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

Abstract

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.

Introduction

Absence of the trapezius muscle is a relatively rare occurrence but instances have been recorded in both cadaveric and in vivo clinical case reports. The two main causes that have been proposed are congenital (Gross-Kieselstein and Shalev, 1987, Adams et al., 2003) and embryonic (Debeer et al., 2002). The absence of trapezius may occur in isolation or in combination with other muscles such as the sternocleidomastoid or the pectoralis major muscles (Sheehan, 1932, Selden, 1935, Horan and Bonafede, 1977, Gross-Kieselstein and Shalev, 1987, Debeer et al., 2002, Adams et al., 2003). Where there is co-involvement with the pectoralis major muscle in particular, the condition is often thought to be a variant of Poland’s Syndrome, which is a rare congenital anomaly characterised by unilateral chest wall hypoplasia and ipsilateral hand deformities (Fokin and Robicsek, 2002). From an embryological viewpoint, the trapezius and sternocleidomastoid muscles are both innervated by the accessory nerve (Pu et al., 2008), and are derived from the mesoderm of the occipital and cervical somites (Carlson, 2004, Nooij and Oostra, 2006). This suggests that the absence of trapezius in isolation may be an incomplete failure of the myotome compartment of either the occipital or cervical somites. Potential genetic links have also been proposed in instances where identical muscle absences have been observed in siblings (two brothers) (Gross-Kieselstein and Shalev, 1987) and in three generations of males within a family (Adams et al., 2003).

Observations of absences of the whole or portions of the tripartite trapezius muscle alone have also been reported. A search of the literature identified six cases (four males, two females) revealed in cadaveric dissections. In five cases, unilateral absences of the left trapezius were reported. All parts of the trapezius muscle were absent in two cases (Allouh et al., 2004, Nooij and Oostra, 2006), the upper portion in one (Rahman and Yamadori, 1994) and the inferior portion in another two cases (Emsley and Davis, 2001, Garbelotti et al., 2001). One case of complete bilateral absence of the trapezii was also reported (Sheehan, 1932). Cadaveric studies, although informative, limit any comment on a possible genetic predisposition in these individuals or any functional or symptomatic relevance as a possible consequence of the absence.

We report three further instances of the isolated absence of the inferior portion of the trapezius muscle which were observed in one family on clinical examination. The clinical observations were verified using structural magnetic resonance imaging (MRI).