The coupling discharge in neurogenic muscular atrophy

Sphincter electromyography and multiple system atrophy

Electromyographic studies of the sphincter in patients with multiple system atrophy have shown increased duration and polyphasia of motor unit potentials. These electrophysiological markers have been used to argue for the selective degeneration of sacral motor neurons in Onuf's nucleus in patients with multiple system atrophy. Studies comparing sphincter electromyographic changes in patients with multiple system atrophy and Parkinson's disease have shown significant differences between these two patient populations. Despite the controversy surrounding this claim, recent studies using quantitative electromyographic techniques support the view that reinnervation of the anal sphincter muscles may be a useful diagnostic marker for distinguishing multiple system atrophy from Parkinson's disease. A critical review of these data is needed to assess the validity and reliability of electromyographic changes in multiple system atrophy.

Satellite potentials as a measure of neuromuscular disorders

The study was carried out to investigate the characteristics of satellite potentials and their validity in clinical electromyography. Conventional needle electromyography was applied to the right biceps brachii and tibialis anterior muscles of 41 controls, 22 neuropathies, and 17 myopathies. Satellites were defined as small extrapotentials, preceding/following the main motor unit action potential (MUAP) component and separated from it by an isoelectrical interval of > 1 ms. The normal mean satellite rate was 1.6% (biceps brachii) and 1.2% (tibialis anterior). In the biceps brachii (tibialis anterior) muscle it was 5 (5) times higher for neuropathies (P = 0.005, P = 0.006) and 5 (6) times higher for myopathies (P = 0.006, P = 0.003). MUAP parameters were not significantly different, whether satellites were considered or ignored. Evaluation of the satellite rate increased detection rates of neuromuscular disorders by up to 13%. The satellite rate proved a valuable and easily available, supplemental electromyographic parameter for the discrimination and detection of neuromuscular disorders.

Motor axon reflex and indirect double discharge: ephaptic transmission? A reappraisal

Causal mechanisms of the motor axon reflex (MAR) and indirect double discharge (IDD) are re-examined. The hypothesis that these indirect intermediate latency responses result from an axono-axonal ephaptic transmission mechanism has recently been suggested. The various conceivable responses of models of axonal cross-talk are compared with actual observations made from the limbs. We conclude that the ephaptic hypothesis does not explain MAR and IDD, and that the earlier understanding that MAR results from axonal branching and IDD from proximal re-excitation on the axon holds true.

Serial action potential analysis in Guillain-Barré syndrome

Action potential analyses were carried out in 62 skeletal muscles of 16 patients with the Guillain-Barré syndrome. The muscles of a good prognosis showed only a slight change in duration, amplitude and phase of action potentials on electromyograms. On the other hand, the muscles of a poor prognosis showed a marked increase in duration and amplitude of action potentials, especially in the chronic stage. The incidence of polyphasic potentials was more frequent in the early stage than in the chronic stage of the muscles of a poor prognosis. The changes in duration in the early stage of a poor prognosis muscles were similar to those of acute beriberi neuropathy and the changes in the later stage were similar to those of amyotrophic lateral sclerosis, both of which showed axonal degenerations. In the early stage, myopathy-like changes in electromyography were often noticed, but the muscle biopsy showed only early neurogenic changes. Coupling or satellite discharges were found most frequently six or eight months after the onset, and decreased in number in the chronic stage. Satellites with blockings appeared earlier than those without blockings. A poor prognosis might be induced by an axonal degeneration partly together with segmental demyelinations.

Double discharges of distal origin: influence on the firing rhythm

Double discharges, or doublets, following the direct M response after peripheral nerve stimulation had a distal origin on the axon and were accompanied bay a back-response. This antidromic wave was demonstrated by collision technique or by its occasional recurrent F wave. Double discharges (DD) with a long interval between the two components had their origin in one subunit and were transmitted to another by motor axon reflex. Increse of the post-doublet interval (PDI) duration during EMG was best explained by invasion of the alpha cell by the back-response and the resetting of a new cycle. This invasion was sometimes followed by a recurrent F wave: a shorter PDI resulted. Normal PDI were probably due to blocking of the back-response at the myelinated-non-myelinated junction, without alpha cell invasion: weak or absent Renshaw inhibitory effect on the alpha cell concerned seemed to play a role.

