The defined and validated anatomo-sonographic landmarks allow the injection of the LC muscle in its cranial portion by transfixing the sternocleidomastoid and splenius capitis muscles, and the injection of the SC muscle in its portion adjacent to the splenius capitis muscle by transfixing the levator scapulae muscle.
Cervical dystonia is the most common focal dystonia. The standard treatment is targeted injections of botulinum toxin, the prerequisite for which is a relevant semiological analysis using the collum-caput concept  and a knowledge of cervical muscle anatomy . The choice of muscles to inject is also based on semiological analysis . It is, therefore, essential to be able to target the chosen muscles as effectively as possible. The injections are carried out under electromyographic guidance, which enables the identification of the dystonic muscles and avoidance of the muscles that are not involved or are compensating. The introduction of the ultrasound tool in the treatment of cervical dystonia is recent  and should allow better targeting of muscles that are difficult to inject. The common lateral muscle mass of the middle layer of the posterior cervical muscles—which includes the LC, SC and longissimus cervicis muscles—is one these difficult areas. Ideally, a combination of electromyography and ultrasound would be used.
The frequency of the presence of the intramuscular tendon at the cranial part of the LC muscle in the population is not specified in the anatomical literature. However, the presence of this tendon has already been observed and reported . It would be interesting to analyze this frequency by ultrasound and/or MRI study of the cervical region. This tendon is a good reference for the injection of the LC muscle but requires the use of ultrasound, as does the injection of the SC muscle through the levator scapulae muscle.
It is possible to propose an injection methodology for the two muscles studied. For the injection of the LC muscle, it is first necessary to place the ultrasound probe on the mastoid process, then move it down and back following the anatomical orientation of the LC muscle. The intramuscular tendon is then visible at the level of the C2 vertebrae. The injection is performed within the cranial portion of the muscle, between the intramuscular tendon and insertion into the mastoid process. In the absence of the intramuscular tendon, it is recommended not to cross the horizontal line passing through the C2 spinous process, below which the LC muscle becomes more difficult to visualize because of its deep location. At this level, it already belongs to the common lateral muscle mass with the longissimus cervicis and SC muscles.
For the injection of the SC muscle, it is necessary to use surface topographic landmarks, consisting of the spinous processes of the C4 and C5 vertebrae and the muscle body of the levator scapulae muscle in the posterior triangle of the neck. The injection is carried out anteroposteriorly, passing through the levator scapulae muscle in a space located in the craniocaudal plane between the horizontal lines that pass through the spinous processes of the C4 and C5 vertebrae.
The choice of injection site at the upper part of the LC does not correspond to the usual maximum concentration area of neuromuscular junctions within a muscle. It is generally accepted that this area is equidistant from distal and proximal muscle insertion tendons. However, this statement has not been validated for all cervical muscles. For example, the distribution of neuromuscular junction areas within the sternocleidomastoid and splenius capitis muscles does not follow this rule [1, 5].
Our study only included a small number of subjects, all of whom had an intramuscular tendon at the cranial part of the LC muscle. The definition of this tendon frequency among the population would reinforce the external validity of our study. Establishing the clinical relevance of these landmarks would require a prospective comparative study of the results of LC and SC muscle botulinum toxin injections under ultrasound guidance in combination with electromyography, compared to the results of injections under electromyographic guidance alone.
The third component of the common lateral muscle mass is the longissimus cervicis muscle. It seemed difficult to us to propose reliable anatomo-sonographic landmarks for the injection of this muscle. Indeed, its morphological characteristics (thinness and entanglement with the surrounding muscles) still limit its detection in ultrasound and MRI.