What do fasciculations look like

Classifying abnormal involuntary movements can be challenging. Myoclonus is a sudden, brief shock-like movement of a joint, the fastest and briefest of all hyperkinetic movement disorders Blindauer, , Espay and Chen, Movements are caused either by muscle contraction, or in the case of negative myoclonus, by muscle inhibition Blindauer, Fasciculations, on the other hand cause visible twitches of a portion of an individual muscle.

As opposed to myoclonus, they are too small to cause movement of a joint, although large distal fasciculations may displace a digit Ropper et al. Fasciculation potentials are generated from the spontaneous depolarization of a single motor axon, or its nerve terminal, with propagation to the remaining arborisation of the motor unit Layzer, Therefore, only a small portion of the muscle contracts with any given fasciculation.

In MND, hyperexcitability of upper motor neurons may also play a role in generating fasciculations de Carvalho et al. Myoclonus involves the contraction or inhibition of a single muscle or group of muscles Zutt et al.

Both myoclonus and fasciculations can be worsened by action. This is particularly seen post-hypoxic cerebral injury Lance and Adams, , Fahn, , although it can be seen in other forms Zutt et al. Fasciculations, although characteristically present at rest, may increase with muscle contraction. Although fasciculations are a well-established feature of MND, myoclonus is not expected to occur in this condition, at least not in typical sporadic MND. However, segmental spinal myoclonus has been described in a slowly progressive familial form of MND Alberca et al.

Muscle ultrasound, combined with electrophysiological studies, may be helpful in trying to decipher myoclonus from fasciculations and other movement disorders. On EMG, fasciculation potentials resemble motor units discharging in an irregular fashion.

Muscle ultrasound continues to gain popularity in the neuromuscular realm with an increasing number of indications. It has been used in patients with possible MND to assess for fasciculations and is even more sensitive than EMG in this context Misawa et al. As Inoue and colleagues have demonstrated, ultrasound has the advantage of being dynamic, able to assess muscle architecture throughout movement, so that fasciculations are detectable even in a contracted muscle, where they can otherwise be obscured by voluntary EMG activity.

The two patients with movements resembling myoclonus had fasciculations in the biceps brachii at rest, which became more florid on ultrasonography with gentle sustained contraction.

This finding suggests that fasciculations are likely to be causing the movements, thus distinguishing them from true myoclonus. Indeed other authors have noted the difficulty in clinically differentiating finger movement due to florid fasciculations from a form of distal symmetrical myoclonus — termed mini-polymyoclonus — which classically occurs in neurodegenerative disorders Bhat et al.

Additional applications of ultrasound in the MND clinic include as an alternative to EMG for the detection of fasciculations in genioglossus, where it is not only less invasive but significantly more sensitive Misawa et al. Ultrasound of the tongue during swallowing may indicate upper motor neuron dysfunction Noto et al. Recent studies have also suggested ultrasonographic differences between MND and multifocal motor neuropathy MMN , with nerve enlargement seen in the latter Grimm et al.

MMN can be difficult to distinguish from MND, even electrophysiologically and ultrasound is therefore, a useful adjunct. Muscle twitching can be a symptom of Myalgic Encephalomyelitis. ISSN PMC PMID Journal of Internal Medicine. Martin ; Bested, Alison C. Categories : Signs and symptoms Muscular signs and symptoms Neurological signs and symptoms. The inter-fasciculation interval was measured.

These double discharges were further classified according to whether the second discharge was the same fasciculation potential DSFP Fig.

The frequency distribution of each variable in the fasciculation potentials of amyotrophic lateral sclerosis and benign fasciculation syndrome was examined for normality using the Normal plot method. Essentially, the cumulative frequency distribution of the data was plotted against that of a normal distribution with the same mean and variance, linearity indicating the likelihood of the distribution being normal. All were found to have skewed distributions Fig. Log transformation, however, rendered the distributions normal, indicated by approximate linearity on the Normal plots Fig.

Thus testing the difference in these parameters between amyotrophic lateral sclerosis and benign fasciculation syndrome and between groups of patients with amyotrophic lateral sclerosis at various stages of the disease was done using a t -test assuming unequal variances on the log-transformed data.

In tables, data are presented for easier understanding as median and range. A The distribution of median discharge interval of benign fasciculations black bars and amyotrophic lateral sclerosis fasciculations open bars.

B Log transformation of intervals renders the distributions approximately normal. The distributions overlap but the difference in median interval is clearly seen. C The median intervals plotted against the normal deviate with the same mean and variance shows a curvilinear relation.

