Intramuscular nerve distribution of the sternocleidomastoid muscle for the botulinum toxin injection

The aim of this study is to define the intramuscular nerve distribution of the sternocleidomastoid muscle (SCM) and the innervation zones (IZ) to describe the optimal botulinum toxin injection sites.

The cricoid cartilage (CC), laryngeal prominence (LP) and hyoid bone (HB) and angle of mandible (AM) were determined as landmarks. The length of the muscles were measured between the sternoclavicular joint and tip of the mastoid process. SCM was evaluated in two parts as anterior and posterior divided by the line where the length of the muscle was measured. Measurements were made to define the relationships of the SCM with common carotid artery, internal and external jugular veins. IZ were described according to these vessels. Afterwards, Modified Sihler's staining technique was applied to expose the intramuscular nerve distribution.

The average length of SCM was 160,1 mm. Motor entry point of the accessory nerve fibers were between the AM-HB lines, in the range of 30-40% of the muscle length, and in the posterior part of the muscles. IZ were between the HB-CC lines in the anterior and posterior part. When this interval was examined according to the vessels, the optimal injection sites were between the LP-CC lines.

This study shows the position of the intramuscular nerve fibers endings of the SCM according to the chosen landmarks and the relationship of the IZ with the vessels to prevent complications. These results can be used as a guide for safe and effective botulinum toxin injections with optimal quantities.