Botulinum toxin A injection is commonly used to treat splenius muscle dystonia; however, the optimal injection site within the muscle remains unidentified. This study identified the optimal target for botulinum toxin A based on the distribution of intramuscular nerves and spindles in the splenius muscles. This study included 24 adult individuals. The curve connecting the external occipital protuberance and the third thoracic spinous process was the longitudinal reference line (line L). The curve connecting the external occipital protuberance and the mastoid process was the horizontal reference line (line H). Modified Sihler's staining showed an intramuscular nerve-dense region in the splenius muscles. Muscle spindle abundance was calculated after hematoxylin and eosin staining. The center of the region of the highest muscle spindle abundance was localized using computed tomography. The projection points (P and P') of the center of the region of the highest muscle spindle abundance behind and in front of the neck, position of P (P_L and P_H) projected onto the L and H lines, and depth of the center of the region of the highest muscle spindle abundance were determined under the Syngo system. P_L, P_H, and depth of the center of the region of highest muscle spindle abundance of splenius capitis and splenius cervicis muscles were 17.33% and 40.59% of the L line, 42.42% and 60.44% of the H line, and 26.30% and 32.60% of the PP' line, respectively. These results will provide morphological guidance for improved efficiency and efficacy of target localization for botulinum toxin A treatment for splenius muscle dystonia.

Keywords: Botulinum toxin A; Intramuscular nerves; Muscle spindle abundance; Splenius muscles; Target localization.

© 2025. The Author(s).