D After log transformation, the plot of log median interval against the normal deviate is approximately linear, confirming the validity of using log-transformed data for statistical testing. At a single recording position up to 15 individual fasciculation potentials could be identified Figs 1 and 2.

Fasciculations occurring just once during the recording amounted to an average of Double fasciculations of both types and evidence of axonal conduction block occurred in both amyotrophic lateral sclerosis and benign fasciculation syndrome. Amplitude, area, duration and turn count all showed skewed distributions in both amyotrophic lateral sclerosis and benign fasciculation syndrome, but none showed evidence of bimodality as might be expected if, as some authors de Carvalho and Swash, have suggested, fasciculation potentials can be segregated into simple and complex or into stable and unstable.

Turn count, for example, a simple measure of complexity, showed a continuous unimodal distribution in both amyotrophic lateral sclerosis and benign fasciculation syndrome fasciculation potentials Supplementary Fig.

The measurements in amyotrophic lateral sclerosis fasciculation potentials and benign fasciculation syndrome fasciculation potentials are set out in Table 1. The time variability Fig. The interval between fasciculation potentials Fig. Evidence of axonal conduction block was found in DSFPs were seen in Similarly, DDFPs occurred in Complex fasciculation potentials may be defined as those having either more than four phases, increased amplitude or increased duration compared to normal values for motor unit potentials in the specified muscle de Carvalho et al.

Using this definition, Fasciculation potentials recorded from patients with amyotrophic lateral sclerosis and benign fasciculation syndrome. A and B Fasciculation potential from a patient with amyotrophic lateral sclerosis showing multifocal distal triggering; the arrowed component fires at different times within the fasciculation potential.

C Fasciculation potential from a patient with benign fasciculation syndrome showing time variability of a single component; the time variability is measured as shown. D Fasciculation potential from a patient with benign fasciculation syndrome showing intermittent conduction block of one component. Fasciculation potential parameters in amyotrophic lateral sclerosis and benign fasciculation syndrome.

Measurements on fasciculation potentials in weak and normal strength muscles are set out in Table 2. DSFPs were found in DDFPs occurred in Fasciculation potential parameters in weak and non-weak muscles in amyotrophic lateral sclerosis. Fasciculation potential parameters at the various stages of muscle weakness and denervation are set out in Table 3. Amplitude, area, turns and time variability of fasciculation potentials showed no significant changes across the stages.

Discharge interval Fig. There were no significant differences, however, in the percentage of fasciculation potentials showing evidence of axonal conduction block across the four stages. DDFPs, however, showed no significant changes in incidence over the four stages. The average inter-fasciculation interval shortens as the degree of weakness and EMG neurogenic change increases.

The incidence of double fasciculations with the same potential in each discharge increases significantly as the degree of weakness and EMG neurogenic change increases. Comparing DSFPs from tibialis anterior in amyotrophic lateral sclerosis and benign fasciculation syndrome, the mean inter-fasciculation interval in this early band was 6. Comparing DSFPs from weak and strong muscles in amyotrophic lateral sclerosis, the mean inter-fasciculation interval in weak muscles was 6.

DSFPs were too infrequent to allow a comparison across the different stages of disease. Of the 48 DSFPs in amyotrophic lateral sclerosis, the waveforms of the two fasciculation potentials in this early peak were identical in 38 Time interval histograms of double fasciculations with the two discharges the same in benign fasciculation syndrome BFS and amyotrophic lateral sclerosis ALS. The data from tibialis anterior and medial gastrocnemius have been pooled in each case.

Time interval histograms of identical double fasciculations in biceps, first dorsal interosseous FDI and tibialis anterior TA in patients with amyotrophic lateral sclerosis.

In five amyotrophic lateral sclerosis muscles, DSFPs were frequent enough to permit a more detailed analysis Fig. It was noted that whereas the first discharge could have a variable waveform suggesting multiple distal axonal triggering, the second discharge was identical on all occasions. Interval histograms of DDFPs were flat, indicating a uniform probability of a different fasciculation potential occurring after the triggering fasciculation potential; this held true in both amyotrophic lateral sclerosis and benign fasciculation syndrome, across muscles in amyotrophic lateral sclerosis and at the various stages of neurogenic change in amyotrophic lateral sclerosis.

The question as to whether benign fasciculation syndrome and amyotrophic lateral sclerosis fasciculation potentials can be distinguished based on their waveform of firing characteristics is first addressed.

