Unilateral Hypoplasia of the Trapezius Muscle in a Female Cadaver.

Dissection of a 67-year-old female cadaver revealed a left-sided trapezius muscle of diminutive size. An examination of nerve supply and vasculature yielded no apparent anomalies, which suggests a developmental etiology. Earlier reports suggest that the trapezius is likely derived from at least two differing cell populations. It is possible that one or both failed to contribute fully to the development of the trapezius muscle in this individual. The rhomboid muscles, which share some functions with the trapezius muscles, demonstrated significant ipsilateral compensatory hypertrophy, likely due to increased use. Although varying abnormalities of the trapezius have been previously observed, this case displayed unique attachment points for a trapezius muscle with congenital hypoplasia. When considered alongside the other reported cases of varying trapezius hypoplasia, this case highlights that trapezius anomalies should be an important differential diagnosis in patients with chronic upper limb weakness and back asymmetry.