Letter to Muscle & NerveUltrasound for Botox Injections
Application of ultrasound-guidance for our various injections and techniques is at the forefront of our research. Our recent contribution to Muscle & Nerve comments on the importance of using ultrasound-guidance for botulinum toxin injections.
OPTIMIZING EFFICACY OF BOTULINUM TOXIN INJECTIONS USING ULTRASOUND
We read with interest the recent article by Yi et al. on the use of Sihler staining to identify optimal locations within the ankle invertor muscles for botulinum toxin injections
on the basis of nerve branching or arborization patterns.
Lepage et al. originally used this approach in the antebrachial muscles with the goal of improving efficacy for surgical and chemical neurolysis and suggested it might be useful for botulinum toxin injections. However, while surgical or chemical neurolysis can be used effectively on nerve trunks, botulinum toxin can only be used effectively at the neuromuscular junction, where it binds exclusively to the presynaptic membranes of unmyelinated terminal motor axons.
Each muscle fiber has a neuromuscular junction. If the goal of treatment is to weaken a muscle while using the least amount of toxin, then the toxin should be distributed among all of the muscle fibers so as to reach as many neuromuscular junctions as possible. For a pennate muscle such as the flexor hallucis longus as depicted in Figure 1 of Yi et al.,1 the muscle fibers at the distal end of the muscle course diagonally upward to end at approximately one-fifth of the muscle’s length.1 A botulinum toxin injection proximal to the distal one-fifth of the muscle will miss these fibers. Similarly, a botulinum toxin injection in the distal fifth of the muscle will miss fibers in the proximal four-fifths.
One must take this into consideration when doing injections and ideally use real-time ultrasound to assess how much of the toxin solution is spreading parallel and perpendicular to the muscle fibers. For muscles such as the tibialis posterior, we typically use 4 injection sites to achieve adequate spread of injectate. For bipennate muscles, in addition to multiple injections proximally and distally, injections must be provided on both sides of the central tendon. A potential way to research how broadly injected toxin is spread throughout a muscle would be to mix it with radiographic contrast dye and assess its distribution shortly afterward using fluoroscopy. Yi et al.1 describe a “motor end plate zone” on the basis of visual assessments of terminal nerve branching, but note that this is a macroscopic assessment of nerve trunks and not their terminal axons, which can only be seen on light or electron microscopy. While nerve trunks are susceptible to phenol or alcohol injections and microsurgical neurolysis, they are not susceptible to botulinum toxin. The toxin targets are the unmyelinated terminal motor axons at the neuromuscular junction.
In summary, an optimally efficacious ultrasoundguided botulinum toxin injection technique would ideally disperse toxin among all of the muscle fibers of a target muscle, requiring multiple evenly spaced injections along the length of a pennate muscle and on both sides of the central tendon of a bipennate muscle.
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