An Intramuscular Lipoma Developing within an Anomalous Cleido-Occipitalis Cervicalis Muscle

An anomalous cleido-occipitalis muscle is an uncommon muscular variant in the posterior triangle that may be unilateral or bilateral. Its reported incidence varies widely between 4% and 33% [1-3]. Embryologically, this muscle originates from a common mixed premuscle mass in the occipital region caudal to the last brachial arch, which normally divides into ventral and dorsal components forming the sternocleidomastoid (SCM) and trapezius muscles respectively [35]. The normal trapezius muscle has three divisions (dorsoscapularis superior, dorsoscapularis inferior and cleido-occipitalis). The clavicular attachment of the trapezius muscle (cleido-occipitalis division) commonly inserts into the lateral third of the clavicle, although sometimes reaches middle third or more rarely the medial third to blend with the SCM [4]. In an anomalous cleido-occipitalis muscle, all or part of the cleido-occipitalis division is separate from the remainder of the trapezius muscle and follows a medial course within the posterior triangle. We present a case of a lipoma developing within this anomalous muscle, which to our knowledge, has not been reported previously.