Accessory cleido-occipitalis muscle: case report and review of the literature.

An aberrant muscular fascicle, the so-called "accessory cleido-occipital muscle", originated from the anterior border of the cleido-occipital portion of the right trapezius muscle, was detected during a routine dissection of a female cadaver. The aforementioned muscular bundle coursing in the posterior cervical triangle, almost parallel to the anterior border of the trapezius muscle, inserted ultimately to the clavicle's medial third. In addition, prior to its insertion, the muscle provided a fibrous arch attached to the midportion of clavicle, overlying the main trunk of supraclavicular nerves. We noted that during abduction of the right arm, the fibrous arch entrapped the supraclavicular nerve trunk, presumably leading to sensory disturbances in nerve's area distribution. We review the relative restricted data in the available literature concerning that muscular variant and elucidate its importance during differential diagnosis of a mass and surgical exploration of the posterior cervical triangle.