Repeater F-waves in ALS are they indicators of UMN or LMN involvement?

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the presence and density of the repeater F waves are mainly related to the degree of loss of LMN and they show no obvious correlation with the UMN system malfunction

In neuroscience, an F wave is the second of two voltage changes observed after the application of electrical stimulation to the surface of the skin above the distal region of a nerve. F-waves are often used to measure nerve conduction velocity and are particularly useful for assessing conduction problems in the proximal region of the nerves (ie, nerve portions close to the spinal cord).

They are almost universally used for the diagnosis of ALS. At the same time ALS patients have often complained about the quality of this review, which regularly leads to incorrect diagnoses. A new article by Akarsu and his colleagues shows that other techniques using the same tools are more accurate. It may even be that in the long term we are reexamining the statement that ALS is a disease affecting both upper and lower motor neurons.

In a typical F wave study, a strong electrical stimulus is applied to the surface of the skin above the distal portion of a nerve, so that the pulse travels both distally (towards the muscle fiber). ) and proximal (to the motor neurons of the spinal cord).

These impulses are also called orthodromic and antidromic, respectively. When the orthodromic stimulus reaches the muscle fiber, it causes a strong response in M ​​indicating muscle contraction. When the antidromic stimulus reaches the motoneuron's cell bodies, a small part of it turns against it and an orthodromic wave descends from the nerve to the muscle.

This reflected stimulus evokes a small proportion of muscle fibers, resulting in a second group of near-simultaneous action potentials from several muscle fibers in the same area, called the F ** wave.

Electrophysiological biomarkers have allowed extensive work to detect and quantify upper motor neuron (UMN) and lower motor neuron (LMN) dysfunction in amyotrophic lateral sclerosis. Neurophysiological index and motor unit number estimation (MUNE) methods have been widely used as potential biomarkers of LLN loss.

The neurophysiological index has been suggested to demonstrate the loss of LML in patients with ALS, even in presymptomatic muscles, and has been shown to be sensitive to the detection of disease progression. Although several MUNE methods and transcranial magnetic stimulation with single pulses and matched impulses have been proposed since the invention of the first technique in 1971, none of them has been accepted as a standard method because of the various inherent limitations. to the technique.

In ALS, a disease affecting both UMN and LMN, cortical and peripheral mechanisms have been proposed to explain F-wave abnormalities. An increase in the number of repeater F waves in the presence of clinical involvement of the UMN has been reported in ALS. On the other hand, it was found that the atrophied muscles, more marked in the thenar region, generated more repeater F waves, which is consistent with the division of the hand that occurs in the same disease.

Overall, the mechanism for generating repeater waves is still discussed.

In the present study, the authors aimed to study repeated F waves in the thenar and hypothenar muscles of patients with ALS and their correlation with other electrophysiological markers to better understand the dominance of the dysfunction of the UMN or LMN in the mechanism of their emergence.

Their results, taken as a whole, suggest that the presence and density of the repeater F waves are mainly related to the degree of loss of lower motor neurons.

In response to the progressive loss of motor neurons, reinnervation intervenes to compensate and the results of these dual processes establish the diagnostic features of ALS. The reduced number of motoneurons in the generation of F waves gives rise to a greater number of repeater F waves. On the other hand, the large F waves and giant F waves of the repeater have been associated with re-innervated motor units.

An earlier study showed that the frequency of repeater F waves was increased in ALS patients with pyramidal signs compared to the non-pyramidal group. The authors therefore divided the groups of patients according to the presence or absence of pyramidal signs and did not use a quantitative tool to determine the involvement of the corticospinal tract.

F-wave studies in UMN-only diseases, such as multiple sclerosis and cerebrovascular disease, have shown an increase in the persistence, amplitude, duration, and latency of the F-wave. but none of these studies studied repeated F waves.

According to their results, the ALSFRS-R and MRC sum scores were not correlated with the F-wave repeat parameters. These clinical scores provide an overall functional assessment in patients with ALS.

In addition, the ALSFRS-R UL score, the ALSFRS-R sub-score addressing upper limb function, also revealed no correlation.

This suggests that clinical scores are less reflective of motoneuronal loss, possibly due to the remanent capacity of the motor system, and that repeater F waves may provide an earlier measure of motor neuron degeneration, as most electrophysiological methods do. on this subject.

Conclusion

Their overall results suggest that the presence and density of the repeater F waves are mainly related to the degree of LLN loss and they do not show any obvious correlation with the UMN network malfunction.

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