Intramuscular tetanus neurotoxin reverses muscle atrophy: a randomized controlled trial in dogs with spinal cord injury

Abstract

Background: Motor symptoms of spinal cord injury (SCI) considerably impair quality of life and are associated with a high risk of secondary diseases. So far, no pharmacological treatment is available for these symptoms. Therefore, we conducted a randomized, double-blinded, placebo-controlled study in dogs with spontaneous SCI due to disc herniation to test whether a reduction of spinal inhibitory activity by intramuscular injections of tetanus neurotoxin (TeNT) alleviates motor symptoms such as muscle atrophy or gait function.

Methods: To this end, 25 dogs were treated with injections of either TeNT or placebo into their paretic hindlimb muscles. Effects of TeNT on muscle thickness were assessed by ultrasound, while effects on gait function were measured using the modified functional scoring system in dogs.

Results: Four weeks after the TeNT injections, muscle thickness of the gluteus medius muscle (before median 1.56 cm [inter-quartile range {IQR} 1.34-1.71 cm] and after median 1.56 cm [IQR 1.37-1.85 cm], P-value 0.0133) as well as of the rectus femoris muscle (before median 0.76 cm [IQR 0.60-0.98 cm] and after median 0.93 cm [IQR 0.65-1.05 cm], P-value 0.0033) significantly increased in the TeNT group. However, there was no difference in gait function between the TeNT and placebo groups. The treatment was well tolerated by all dogs without any signs of generalized tetanus symptoms or any spreading of effects beyond the lumbar level of the injected hindlimbs.

Conclusions: With regard to the beneficial effects on muscle thickness, intramuscular injections of TeNT represent the first pharmacological approach that focally reverses muscle atrophy in SCI. Moreover, the study data support the safety of this treatment when TeNT is used at low dose.

Keywords: Dogs; Muscle thickness; Paraparesis; Paraplegia; Spinal cord injury; Tetanus neurotoxin.

Figure 1

CONSORT flow chart. Study design and number of dogs recruited to each treatment group are documented.

Figure 2

Ultrasound measurement of muscle thickness before and after TeNT injections. (A) Exemplary ultrasound image (ultrasound machine Esaote, MyLab, 10 MHz) of the rectus femoris muscle of dog #25 before (7.1 mm) and 4 weeks after (10.1 mm) TeNT injection. (B) Effect of TeNT injections on the thickness of rectus femoris and gluteus medius muscles assessed by ultrasound measurements as compared to placebo injections. Original values of muscle thickness before and 4 weeks after injection are shown for each individual muscle (rectus femoris muscle: placebo n = 23, TeNT n = 19; gluteus medius muscle: placebo n = 23, TeNT n = 20). Data are represented as original values (cm); p‐values (Wilcoxon matched pairs test) are stated above the brackets of the dot blot. The asterisk indicates significant results (p < 0.05*) and the two asterisks represent highly significant results (p < 0.01**). (C) Relative change of muscle thickness of gluteus medius muscle and rectus femoris muscle before injection and 4 weeks after the injection was calculated as delta values in per cent (rectus femoris muscle: placebo n = 23, TeNT n = 19; gluteus medius muscle: placebo n = 23, TeNT n = 20). Data are represented as mean with standard error of the mean. Neither the investigator nor the dog owner were aware of the treatment condition.

Figure 3

Effect of TeNT injection on gait performance. The change in the modified functional scoring system in dogs (mFSSD) before and after the injections is stated for both sides individually. A higher score indicates an improvement of gait function. Mann–Whitney U test was performed to test for significant differences between placebo‐injected hindlimbs (n = 23) and TeNT‐injected hindlimbs (n = 24). The P‐value is stated above the bracket of the dot plot diagram.