Botulinum toxin injection,dilution confusion: The impact of toxin diffusion on clinical practice

Botulinum toxins (BoNT) are considered standard of care of spasticity management for a variety of neuromuscular conditions in the pediatric community. BoNT injections offer a reliable, effective, and minimally invasive way to relax skeletal muscles by inhibiting the release of acetylcholine from the neuromuscular junction (NMJ). Many Pediatric Physiatrists find themselves advising patients and families on the risk and benefits of BoNT in spasticity management. Through their training physiatrists become skilled at targeting specific muscles for these injections. In order to achieve an adequate clinical effect it is believed that a sufficient number of units of BoNT must be delivered to the NMJ, and accurate localization of the target muscle must be achieved [1]. There are a variety of resources to increase accuracy of injections including anatomic landmarks, knowledge of motor endplates, electrical stimulation devices, electromyographic amplifiers, and sonography, computer topography and magnetic resonance imaging. Over time providers become accustomed to the pharmacological differences between different BoNT formulations. In addition, providers must also consider extrinsic factors that may affect BoNT delivery including the injection technique, motor endplate localization, and reconstitution and dilution of the neurotoxin; all of which can affect the efficacy of these injections. Ultimately, the decisions regarding quantity, dilution, and localization technique to utilize in the administration of BoNT is up to each provider’s discretion. Providers may choose to develop a standard dilution pattern for their preferred toxin that provides a consistent and reliable dose of BoNT per volume or they may choose to vary the dilution of BoNT on a case by case basis. As part of these calculations and considerations providers must account for the distribution or spread of BoNT after it is injected. In theory, increasing the amount of normal saline used to reconstitute the BoNT can strategically vary the spread of the BoNT within a target muscle, modifying the dilution of BoNT during reconstitution to either maximize or minimize the effect of BoNT on NMJs [2, 3].

In the literature there is limited evidence regarding the clinical impact of variable dilution of BoNT. In both adult and pediatric trials comparing high-volume and low-volume dilutions of BoNT, researchers have found no statistically significant difference between groups in electrodiagnostic results, spasticity grading, range of motion, or gait characteristics [4, 5, 6]. In his review of BoNT dosing and dilution, Francisco speculates that the lack of detectable impact between high and low volume dilutions (e.g. 100 U/ml v. 20 U/ml) may be due to the fact that the differences in these volumes may not have been significant enough [4]. He goes on to propose that larger volumes of dilute toxin may be needed to detect a clinically significant effect between low-volume and high-volume BoNT injections [4]. Nonetheless, to date there is no evidence for diagnosis or muscle group specific benefits from higher or lower volume dilutions injections of BoNT that has demonstrated clinically significant outcomes [4]. This likely reflects many confounders including a lack of high-quality research, the heterogeneity between individuals with spasticity within these studies, and a lack of sensitivity of our exams and clinical outcome measures.

In practice, the most typical concentration of onabotulinumtoxinA that is utilized is 100 U/ml saline [7]. The factors of size and anatomy can guide provider’s decision to change the reconstitution of BoNT from his or her common practice. A provider can manipulate the size or total volume of injectate that is administered. In theory, the volume of solution used to deliver BoNT can affect the diffusion distance of injectate from the injection site. It has been demonstrated in animal models that injecting higher volume of low concentration BoNT was more effective at relaxing skeletal muscles. It is believed that this occurs as higher volumes allow the BoNT to distribute into a greater area within the target muscle, leading to the inhibition of more NMJs and increased paralysis [2, 3]. This is in contrast to the current standard of multiple injections sites within a muscle to deliver the BoNT to discrete NMJs in a controlled manner. There is promising data to suggest that this conclusion might be true although the clinical evidence has yet to demonstrate this effect in humans [4, 8].

All formulations of BoNT have weight-based standards. The weight of the patient determines the total number of units necessary. Thus, the size or weight of the child is a potentially important factor to be considered for varying the dilution strategy. When an individual’s weight limits the number of units of BoNT, providers may choose to reconstitute the toxin twofold, or as is suggested in the literature, by even more dilute concentrations (i.e. four to five-fold as has been tested in clinical trials). Dilution increases the diffusion of the toxin across a greater surface area potentially maximizing the toxins effect, albeit the clinical impact of this has proven to be minimal [5, 8].