Intranervous regeneration. The study of motor axon reflexes

More than 1000 motor axon reflexes (ARs) have been studied. The AR often has a long latency and the first excited branch almost always has the fastest conduction velocity. Sometimes the potentials of the two branches are recorded at the same needle placement, giving the appearance in the EMG tracing of a coupling discharge. Stimulation of the axons in the muscle produces more ARs than stimulation of the nerve trunk. ARs are recorded in denervated muscles but never in normal subjects. All kinds of lower motor neuron lesions produce ARs. They are more frequent in the Charcot-Marie-Tooth syndrome and hereditary neuropathy with liability to pressure palsies where the axonal lesions are mainly located at the traditional sites of entrapment. The possibility of neurolysis in some of these cases has to be considered. Obstetrical brachial plexus palsies furnish many ARs linking two muscles, thus explaining the associated contractions in these cases.

An analysis of double discharges in the human electromyogram

Different characteristics of double discharges (doublets) of motor units in m. biceps brachii and tibialis anterior were examined. Doublets may occur physiologically at the beginning and termination of muscular effort and at the minimum rhythmic firing frequency, but doublets arising within a regular sequence of motor unit potentials can be regarded as abnormal. In neuromuscular diseases about 3% of motor units showed this firing pattern, but it was never found in intact limb muscles. Most of the abnormal doublets have the following characteristics: the interval between the first and the second component is 9 msec or less (longer intervals up to 22 msec occur very rarely); both components have nearly the same form; the amplitude of the second component is slightly smaller than that of the first; a stepwise or continuous jitter between doublet components is common; doublets recur randomly during a sequence of motor unit potentials; the interval between the doublet and the next MUP is usually about 50% longer than the mean MUP interval. A number of doublets differ, however, in one way or another from the above. The second component of the doublet may differ completely in amplitude and even in form from the first component; or they may fuse. It is thus hardly possible to define the lower limit of the interval between doublet components. Sometimes the post-doublet interval increase is absent. When applying all the criteria there is seldom any uncertainty about the identification of doublets.

The differential diagnosis of scapuloperoneal amyotrophy

This report deals with a scapuloperoneal syndrome which developed simultanously with pain and distal paresthesias. In addition there was a slight sensory disturbance of glove and stocking type distribution. Motor conduction velocity was within normal limits and all distal latencies of response were normal; only the sensory conduction velocity of the left median nerve was found to be decreased (42.1 m/s). Electromyographic investigations revealed only signs of myopathy. Histological findings (m. deltoideus, m. tibialis anterior) favoured a primary myopathic process. Biopsy of the n. suralis revealed no certain pathological changes. The affection appears to have an autosomal dominant mode of inheritance. The sensory disturbance and decreased reflexes indicate an involvement of the nervous system, but the question of relationship to the scapuloperoneal muscular atrophy cannot yet be answered.

"Satellite potentials" and the duration of motor unit potentials in normal, neuropathic and myopathic muscles

Time-locked activity in the EMG was studied in normal, neuropathic and myopathic muscles, using the averaging technique. Normal and neuropathic muscles seemed in most cases not to differ with regard to the frequency nor the quality of "satellite signals", i.e. small and short potentials joining the main component of the MUP with a constant interval, up to 26 ms in duration. In normal muscles 10% of the MUPs and in neuropathic muscles 12% had a satellite, which, when studied, showed no signs of blocking. However, in 2 patients with an apparent old spinal affection both an increased number and an increased latency of satellite signals were found. In the different myopathies studied 45% of the MUPs recorded consecutively were preceded or followed by an often increased number of tiny satellites. Sometimes the satellites joining the myopathic MUP formed a long (up to 60 ms) "tail" after the triggering component. On the basis of our results we conclude that in myopathies the total duration of MUPs is in certain cases pathologically incrased rather than decreased.

Transmission block in terminal nerve twigs: a single fibre electromyographic finding in man

Single fibre electromyography has been performed in patients with partial nerve lesions, amyotrophic lateral sclerosis, progressive spinal muscle atrophy, muscular dystrophy, and distal hereditary myopathy. The recorded action potentials were often more complex than in the normal muscle due to increased fibre density in the motor unit and the individual spike components showed a large jitter and occasional blockings. Sometimes two or more spikes in a complex disappeared and reappeared simultaneously upon successive discharges. This phenomenon, called `paired blocking', has been further investigated. The jitter of the blocking potentials in relation to the rest of the complex was large, up to 500 μsec. The degree of blocking increased with increasing innervation frequency until it eventually proceeded to total block during continuous activity. Sometimes a slight effect on blocking was seen after edrophonium. This type of block is probably localized in the terminal nerve twigs, perhaps in newly formed sprouts. The phenomenon of neurogenic blocking may contribute to the fatigue clinically experienced in different denervation-reinnervation cases.