximal muscular antrophies

The development of the extensor digitorium brevis (EDB) muscle was studied in 39 healthy subjects, 27 patients with progressive proximal spinal muscular atrophy, 20 patients with limb-girdle muscular dystrophy and three with facioscapulohumeral muscular dystrophy. The EDB muscle is spared and usually hypertrophic in patients with muscular dystrophy, whereas it is often atrophic and weak in patients with spinal muscular atrophy. It is proposed that the degree of development of the EDB muscle can be used as a clinical sign in the differential diagnosis of progressive muscular atrophies.

Further motor unit studies in Duchenne muscular dystrophy

Numbers of functioning motor units have been estimated in 124 muscles of boys with Duchenne dystrophy; some of the patients were studied on several occasions. In the distal muscles examined (extensor digitorum brevis, thenar, and hypothenar muscles) the losses of units were probably present at birth and did not decrease with age. In contrast, the numbers of units and of excitable muscle fibres in the soleus muscles declined significantly, especially between the ages of 9 and 12 years.

Spinal cord limb motor neurons in dystrophia myotonica

The total numbers of limb motor neurons have been counted in 20 micron sections of the lumbosacral spinal cord obtained a autopsy from 5 control subjects who showed no evidence of neuromuscular disease and from 5 patients with dystrophia myotonica ranging in age from 42 to 64 years. No significant reduction in the total number or distribution of such neurons was found in the cases of myotonic dystrophy and the quantity of lipofuscin present in the cells was similar to that in controls. Glial cell numbers were, however, significantly increased in the cases of dystrophia myotonica and this increase was not due to shortening or "crowding" of the relevant cord segments; in 2 patients with dystrophia myotonica the cell body area of the motor neurons was reduced by comparison with normal controls.

The relationship between surface potentials and the number of active motor units

One of the assumptions inherent in a technique recently devised for enumerating motor units in human muscles is that the surface potentials from active motor units summate in a linear fashion. We present an electrical model of a muscle which predicts that a linear relationship between the number of active units and the electrical response recorded at the surface overlying the muscle would not be expected. The extent of the non-linearity, and hence the error in the calculation of the number of motor units in a given muscle, depends upon the ratio between the mean conductance of the motor units themselves and that of the external conduction pathway through which the electrical signal is fed (Gu/Ge). The extent of non-linearity is assessed experimentally in human hypothenar muscles using a "collision" technique. The average underestimate introduced into the calculation of the number of motor units in this particular case was concluded to be 26%. The value of Gu/Ge derived from these experiments, in 2 subjects, was checked by simulating an intramuscular action potential and determining the attenuation at the surface. The 2 independently obtained values were sufficiently close to suggest that the model may be a valid one. We conclude that caution should be employed in the interpretation of experiments which purport to determine the number of motor units in a muscle by means of surface recordings.

Dystrophia myotonica. A model of combined neural and myopathic muscle atrophy

Reports on clinical, electrophysiological and histological studies in dystrophia myotonica have shown that the peripheral nerves are usually not intact. In a few patients the neuropathy exceeds the myopathy. On the basis of these findings some authors believe that muscle changes in dystrophia myotonica are entirely due to the nerve lesions. However, despite the commonly found evidence of neuropathy, electromyographic and muscle biopsy findings are reported usually as "myopathic". Furthermore, clinical and laboratory findings indicate that the degree of muscle atrophy and peripheral nerve changes are unrelated. This report attempts to explain the discrepancies by suggesting that the "myopathic" and "neuropathic" changes in the muscle fibres in dystrophia myotonica are independent processes.

The lack of deep reflexes in myotonic dystrophy

Clinical and electrophysiological observations have been carried out on 12 patients with myotonic dystrophy. Neurological examination showed that the tendon reflexes were absent or weak in almost all cases, whereas the cutaneous reflexes were normal. Examination of both deep and superficial sensibility gave normal results. Electromyography confirmed widespread "myopathic" activity and myotonic discharges were recorded on insertion of the needle electrode and at rest. Motor and sensory conduction velocity in the ulnar nerve and motor conduction in the peroneal nerve proved to be normal. Repetitive supramaximal nerve stimulation showed in 10 cases a decrease in potential amplitude, more evident at higher frequencies of stimulation. In the 2 other cases, by contrast, an increase in amplitude was observed, and this was suggestive of a partial presynaptic block. The jaw reflex was absent in 5 cases and reduced in amplitude in the 7 other cases. The results of blink reflex investigations were normal, with the exception of 2 cases where no early response was elicited. Spinal monosynaptic reflexes were absent in 7 cases after both electrical (H reflex) and mechanical stimulation (T reflex), whereas the response to direct stimulation of nerve motor fibres (the M response) was always present, even though reduced in amplitude. Such data lead one to reject the hypothesis that the absence of deep reflexes is due to pathological change in the muscle spindles. It seems more likely that the selective atrophy of Type 1 muscle fibres, known to be involved in deep reflex responses, is responsible for the early disappearance of the tendon reflexes.

Dystrophia myotonica. Peripheral nerve involvement and pathogenetic implications

A comparative electrophysioloical study of patients with dystrophia myotonica and control subjects is presented. The study includes estimation of the number of motor axons innervating the extensor digitorum brevis muscle and measurements of the conduction of M- and F-waves along the deep peroneal nerve. There is unequivocal electrophysiological evidence of nerve involvement in the disease. This was indicated by (a) prolongation of the terminal latencies and slowing of the motor conduction of the deep peroneal nerve (P less than 0.001), (b) delayed conduction of the F-wave along the proximal segments of the nerves (P less than 0.02-0.01), (c) reduced number of motor axons innervating the extensor digitorum brevis muscle (P less than 0.001), (d) high amplitude motor unit potentials and discrete EMG activity on the maximal volitional contraction of minimally affected muscles in one patient. However, if the muscle changes were secondary to the nerve involvement occurring in the disease, one would expect that the electrophysiological findings of neuropathy would become more prominent in patients with severe muscle wasting and weakness. Our results showed that in some patients with marked muscle atrophy and weakness there was no evidence of nerve involvement while in other patients with slight to moderate degree of muscle weakness the electrophysiological studies indicated peripheral neuropathy. Therefore, it appears that both the nerves and the muscles are independently affected by the pleiotropic action of the responsible gene and the evidence does not suggest that the muscle atrophy of patients with dystrophia myotonica is entirely neural.

Computer method for the analysis of evoked motor unit potentials. 2. Duchenne, limb-girdle, facioscapulohumeral and myotonic muscular dystrophies

Single motor unit potentials recorded from surface electrodes over the extensor digitorum brevis muscle and evoked by stimulation of the anterior tibial nerve at the ankle were obtained by a computer subtraction method. Their latencies, durations, amplitudes, and areas were measured in control subjects and patients with Duchenne, limb-girdle, facioscapulohumeral, and myotonic muscular dystrophy. Lateral popliteal motor nerve conduction velocities were also recorded. In the muscular dystrophies there was a significant increase in both the latencies and durations of motor unit potentials, the latter in notable contrast with the findings of conventional needle electromyography. Fastest motor conduction velocities were significantly reduced in the limb-girdle, facioscapulohumeral, and myotonic muscular dystrophy patients, while the shortest distal motor latencies were significantly prolonged in these patients and those with Duchenne muscular dystrophy. The results support the presence of a definitive neurogenic influence in the muscular dystrophies.