Absence of the inferior portion of the trapezius muscle in three family members.

Absence of the trapezius muscle is regarded as relatively rare. This report presents the clinical observations for absence of the inferior portion of the trapezius muscle in three family members that were later confirmed in vivo with magnetic resonance imaging. The absences occurred in two sisters and the son of one sister. The inferior portion of trapezius was absent bilaterally in the sisters but was absent on the left side only in the son. These findings support the notion of a genetic link as one of the possible causes of this aplasia. There were overt functional implications for the absence of the lower portion of the trapezius in one female where there was high demand on the upper limbs in competitive swimming.