In 2009, a randomized clinical trial (RCT) on the effects of BoNT-A injection into the calf muscle in typically developing adult volunteers was reported. The lateral head of the gastrocnemius was injected either with a standard clinical dose of BoNT-A or with normal saline solution43. The outcome was studied with MRI scans at 3, 6, 9, and 12 months after injection and a muscle biopsy at 12 months after injection. At 3 months after the injection, an abnormality of the high signal intensity pattern (HSIP) in the MRI STIR (short tau inversion recovery) sequence and a reduction of muscle cross-sectional area were identified, which ranged from 81% to 86% of the initial cross-sectional area when compared with the contralateral control muscle. These changes also were apparent at 6, 9, and 12 months following the injection. Histopathology revealed neurogenic atrophy of small groups of muscle fibers that amounted to 30% in the BoNT-A-injected muscle. There also were increases in the number of perimysial fat cells and the amount of connective tissue surrounding the atrophic muscle fibers43.
In a more recent study using serial MRI scans, 46% to 48% atrophy of the procerus muscle was demonstrated after a single dose of BoNT-A, which was still present at 12 months after injection, long after the clinical effect had worn off44. In another study using serial MRI scans, changes to the piriformis muscle were studied prospectively following injection of BoNT-A for piriformis syndrome. Serial MRI scans showed muscle atrophy and fatty infiltration45. All 3 of these studies share a common finding: intramuscular injection of BoNT-A in standard therapeutic doses resulted in acute muscle atrophy with deficits that were evident on MRI scans at the 12-month follow-up, long after the clinical effects had worn off43-45.
A number of small mammals have been studied to elucidate the effects of injection of BoNT-A46. In 2011, Fortuna et al. reported changes in the contractile properties of the quadriceps muscles in New Zealand white (NZW) rabbits following injection of BoNT-A47. Their principal findings included an acute reduction in muscle mass to 80% and 64% at 3 and 6 months after injection, respectively, with contractile material replaced largely by fat. Similar but less-pronounced results were reported in the contralateral noninjected limb47. In 2013, the same research team investigated whether skeletal muscles recover following repeated injections of BoNT-A48. They reported that neither the injected nor the contralateral noninjected muscles had recovered by 6 months after injection48. In 2015, the same research team reported that muscle strength and contractile material had not recovered by 6 months after BoNT-A injection. In addition, the messenger (m)RNA expression phenotype remained altered in favor of fibrotic response molecules9. The authors concluded that BoNT-A-induced weakness and muscle atrophy lasts much longer than had been previously thought. As such, they advised caution in treating the skeletal muscles of children with cerebral palsy, which are already grossly abnormal. In 2018, this same team reported mRNA elevation for inflammatory molecules, proteinases, adipokines, and mesenchymal stem cells (MSCs) after BoNT-A injection, theorizing that these molecules and cells contributed to a lack of muscle recovery and promoted the development of fatty tissue infiltration49 (Fig. 2). In addition, the extent of mRNA expression increased as the number of injections increased, suggesting a dose-dependent response. Given these findings, the authors expressed additional concern regarding the long-term effects and possible complications after BoNT-A injections in children with cerebral palsy.
In 2015, Minamoto et al. reported a 50% reduction in muscle torque after a single injection of BoNT-A in a rat model and a 95% reduction in torque after a second injection 3 months later51. The same group reported recovery of muscle size but not contractile function at 12 months after a single injection of BoNT-A52.
Despite targeting different muscles in different animal models and using different combinations of biomechanical, morphological, imaging, and molecular techniques, the results of the 3 research teams described above are broadly similar. Injection of BoNT-A results in acute muscle atrophy, reduction in torque, damage at the ultrastructural level, and loss of contractile function and induction of fibrosis47-52 (Fig. 2). It is not known how long the gross morphological changes or the molecular changes last, but the changes would seem to exceed the expected clinical duration of benefit by a substantial margin. These studies should raise concerns for clinicians who inject BoNT-A and for all clinicians who counsel parents of children who are affected by cerebral palsy8,18,26.
