The sternalis muscle is a very rare chest wall muscle variant with no apparent function and embryological origin. Familiarity with its appearance is essential for avoiding confusion with a wide range of lesions, such as breast carcinoma, extra-abdominal desmoid tumours, granular cell tumours, diabetic mastopathy, abscesses, hematomas, sclerosing adenositis, lymphadenitis, fat necrosis, and surgical scars. It also may lead to the misdiagnosis of breast cancer recurrence during post-treatment checkups. The presence of the sternalis muscle may alter the depth at which the internal mammary nodes are irradiated in cases of breast carcinoma, especially those lesions infiltrating the medial quadrants [1–3]. In addition, breast tissue extending deep to the sternalis muscle may be neglected during mastectomy [4]. This muscle has never been related to any clinical symptoms; however, its presence may evoke alterations in the electrocardiogram and may cause breast or chest asymmetry [5, 6].

The sternalis muscle is occasionally observed as an unusual bulge in the medial aspect of craniocaudal mammograms. This muscle can also easily be identified using CT or MRI scans. CT shows a flat or oblique strap-like structure that overlies the pectoralis major muscle and is isodense with other anterior thoracic muscles. The incidental finding of a sternalis muscle in mammography, CT, or MRI studies must be documented in a patient’s clinical records, as it can be an ideal candidate for utilisation in plastic reconstruction of the head, neck, and anterior chest wall [3, 7]. For example, use of a conjoined sternalis-pectoralis muscle flap in immediate tissue expander reconstruction after breastectomy has been proposed by reconstructive surgeons [8].

The existence of the sternalis has not been reported in Iranians. This is the first report in which the gross morphometry of a rare subtype of sternalis muscle, biceps sternalis, is described. The rarity was observed in the cadaver of an Iranian adult male. The muscle’s probable embryological origin is also discussed.

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.

This case report illustrates two important concepts. First, an extra muscle may exhibit different morphologies, depending on gender and ethnic background. Second, a muscular anomaly may be associated with other vascular and visceral anomalies. An awareness of these types of anatomical variations is essential for cardiovascular, thoracic and general surgeons in order to avoid post-operative patient discomfort. Further research is needed to understand the association between muscular anomalies of the thoracic wall and congenital anomalies of the abdomen. Importantly, investigations into whether it is possible to anticipate associated anomalies based on the accidental observation of a particular anomalous structure would be informative for clinical practice.

Written informed consent was obtained from the cadaver’s next of kin for publication of this Case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.