The sternalis muscle has been denoted variously [11, 12]. A new nomenclature, “biceps sternalis”, can be used for this subtype, owing to its continuity with the sternocleidomastoid muscles of both sides and its longitudinal orientation in the anterior thoracic wall.
Origination from the sternum is one of the main criteria for a muscle to be categorised as sternalis [13]. The biceps sternalis in this case lacked any attachment to the sternum; however, its position in the subcutaneous plane, insertion and innervation support its categorisation as a sternalis muscle.
It seems that the sternalis muscle in this case has similarities to the cases presented in other studies, but to our knowledge, there is no identical case in the literature.
The unicep sternalis was first identified by Bradley et al., who described it in six women during the screening and diagnostic mammographic imaging of 32,000 women [14]. Arraez-Aybar et al. and Deepali et al., in cadaveric studies, observed a unilateral left-sided sternalis muscle [7, 15]. This accessory muscle was also observed bilaterally [16, 17].
The right-sided biceps sternalis, in this male case, had a regular flame shape along its longitudinal and parasternal course, which is similar to that of the muscle reported previously [14]. However, the contour of the unicep muscle in that report was shorter and irregular.
The pattern closest to this in the literature was mentioned in a report by Mehta et al. but with a different muscle origin, insertion and dimensions [18]. Another similar case was observed by Raikos et al., except in their case, they found that the muscle originated from the sternal head of the right sternocleidomastoid, crossed to the opposite side, and split into 2 parts that inserted into the left subcostal arch region [5].
Association of the sternalis muscle with several clinical conditions such as anencephaly and anomalies of the adrenal gland has been reported [19]. In this case, in addition to vascular anomalies of the adrenal gland, anomalies of the hepatobiliary system and anatomic variations of the renal and testicular vessels were also observed [9, 10].
Two possible theories are suggested to explain the embryological origin of the sternalis muscle. The first is based on muscle innervations, and the second is based on muscle fibre arrangement. The innervation of the sternalis muscle depends on local signals [20]. Thus, varying innervations of the muscle are to be expected. These include the pectoral [19] or, less frequently, the intercostal nerves [16, 17]. If the sternalis muscle is supplied by the pectoral nerves, it could originate from the pectoralis major. If the sternalis is supplied by the intercostal nerves, similar to this case, it could arise from the rectus abdominus muscle [13].
With regard to muscle fibre arrangement, the sternalis muscle may be an aberrant extension of the adjacent muscles or their blastemas. These muscles include the sternocleidomastoid muscle or the rectus abdominis muscle; however, the sternalis muscle is always superficial to the rectus abdominis and not continuous with it [11, 21]. In addition, although the sternalis muscle is accompanied by a partial deficiency of the pectoralis major [22], the ipsilateral pectoralis major was observed to have additional muscle fibres (i.e., a pectoralis inferior) in this case. Furthermore, the sternalis was the downward continuation of the left sternocleidomastoid muscle. Hence, it is also possible that the present sternalis is a derivative of the sternocleidomastoid muscle.
According to these theoretical explanations, the sternalis muscle may have evolved from two different origins: superiorly from the sternocleidomastoid muscle and inferiorly from the rectus abdominis muscle. Finally, regarding muscle location and fibre direction, this biceps sternalis may assist in elevating the lower ribs.