The introduction of BoNT-A as a therapeutic agent may be unique in the history of drug development in that the majority of clinical trials have been conducted by clinical investigators, with a dearth of relevant clinical data provided by the manufacturers of the various BoNT-A preparations27,53. For example, experimental work in animals suggesting a functional recovery of endplate function by 6 to 12 weeks after injection led to early recommendations supporting repeat BoNT-A injections for various clinical conditions every 3 months54. In the only 2 RCTs to date of injection frequency in children with cerebral palsy, both studies confirmed that injection of the gastrocnemius-soleus complex for spastic equinus was as effective when performed once per year compared with 3 times per year (every 4 months)55,56.
Despite the widespread use of BoNT-A injections in ambulant children with cerebral palsy, the study of changes after injection has largely been confined to measures of spasticity, such as the Modified Ashworth Scale (MAS) or the Modified Tardieu Scale (MTS). In a recent large international multicenter trial, the MAS was used as the primary outcome measure57. The MAS is a surrogate outcome for the measurement of spasticity. It should not be accepted as a primary outcome measure because of its poor correlation with meaningful functional measures such as gait and function58. Fewer studies have reported functional outcomes (including changes in gross motor function and gait) after BoNT-A injection58,59. Similarly, very few studies have investigated changes in muscle volume, changes in muscle strength, or changes in muscle ultrastructure or molecular events after BoNT-A injection in children with cerebral palsy. The historical context is important. The ability to image muscles in children without ionizing radiation had not been developed when BoNT-A was first released12,13. Knowledge regarding the small size of muscles in children with cerebral palsy at that time also was rudimentary. Muscles in children with cerebral palsy were known to be short, but little was known about size8,25,26.
In 2013, using serial MRI scans, Van Campenhout et al. demonstrated 20% atrophy of the psoas muscle following injection of BoNT-A. Six months after injection, the muscle atrophy was still present on the final MRI scan. Given the short-term follow-up, it is not known if or when the muscle atrophy might have resolved60.
Also in 2013, Williams et al. demonstrated a 5% reduction in gastrocnemius volume with serial MRI scans in children with cerebral palsy after a single injection of BoNT-A in the gastrocnemius61. However, this decrease was accompanied by a 4% increase in the volume of the soleus muscle, which may have been a compensatory phenomenon. In 2018, the same group reported a 7% reduction in gastrocnemius muscle volume after injection of BoNT-A at 25 weeks after injection62. Although the volume of the plantar flexors was maintained by hypertrophy of the soleus, this cannot be taken as reassuring. Injections for equinus gait in many centers include the injection of the gastrocnemius and the soleus, with injections repeated every 4 to 6 months18,53. With a combination of small numbers and short-term follow-up, these studies do not provide robust evidence for safety.
The reduction in muscle volume in these clinical studies is much less than in both previously described groups (typically developing adults and animal models). Nevertheless, given that children with cerebral palsy are subjected to intramuscular injections of BoNT-A several times per year throughout childhood, the smaller degree of acute muscle atrophy cannot be taken as completely reassuring18,53.
The traditional view of BoNT-A therapy was that injection resulted in a reduction in spasticity in the injected muscle, with full recovery of muscle function at 3 to 6 months following the injection (Fig. 3). Unfortunately, the effects of BoNT-A are much less precise. The current view, supported by the evidence in this review, is that injection of BoNT-A is followed by acute muscle atrophy, reduction in hypertonia and strength, decreased muscle stiffness, and increased range of motion in the distal joints (e.g., ankle dorsiflexion) (Fig. 4). Most studies report improvement in measures of spasticity with the MAS or the MTS57. However, these are surrogate measures rather than valid or reliable measures of gait or function18,27,56,58. In terms of outcome measures that are meaningful to patients, the desired effects are both temporary and small59. When gold standard measures of function such as the Gross Motor Function Measure (GMFM) are utilized, some RCTs report no improvements63. In studies utilizing gait analysis, improvements in targeted muscle function are frequently recorded, including increased ankle dorsiflexion and improved foot contact following BoNT-A injection for spastic equinus64,65. However, most studies have relied on observational gait analysis or video recordings of gait (supplemented by rating scales such as the Physician Rating Scale [PRS] or the Edinburgh Visual Gait Score [EVGS]) rather than a full 3D gait analysis (3DGA)66,67. There are a relatively small number of studies utilizing 3DGA and, to our knowledge, only 1 study in which an overall measure of gait function, the Gait Profile Score (GPS), has been reported58. Selective reporting of gait parameters such as increased ankle dorsiflexion is not sufficient given that this may be achieved at the expense of diminished knee extension (i.e., impaired plantar flexion, knee extension coupling).
