Botulinum neurotoxin (BoNT) injections are the main medical treatment of writer's cramp. When the outcome is favourable, patients usually receive injections several times per year in the long-term. However, we know little about the course of BoNT doses and nothing about the impact of the guidance method on the clinical outcome or injection strategy.

We studied, in the long-term, the doses of BoNT and the target muscles in a group of patients with writer's cramp, according to the guidance method (electrical stimulation or ultrasound). Patients received at least three injection cycles guided by electrical stimulation, followed by at least three injection cycles guided by ultrasound.

Twenty-four patients were included. More target muscles were injected after switching to ultrasound guidance, especially the flexor carpi ulnaris and the flexor carpi radialis. The mean dose by muscle was lower when ultrasound guidance was used. When using electrical stimulation guidance, the dose in the flexors of the fingers decreased in the long-term, but increased in the flexors of the wrist. The course of the BoNT doses and of the number of target muscles per cycle were not the same during the first period (electrical stimulation) and the second period (ultrasound).

Switching to ultrasound guidance, the BoNT dose decreased, mainly in the flexors of the wrist. Based on the results of our study, we suggest a starting dose in several muscles (flexor carpi ulnaris, flexor carpi radialis, flexor digitorum profundus and flexor pollicis longus).