Contribution of Single-Fiber Evaluation on Monitoring Outcomes Following Injection of Botulinum Toxin-A: A Narrative Review of the Literature

Abstract

Botulinum toxin-A (BoNT-A) blocks acetylcholine release at the neuromuscular junction (NMJ) and is widely used for neuromuscular disorders (involuntary spasms, dystonic disorders and spasticity). However, its therapeutic effects are usually measured by clinical scales of questionable validity. Single-fiber electromyography (SFEMG) is a sensitive, validated diagnostic technique for NMJ impairment such as myasthenia. The jitter parameter (µs) represents the variability of interpotential intervals of two muscle fibers from the same motor unit. This narrative review reports SFEMG use in BoNT-A treatment. Twenty-four articles were selected from 175 eligible articles searched in Medline/Pubmed and Cochrane Library from their creation until May 2020. The results showed that jitter is sensitive to early NMJ modifications following BoNT-A injection, with an increase in the early days’ post-injection and a peak between Day 15 and 30, when symptoms diminish or disappear. The reappearance of symptoms accompanies a tendency for a decrease in jitter, but always precedes its normalization, either delayed or nonexistent. Increased jitter is observed in distant muscles from the injection site. No dose effect relationship was demonstrated. SFEMG could help physicians in their therapeutic evaluation according to the pathology considered. More data are needed to consider jitter as a predictor of BoNT-A clinical efficacy.

Some reasons for frowning upon the effects of botulinum toxin injections in facial muscles

“…Recently, single fiber electromyography (SFEMG) studies are widely used to search the neuromuscular junction (NMJ) functions in patients who applied BoNT and showed the increase in jitter frequently associated with blocks in the muscle injected BoNT. These SFEMG findings have been shown in the muscles near and far from the application site ( Lispi et al, 2018 , Moron et al, 2021 ). Unfortunately, the results of the effects of BoNT on the NMJs are interrelated with each other and are severe enough to lead to both clinical and electrophysiological difficulties in the diagnosis of NMJ diseases.…”

“…If botulism occurs because of cosmetic or therapeutic procedures using BoNT, it is called iatrogenic botulism. In addition to iatrogenic botulism, temporary weakness or myasthenia-like weakness such as dysphagia, dysarthria, unilateral ptosis, and neck extensor muscle weakness may also occur when a patient who received even therapeutic doses of BoNT ( Moron et al, 2021 ).…”

“…Single-fibre electromyography is such a sensitive method of demonstrating NMJ disorder that it has been confirmed to show NMJ disorders even in cases of botulism without clinical signs of general muscle weakness ( Ehrenreich et al, 1989 , Moron et al, 2021 ). The jitter parameter of SFEMG technique measures the variability of the interpotential interval of two muscle fibres from the same motor unit as microvolt.…”

Lambert-Eaton Myasthenic Syndrome and Botulism

“…When evaluating for a disorder of neuromuscular transmission, it is particularly important to closely question a patient about prior exposure, as even a single exposure to targeted botulinum toxin can manifest with distant systemic symptoms. 58 EMG signs of prior exposure include abnormal spontaneous activity and short-amplitude, short-duration motor unit potentials with early recruitment or tall, thin, longer-duration motor unit potentials with reduced recruitment. If a study is performed after the recent administration of botulinum toxin, motor unit potentials may be unstable.…”

“…Iatrogenic botulism is due primarily to overdose from botulinum toxin use in the treatment of migraines, dystonias, blepharospasm, spasticity, sialorrhea, and cosmetic use. When evaluating for a disorder of neuromuscular transmission, it is particularly important to closely question a patient about prior exposure, as even a single exposure to targeted botulinum toxin can manifest with distant systemic symptoms 58 . EMG signs of prior exposure include abnormal spontaneous activity and short-amplitude, short-duration motor unit potentials with early recruitment or tall, thin, longer-duration motor unit potentials with reduced recruitment.…”