Electrical stimulation guidance is generally used nowadays, and is recommended over the use of palpation and anatomical landmarks. However, electrical stimulation has its limits. It is sometimes poorly tolerated by patients as it causes pain, which is greater in muscles less responsive to such stimulation and requiring a stronger electrical current. The pain also increases with the number of injections performed in a single session. Electrical stimulation does not guarantee for certain that the needle is placed within the muscle, as a stimulation of the aponeurosis may cause the muscle to contract through the excitation of motor branches entering the muscle. The injection of botulinum toxin in the aponeurosis does not provide a satisfactory effect and may render the injection ineffective. In some patients, the muscle to inject may present some degree of retraction with fat degeneration, which reduces its sensitivity to electrical stimulation. Individual patients may also respond poorly or not at all to electrical stimulation, which causes technical and treatment problems. Finally, certain muscles are difficult to locate by electrical stimulation because their contraction produces only weak mechanical effects (e.g. short muscles such as the interossei muscles). All these factors explain why ultrasound imaging provides an interesting alternative guidance technique for botulinum toxin injections. In most cases, it can replace electrical stimulation altogether, and thus avoid the pain it causes. Ultrasound guidance is already recommended in pediatric patients to improve their comfort during the injections. In our study, the patient's comfort during the injection was chosen as the main endpoint because it has already been demonstrated in children and because the investigators feel it is an essential aspect of our routine practice. Moreover, ultrasound imaging helps locate accurately the muscle to be injected. Once the tip of the needle has been located, it can be tracked during the injection, and guided to the muscle belly. Previous studies have already shown that this technique improves the efficacy of the injections. Ultrasound imaging is also interesting when the muscles to be injected are located in a deep plane or close to vital structures, such as nerves, arteries, or veins (e.g. piriformis or psoas). All these considerations have been studied in children and should be validated in adult spastic patients as well.

Therefore, the investigators will compare ultrasound guidance to electrical stimulation guidance in routine clinical practice. This study will be carried out in adult spastic patients who may receive botulinum toxin injections in the Service of Physical Medicine and Rehabilitation of University Teaching Hospital. The aim is to analyse the advantages of this injection technique under current practice conditions.

The evaluating physician (blinded) will conduct a clinical examination of the patients before the injections, and 6 to 8 weeks after the injection of botulinum toxin. The pain associated with the toxin injections will be evaluated after the injections and during the follow-up visit, using the vertical indexed VAS and the Face Pain Scale. The spasticity will be evaluated on the Tardieu scale before the injections and during the follow-up visit. The duration of the botulinum toxin injections will be measured. Finally, functional objectives will be determined prior to the injections. The GAS (Goal Attainment Scaling) methodology will be used during the follow-up visit to determine whether these objectives have been met or not.