Beyond muscle size, the developing pediatric musculoskeletal system has a higher density of NMJs compared to the adult population [1, 9]. Given the mechanism of action of BoNT at the NMJ, understanding the differences in the developing musculoskeletal system can provide important insight for dosing and dilution recommendations in children. Our current standards for BoNT dosing are largely based on adult data and consensus opinion. To date, the pediatric research has not specifically evaluated BoNT dosing based on the number of NMJs within a muscle [1, 9]. At least in theory it may take more toxin to saturate the NMJs in younger children than older youth and adults, before achieving similar levels of chemodenervation or before the possibility of unintentional spread to nearby structures [9]. As such, we find the muscle groups of these children are small but mighty.

Muscle bulk is another important consideration that could affect the diffusion of BoNT to target NMJs. Molecular and cellular characteristics of muscle changes through typical development from childhood to adulthood. Generally, the bulk of children’s muscle groups are smaller than adults and the majority of the increase in muscle bulk that occurs in adulthood represents myocyte hypertrophy rather than hyperplasia. Nonetheless, muscle size acts at least as a partial surrogate for force generation and influences dosing recommendations for BoNT [10, 11]. An important distinction between adult’s and children’s muscles occur at the cellular level. In children there are differences in the density and topography of NMJs when compared to adults which may benefit from further investigation when trying to understand the effect of BoNT in the pediatric population [1]. While pediatric muscles generally are smaller than their adult equivalents the composition of NMJ sites within each muscle places this population in the unique position of having large force generating muscles with higher concentrations of functional NMJs that must be inhibited by BoNT to achieve adequate neuromuscular blockade. As a general practice larger muscles tend to have higher standard dosing recommendations than do smaller muscles. Given the anatomic, physiologic and topographic characteristics of the pediatric muscle and the mechanism of action of BoNT, in theory these larger muscle groups with their substantive bulk would respond beneficially from a higher volume of lower concentrated toxin, though no research is available at this time.

Finally, it should come as no surprise that the guiding principle of medicine, ‘first do no harm,’ is necessary to invoke when considering utilization of BoNT therapy. Risk and harm reduction are at the core of dosing recommendations for the neurotoxin. Beyond dose limitations, due to the nature of delivery by injection, providers must also consider the risk that comes with delivering this medication to unintended locations as well as those associated with unintentional spread. These risks have to be evaluated and weighed by the provider for each injection site being undertaken. For example, in smaller individuals where muscle surface area is low, providers may benefit from concentrating the toxin from their standard dilution, thereby minimize the risk of unintentional spread of the toxin to NMJs of adjacent structures. In the extremities these adjacent structures are often nontarget appendicular muscles. However, in cervical injections, unintentional injection or spread of BoNT can lead to difficulty breathing and swallowing. Thus, understanding the risk of injections and site specific adjacent anatomic structures is critically important. When there is less margin for error, I saw providers during my residency training concentrate their standard dosing by half (i.e. for Botox a dilution of 100 U/0.5ml saline) in an attempt to further decrease the risk of adverse effects from unintentional spread.

BoNT has a remarkable impact on the lives of children and adults with spasticity by improving range of motion, functional mobility, and decreasing care giver burden. Within, this article I have presented three factors that I have used in my practice to help guide the decision to change from a standard dilution of BoNT (Table 1), as well as reviewed the literature to provide some of the evidence based support for the consideration of such practices. It is my opinion that we are still in the early days of understanding the nuances of dilution for targeted BoNT therapy. Further controlled trials are needed to determine the effectiveness and safety of varied BoNT dilutions in the management of spasticity in the pediatric population. BoNT offers flexibility for injectors to target specific muscles with spasticity, the pattern and severity of which is unique to each individual child. The nature and pattern of the child’s neuromuscular conditions, the delivery method of the BoNT, and the mechanism of action, including spread of the BoNT, allow providers flexibility in optimizing chemodenervation strategies for each individual. The rationale for varying dilutions of BoNT should be justified by physical factors but we should all feel encouraged to think creatively about dilution as a way to maximize the toxin’s effect while minimizing harm.