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

Motor unit estimation: anxieties and achievements

The history of motor unit number estimation (MUNE) is given, together with brief descriptions of the various methods presently available. A small muscle of the hand contains about 100 motor units and greater numbers are found in larger muscles; beyond 60 years the numbers begin to decline. In ALS approximately half the motor units cease to function within 6 months of the involvement of the motoneuron pool, while in adult spinal muscular atrophy further loss may not occur over several years. The reduction in MUNE values in myotonic dystrophy remains an enigma, but even more curious are the losses and subsequent recoveries occasionally observed in hyperthyroidism and chronic renal failure; possibly, nontransmitting ("silent") synapses are involved. MUNE may also be used to study CNS problems such as hemiplegia and congenital brachial palsy. The availability of more powerful computers for EMG should lead to advances in MUNE.

Invited review: motor unit estimation: methods, results, and present status

The renewed interest in motor unit estimation (counting) has coincided with the introduction of computer-based methodology and with the application of the technique to proximal as well as distal muscles. The advantages and disadvantages of the different methods are considered, together with the assumptions inherent in this type of examination. In normal subjects, the extensor digitorum brevis (EDB) muscle has approximately 200 motor units while each of the intrinsic muscles of the hand has about 100 units; larger muscles in the limbs contain greater numbers of units. Beyond the age of 60 years, there is a decline in the number of functioning motor units in both proximal and distal muscles. In denervating disorders, motor unit estimation is useful for diagnosis and assessment; abnormal values may often be observed in muscles judged clinically to be unaffected. Serial studies have enabled the rate of motor unit loss to be determined in ALS and in spinal muscular atrophy. Depletion of motor units has also been found following upper motoneuron lesions caused by injury to the spinal cord or by cerebral hemorrhage; trans-synaptic dysfunction has been presumed responsible. Rather surprisingly, reduced numbers of motor units have been observed in a variety of myopathic disorders; of these, the most consistent abnormalities have been reported in myotonic muscular dystrophy.

Short-latency somatosensory evoked potentials in dystrophia myotonica

Somatosensory evoked potentials (SEPs) were recorded in a group of 21 patients with dystrophia myotonica and in a group of controls. Those with dystrophia myotonica had longer absolute peak latencies due to slower peripheral conduction. SEP abnormalities revealed peripheral and/or central conduction delays in 33% of the dystrophia myotonica subjects. There was no apparent relationship between the clinical severity of the disease and SEP abnormality.

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.

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.

A quantitative electrophysiological study of uraemic neuropathy. Diabetic and renal neuropathies compared

The numbers of functioning motor units and the parameters of the electrically evoked motor unit potentials in the extensor digitorum brevis muscles of 30 patients on chronic haemodialysis for renal failure were obtained using recently introduced quantitative electrophysiological techniques. Measurements of the distal motor latencies and motor nerve conduction velocities in the lateral popliteal nerves of the same patients are also presented. The results support the presence of a "dying back" type of neuropathy in uraemia, with a preferential involvement of the fastest conducting motor axons. Collateral reinnervation and compensatory increase in size of surviving motor units is relatively impaired in uraemic neuropathy in comparison with a similar study on diabetic neuropathy. The significance of paranodal and segmental demyelination in producing abnormalities of conduction velocity in these neuropathies is discussed.

Axonal dysfunction in the neuropathy of diabetes mellitus: a quantitative electrophysiological study

The extent of axonal dysfunction in a group of patients with diabetes mellitus has been investigated using recently introduced quantitative techniques of electromyography. In 40 diabetic patients, estimates of motor unit numbers in the extensor digitorum brevis muscles and measurements of the parameters of electrically evoked motor unit potentials therein are presented, along with significant correlations between these parameters and motor nerve conduction velocities and distal motor latencies in the lateral popliteal nerves. The results indicate that axonal dysfunction and concomitant collateral reinnervation are prominent and integral features of the neuropathy of diabetes mellitus. Axonal dysfunction is present in proportion to and parallels the severity of the demyelinative lesion. It is also extensive in patients without clinical evidence of neuropathy in whom the accompanying collateral reinnervation is of sufficient magnitude to mask the clinical signs of motor involvement.

F-wave studies on the deep peroneal nerve. Part 2.--1. Chronic renal failure 2. Limb-girdle muscular dystrophy

The F-wave has been used for estimations of motor nerve conduction velocity (MNCV) along the proximal segment (anterior horn cells to the knee) of the axons of the deep peroneal nerve in patients with renal failure and limb-girdle muscular dystrophy. MNCV and terminal latencies with conventional methods have also been estimated. Patients with chronic renal failure show marked F-chronodispersion (i.e. F-wave temporal dispersion) and impaired conduction along proximal and distal segments of the deep peroneal nerve. There is no evidence of motor nerve impairment, with conventional and F-wave methods, in patients with limb-girdle muscular dystrophy. It is shown that F-wave measurements are useful in electrophysiological studies of patients and may be more sensitive than conventional MNCV methods.

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.

A quantitative electrophysiological study of acute idiopathic polyneuritis

The motor unit content and the dimensions of individual motor unit action potentials were studied in 17 patients with acute idiopathic polyneuritis from one week to 9 1/2 years after the onset of the illness. An initial decrease in motor unit numbers is followed by a progressive increase with the passage of time from the onset of the illness. The latencies, areas, amplitudes, and durations of individual motor unit potentials were increased above normal values. The results suggest the presence of significant axonal damage in the majority of cases of acute idiopathic polyneuritis. The intramuscular nerve fibres are the site of most severe electrophysiological dysfunction in this study.