Given that the benefits of BoNT-A injection are so small and so short-lived, it would be reasonable to question its therapeutic value. From very early childhood, skeletal muscles in the ambulant child with cerebral palsy are smaller than in typically developing peers, and progressive sarcopenia is noted by early adult life17,25. Any intervention that further increases sarcopenia and reduces skeletal muscle reserves must be viewed with concern26. In this regard, the clinical study designs have been lacking by focusing almost entirely on a single BoNT-A injection cycle. This may be the reason for the false sense of safety in the literature to date60-66. Sarcopenia and injection-related fibrosis will only become clinically apparent at long-term follow-up. To our knowledge, there are no such studies in the literature to date, and they are urgently needed. Until then, BoNT-A should be used with caution and there should be consistent monitoring of muscle volume and strength until it can be demonstrated that the long-term benefits outweigh the risks. As a simple starting point, injection frequency should be reduced to once every 12 months rather than the current widely used 4 to 6 monthly injection protocols, as supported by the only 2 clinical RCTs to date55,56. This approach is also in accordance with the animal studies that have been discussed above.
If there is a persistent small but cumulative deficit in skeletal muscle volume, morphology, and function with each repeat injection of BoNT-A, there is the possibility that a slow and insidious reduction in function in key muscle groups (e.g., the gastrocnemius-soleus complex) will surface. More than 50% of gastrocnemius-soleus complex moment-generating capacity is required during normal gait, and the deficits in older children with cerebral palsy have been shown to be nominally in the region of 40% to 60%25,26. Thus, there is no reserve available to allow for any additional deleterious effects on muscle function to occur. Accordingly, in the effort to achieve “foot flat” during gait by injection of BoNT-A for spastic equinus, an increase in dorsiflexion at the ankle may be accompanied by an increase in knee flexion, crouch gait, and a decrease in overall gait function as measured by a global kinematic index (e.g., the GPS)58.
The relationship between muscle and bone has been extensively explored in recent years, and injection of BoNT-A has become a standard model in animal studies to investigate disuse osteopenia. Animal studies utilizing this disuse model demonstrate that BoNT-A-induced paralysis precipitates an acute profound catabolic effect on the neighboring bone, with prolonged and incomplete recovery of bone density following the return of muscle function68-70. The cause-and-effect relationship between muscle and bone is based on 2 consistent findings in recent studies. First, loss of bone mass is preceded by the onset of BoNT-A-induced muscle paralysis, and, secondly, return of bone mass is preceded by recovery in muscle function. Thus, any factor that compromises muscle function, such as injection of BoNT-A, will confer a deleterious effect on adjacent bone in animal models. We are not aware of any clinical studies on BoNT-A-associated osteopenia in children with cerebral palsy, but these are clearly needed.
We should focus on interventions that increase the volume, the strength, and the reserves of skeletal muscle in children with cerebral palsy. These may include strengthening programs, enhanced nutrition, and novel molecular therapies that have not yet been developed17,26,71. In the meantime, children with cerebral palsy will develop fixed contractures with or without prior injections of BoNT-A, and surgical correction of fixed deformities will be needed for the foreseeable future. In 1 study, gastrocnemius recession was associated with an increase in calf muscle volume at 12 months after surgery72.
Given the short-term follow-up in studies to date, it is not known if complete muscle recovery ever occurs after injection of BoNT-A in terms of gross muscle morphology60-62. The corresponding duration of the upgraded inflammatory and fibrotic pathways in both children with cerebral palsy and in experimental animals is also unknown at this time47-52. Additional studies are required, and caution is needed with the administration of BoNT-A. We acknowledge the role of BoNT-A as a temporizing measure. However, therapeutic benefit must be balanced against long-term sarcopenic effects, and the necessity of BoNT-A injections must be reviewed continuously throughout treatment. For children who are enrolled in regular injection programs, we suggest that measurement of muscle volume at baseline and at intervals throughout the injection schedule should be performed to identify and limit potential loss of muscle function73,74. Novel therapies to limit contracture development also are required, and injections of collagenase have been proposed as one such therapy75.
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