The amplitude, area, time variability of components and complexity as measured by the coefficients of variation of amplitude or area are not different. Evidence of axonal conduction block is also present in both conditions; so are DSFPs, but they are far more frequent in amyotrophic lateral sclerosis.

Firing interval is random in both conditions but is significantly longer in benign fasciculation syndrome. Clearly then, in the individual case, there are no features distinguishing the two conditions. Instability and complexity have generally been held to be characteristic of the fasciculation potentials seen in amyotrophic lateral sclerosis, whereas those in benign fasciculation syndrome are thought to have relatively simple waveforms.

The current results refute this and show that in both conditions fasciculation potentials can be highly complex. It is worth considering the sources of this complexity, and three main factors should be considered. Assuming that most fasciculation potentials arise from distal generator sites, one source of such complexity is intermittent axonal conduction block evidenced by some components of the fasciculation potential making an intermittent appearance Figs 1 , 2 and 5.

Evidence of axonal conduction block has been found in the present study in about equal degrees in both amyotrophic lateral sclerosis and benign fasciculation syndrome. This would argue that membrane abnormalities making axons intermittently subject to block are present in both amyotrophic lateral sclerosis and benign fasciculation syndrome, although the mechanisms of block may not necessarily be the same in each condition.

A second factor contributing to complexity is variability in axonal conduction time, manifesting as variability in the discharge of an identifiable component of the fasciculation potential.

Again, evidence of such variability has been found in both amyotrophic lateral sclerosis and benign fasciculation syndrome Fig. The time variability parameter, measuring the maximal time variation of a fasciculation potential component in relation to the triggering component, was not significantly different between the two conditions.

Thus factors causing variation in axonal conduction, which may be the same as the mechanisms causing axonal block, are also present in both conditions. Factors such as axonal diameter, internodal distance, state of myelination, temperature and so on, clearly play no role in these short-term variations in axonal conduction. It is more probable that there are changes in membrane potential due to imbalance of ionic conductances.

A third factor that may contribute to complexity of fasciculation potentials is multifocal and possibly intermittent distal axonal triggering of fasciculation potentials. The effect of this will be to produce a different order of firing of the components of a given fasciculating motor unit Fig.

Distinguishing multifocal distal triggering from extreme timing variability due to insecurity of axonal conduction is difficult. However, the fact that up to 15 identifiable waveforms can be recognized at a single recording site sheds some light on this.

In muscles undergoing denervation and reinnervation, grouping of fibres will further reduce the number of motor units within the pick-up volume. However, if each fasciculating motor unit has 2 or more generator sites, then the percentage of fasciculating units is reduced commensurately.

This latter argument also has an effect upon what has hitherto been referred to as fasciculation discharge interval; in effect what is probably being reported is the discharge interval of individual generator sites.

Furthermore, the fact that many identifiable waveforms at a single needle site are seen in both amyotrophic lateral sclerosis and benign fasciculation syndrome suggests that distal multifocal triggering also occurs in both conditions. In both amyotrophic lateral sclerosis and benign fasciculation syndrome, double fasciculation potentials with the two discharges from the same motor unit were seen in all muscles investigated. The incidence of these DSFPs was significantly higher, however, in amyotrophic lateral sclerosis.

It is tempting therefore to attribute the early range DSFPs to axonal superexcitability. If the axon containing the fasciculation potential generator is situated proximal to the terminal arborization within the muscle and before branch points, then the fibres of the fasciculating unit will fire in the same order on each occasion and therefore the fasciculation potential waveform will be invariant ignoring axonal conduction block and variation in fibre conduction time.

Similarly, if there is just one fasciculation potential generator situated in one of the branches of the terminal arborization, then again the fasciculation potential waveform will be the same on each occasion.

If however, there are more than one fasciculation potential generators in the terminal arborization, then the fasciculation potential waveform will vary because of a different order of fibre activation, dependent on which fasciculation potential generator is active.

Thus the fact that short interval DSFPs were almost always of identical waveform suggests that the double discharge arose from the same point in the motor unit arborization. Purely on timing grounds, this could represent either an F-response or a monosynaptic reflex transmitted over fast conducting afferent fibres. A fasciculating motor unit could activate stretch or tension receptors in the muscle. The lack of interaction between different fasciculation potentials, however, argues against a reflex cause for double fasciculation potentials.

From Fig. If we postulate that after generation in the terminal arborization, impulses propagate antidromically to the motor neuron and then back to the muscle, then the order of fibre activation would be identical for the latter but may vary for the first fasciculation potential.

Double fasciculations in amyotrophic lateral sclerosis in the early and later phases of increased firing probability.