Therapeutic effects of Tetanus neurotoxin in spinal cord injury: a case series on four dogs

Restoration of standing and walking after spinal cord injury (SCI) is a major therapeutic goal to improve daily functioning and quality of life. Experimental approaches to restore locomotor central pattern generator activity include electrical stimulation and intraspinal pharmacological treatment, but clinical translation remains challenging. Tetanus neurotoxin (TeNT) facilitates spinal motor neuron activity by blocking inhibitory neurotransmitter release (glycine and GABA) via VAMP/synaptobrevin cleavage in inhibitory interneurons, which can locally and dose-dependently increase muscle tone. The study explores whether low-dose intramuscular TeNT can alleviate centrally originated muscle weakness and improve stance and gait in dogs with SCI due to thoracolumbar disc herniation, a condition common in chondrodystrophic breeds. The research question is whether localized low-dose TeNT injections into paretic hind limb muscles can improve motor function and muscle mass without inducing generalized spasms or systemic side effects.

Prior work has targeted depressed spinal locomotor circuits after SCI using electrical stimulation and pharmacologic strategies, but clinical implementation is limited. TeNT, a 150 kDa protein from Clostridium tetani, is retroaxonally transported to the spinal cord and, after transcytosis into inhibitory interneurons, cleaves VAMP/synaptobrevin to block glycine and GABA release, disinhibiting and facilitating alpha-motor neuron activity. While generalized tetanus causes widespread spasms, very low, localized TeNT doses can produce temporary, focal increases in tone and have been proposed as a therapy for central muscle weakness. A preceding in vivo mouse study quantified TeNT-induced focal increases in muscle tone and informed dose scaling used in this report.

Design: Compassionate-use clinical case series of four dogs (dog #1–#4) with SCI from non-traumatic thoracolumbar disc herniation (levels T13/L1 or L1/L2) confirmed by spinal MRI, referred from different Berlin veterinary clinics. Time from SCI to TeNT ranged 2–157 weeks; all had prior surgical decompression (1–52 weeks after SCI) without adequate recovery and had undergone physiotherapy. Ethics: Owner informed consent obtained; procedures complied with German animal protection laws and were verbally approved by the Institute of Animal Welfare, Department of Veterinary Medicine, Free University of Berlin. Screening: Other causes such as radiculopathies were excluded via bilateral EMG of gluteus medius, quadriceps femoris, gastrocnemius, and tibialis cranialis. Proprioception and nociception were evaluated by postural reactions (proprioceptive positioning) and nail bed pressure (deep pain sensation). TeNT preparation and dosing: TeNT aliquots (5 ng/ml) supplied by Hannover Medical School were thawed and diluted to 625 pg/ml in PBS with 0.1% BSA immediately before injection. Doses were scaled from a prior mouse study based on body weight and motor deficits. Injection volumes were 200–600 µl per muscle via 30 G needle. Injection sites: Dogs #1, #3, #4 received bilateral injections into gluteus medius, vastus medialis, vastus lateralis, and medial/lateral heads of gastrocnemius. Dog #4 was treated twice 16 weeks apart (#4a and #4b). Dog #2, with right drop paw, received right-sided injections into tibialis cranialis and quadriceps femoris. Example total doses: #1 total 3750 pg (375 pg per muscle); #2 total 312.5 pg; #3 total 2187.5 pg (187.5–250 pg per muscle); #4a and #4b each total 2187.5 pg (250 pg right, 187.5 pg left per muscle). Sedation: Dexmedetomidine 5 µg/kg IV plus butorphanol 100 µg/kg IV. Outcomes and assessments: Follow-ups at weeks 4 and 6 post-injection. Side effects: Owners queried and clinical exams performed to detect painful spasms or spread beyond injected regions. Stance and gait: Primary outcome measured by modified Functional Scoring System in Dogs (mFSSD; 0–14 scale, higher is better), scoring each hind limb separately from standardized 30 s videos standing/walking; if non-weight-bearing, assessed during crawling or with sling support. Blinded physiotherapist scorer. Muscle thickness: Bilateral rectus femoris thickness measured by ultrasound (Esaote MyLab; 6 MHz) at midpoint between patella and hip joint immediately before injection and at weeks 4 and 6. Baseline set to 100%; changes expressed as mean ± SEM. Statistics: Paired t-test; significance threshold P < 0.05.

Safety and tolerability: All dogs tolerated injections; no negative side effects, and no focal or generalized muscle cramps were observed. Stance and gait (mFSSD): All dogs improved weight-bearing and pelvic limb gait compared with baseline at 4 weeks and sustained improvements at 6 weeks, though some showed slight decline from week 4 to week 6. Individual mFSSD scores (Left/Right): #1 baseline 0/0; week 4 0/5; week 6 0/3. #2 baseline 14/8; week 4 14/12; week 6 14/11. #3 baseline 5/6; week 4 10/10; week 6 10/10. #4a baseline 4/3; week 4 5/5; week 6 4/4. #4b baseline 5/5; week 4 9/9; week 6 9/9. Functional observations: #1 progressed from paraplegia to brief unsupported standing (up to 30 s) with right limb assisting crawling push-off; no sustained walking. #2 right drop paw improved to timely, full weight-bearing steps >50% of time; corrected paw placement; chronic paw skin lesion healed. #3 improved to standing >1 min and 10–15 consecutive full weight-bearing steps at weeks 4 and 6. Muscle thickness: Rectus femoris diameter increased versus baseline: 148.7% ± 10.9% at 4 weeks and 137.1% ± 7.9% at 6 weeks (baseline = 100%). Overall, three of four dogs improved gait, and all improved stance and/or muscle mass, without signs of toxin spread.

The case series indicates that localized, low-dose intramuscular TeNT can facilitate alpha-motor neuron activity sufficiently to improve weight-bearing, gait scores, and muscle thickness in dogs with chronic or subacute SCI following thoracolumbar disc herniation, even when prior decompressive surgery and physiotherapy were insufficient. The improvements in mFSSD scores and objective gains in rectus femoris thickness suggest partial reversal of disuse atrophy and functional enhancement of spinal motor circuits, consistent with TeNT’s disinhibitory action on spinal inhibitory interneurons. The absence of generalized or painful muscle spasms and the lack of detectable spread beyond injected muscles support a favorable safety profile at the administered doses. These findings address the therapeutic hypothesis that targeted TeNT can augment motor output in central paresis and highlight TeNT’s potential as a pharmacologic adjunct for SCI-related motor deficits. Given the diversity of chronicity (2–157 weeks post-injury) and persistence of benefits through 6 weeks, TeNT may offer utility across different stages post-SCI, warranting more rigorous evaluation.

Low-dose, intramuscular TeNT injections into paretic hind limb muscles in four dogs with SCI improved stance and/or gait and increased rectus femoris muscle thickness without observed adverse effects, supporting the safety and potential efficacy of this approach. TeNT may represent a promising therapeutic option for centrally originated muscle weakness, including in SCI, and potentially in other conditions such as stroke or multiple sclerosis. Future research should include controlled, larger-scale studies to define optimal dosing, injection targets, duration of effect, safety margins, and translational applicability to human patients.

This was an uncontrolled, small case series (n=4; one dog treated twice) under compassionate use, with heterogeneous injury chronicity, prior surgical histories, and individualized dosing and target muscles. Follow-up was short (4–6 weeks), and outcomes relied on clinical scoring and ultrasound metrics without additional objective biomechanical or neurophysiological endpoints. These factors limit generalizability and preclude causal inference.