Writer's cramp (WC) is a rare form of focal, idiopathic dystonia [1], [2]. Although botulinum neurotoxin (BoNT) lacks approval in this condition, this drug is often considered the first-line treatment. Good clinical outcome was reported in a large number of open label and retrospective studies, but also in some double-blind randomised clinical trials [3], [4], [5], [6].

Several techniques are used to perform intramuscular injections of BoNT. Target muscles can be localised by palpation of anatomical landmarks (surface anatomy), electrical stimulation (ES), electromyographic recording (EMG) or ultrasound (US). As forearm muscles are small, numerous and overlapping, surface anatomy might be inaccurate [7], [8]. In Picelli et al.’s study (41 spastic patients) [8], the accuracy of surface anatomy guided injections was worse in the wrist extensors (39%) than in the flexors (63%). Henzel et al. [9] compared the localisation of flexor forearm muscles according to surface anatomy and US in a group of 18 spastic patients. Depending on the method (US or surface anatomy), they found different coordinates for flexor pollicis longus, pronator teres, flexor digitorum superficialis and flexor carpi radialis. In WC patients, ES is commonly used to inject forearm muscles, while EMG is mainly used to inject larger muscles, for example in cervical dystonia. For BoNT injections in the limbs, US-guided techniques were developed more recently than electrophysiological targeting. The outcome of US-guided injections has mainly been described in the field of spasticity: two studies found better outcome when instrumental guidance was used [10], [11]; Picelli et al. [10] found no difference between an ES group and a US group.

The impact the guidance method has on injection strategy, including BoNT dosage, has not been explored in the field of functional dystonia. The main objective of our work was to determine, in WC patients presenting with finger or wrist flexion, whether BoNT doses were modified after changing the guidance method from ES- to US-targeting. We also studied the number of target muscles, the type of the finger and wrist flexor target muscles, the interval between two injections, and the frequency of side effects. The secondary objective was to determine the injection method effect on long-term changes in BoNT dosage.