European Journal of Cardiovascular Medicine

The rectus sternalis muscle is an uncommon anatomical variant located in the anterior thoracic region, superficial to the pectoralis major muscle. First documented by Cabrolius in 1604 and later by Dupuy in 1726, this muscle has been recognized under various names, including musculus sternalis, presternalis, and sternalis brutorum (1, 2). Positioned parasternal or along the sternum, it typically extends from the jugular notch to variable points in the costal or abdominal regions, presenting with diverse morphological characteristics such as unilateral or bilateral occurrence, and variability in length, width, and thickness across populations (3, 4). Its prevalence is reported to range from 0.7% to 11.5%, with differences noted across ethnic groups, sexes, and geographic regions, such as higher rates in Asian populations (up to 11.5%) compared to Europeans (4.4%) or Africans (8.4%) (5, 6).

The embryological origins of the rectus sternalis remain a subject of debate. Some authors propose it as a derivative of the pectoralis major, supported by shared innervation patterns from pectoral nerves in approximately 68.5% of cases (7, 8). Others suggest connections to the rectus abdominis, citing continuity with abdominal musculature or innervation by intercostal nerves (26.7% of cases) (9, 10). Alternative theories link it to vestigial structures, such as the panniculus carnosus of mammals, or propose it arises from aberrant migration or incomplete rotation of pectoral muscle segments during development (11, 12). These hypotheses are supported by observed connections with adjacent muscles, such as the sternocleidomastoid or external oblique aponeurosis, though definitive innervation studies are often limited by the muscle’s small size and delicate nerve supply (13, 14).

Clinically, the rectus sternalis holds significant implications. Its presence can mimic pathological conditions, such as breast tumors, lipomas, or lymphadenitis, on imaging modalities like mammography or computed tomography, potentially leading to diagnostic errors (15, 16). For instance, studies have reported its identification in only 4 out of 32,000 mammography screenings, underscoring its rarity but critical diagnostic relevance (17). Additionally, its superficial position may cause electrocardiogram (ECG) alterations or chest wall asymmetry, affecting nipple and areola positioning (18, 19). Surgically, the muscle’s utility as a flap in breast reconstruction or head and neck surgeries highlights its therapeutic potential, yet it may complicate procedures like mastectomy if unrecognized, as residual breast tissue beneath the muscle could be overlooked (20, 21). Associations with congenital anomalies, such as anencephaly or renal variations, further emphasize its developmental significance (22, 23).

Given the variability in its presentation and the limited data from specific populations, regional studies are essential to map its prevalence and characteristics. In India, reported incidence ranges from 0.7% to 8%, with sparse data from the Rayalaseema region (24). This study aims to investigate the rectus sternalis muscle in adult cadavers from this region, documenting its prevalence, morphometric features (length and width), and anatomical relationships (origin and insertion). By comparing these findings with global literature, the study seeks to enhance understanding of this variant’s clinical and surgical relevance, contributing to improved diagnostic accuracy and surgical planning.

This cross-sectional observational study was conducted in the Department of Anatomy at Sri Venkateswara Institute of Medical Sciences-Sri Padmavathi Medical College for Women (SVIMS-SPMCW), Tirupati, Andhra Pradesh, India, as part of routine anatomical education for first-year MBBS students. The study involved the dissection of 18 adult human cadavers, comprising 12 males and 6 females, which were obtained through the institution’s body donation program. All cadavers were well-preserved using 10% formalin fixation, ensuring adequate tissue integrity for detailed anatomical examination. The cadavers were estimated to be from individuals aged 50–80 years at the time of death, and all were of local ethnic origin from the Rayalaseema region of Andhra Pradesh. No specific inclusion or exclusion criteria were applied beyond the availability of cadavers for dissection during the study period.

Dissections were performed following standardized protocols outlined in Cunningham’s Manual of Practical Anatomy, focusing on a layered approach to expose the anterior thoracic region. The skin and superficial fascia were carefully incised and reflected to reveal underlying musculature, with particular attention to the pectoral region superficial to the pectoralis major. The presence of the rectus sternalis muscle was identified through visual inspection and palpation, noting its position relative to the sternum, pectoralis major, and adjacent structures. When detected, the muscle was meticulously isolated to preserve its attachments and photographed using a high-resolution digital camera for documentation. Morphometric measurements, including the length (from origin to insertion) and width (at the widest point), were recorded in centimeters using a flexible plastic measuring tape, ensuring accuracy to the nearest 0.1 cm. The origin, insertion, laterality (unilateral or bilateral), and side (left or right) were documented, and variations were classified according to established typologies, such as those proposed by Jelev et al. (5). Due to resource constraints, no histological analysis or detailed neurovascular tracing was performed, limiting observations to gross anatomical features. All dissections were conducted by trained anatomists, and data were recorded systematically to minimize observer bias, although embalming artifacts were noted as a potential limitation in identifying subtle tissue details.

Dissection of 18 adult human cadavers (12 male, 6 female) revealed the presence of the rectus sternalis muscle in 2 cadavers, resulting in an overall prevalence of 11.1%. The prevalence was 8.3% among male cadavers (1 out of 12) and 16.7% among female cadavers (1 out of 6). In both cases, the muscle was unilateral and located on the right side, with no bilateral or left-sided instances observed. All dissections were completed without loss of specimens, and no exclusions were necessary due to tissue degradation or other factors.

In the male cadaver, the rectus sternalis muscle was observed on the right side, positioned parasternal, lying deep to the skin and superficial fascia but superficial to the pectoralis major and its fascia. The muscle measured 12.5 cm in length (from origin to insertion) and 3.5 cm in width at its widest point. It originated inferiorly via tendinous slips from the aponeurosis of the external oblique muscle and the fascia overlying the pectoralis major at the level of the right 5th and 6th costal cartilages. The muscle ascended in a slightly oblique trajectory, becoming narrower and tendinous at the level of the right 2nd costal cartilage. Superiorly, it terminated by blending with the tendinous fibers of the sternal head of the sternocleidomastoid muscle on both sides.

In the female cadaver, the rectus sternalis was also right-sided, measuring 8.5 cm in length and 2.5 cm in width. It was fleshy at its inferior origin, arising from the aponeurosis of the external oblique muscle and the pectoral fascia at the level of the right 6th and 7th costal cartilages. The muscle coursed vertically upward, slightly lateral to the right border of the sternum, in front of the pectoralis major. At the level of the right 2nd costal cartilage, it abruptly angled toward the left, terminating superiorly by blending with the clavicular fibers of the pectoralis major and the sternal head of the sternocleidomastoid muscle on both sides.

Both observed variants were classified as Type A according to the typology of Jelev et al. (5), characterized by a unilateral, right-sided muscle located lateral to the sternum with no corresponding muscle on the left side. Visual documentation included photographs of the male cadaver’s rectus sternalis (Figure 1) and a schematic diagram illustrating the right unilateral rectus sternalis (Figure 2), adapted from established anatomical classifications. Morphometric measurements were consistent across repeated assessments, with no discrepancies noted during data collection.

FIG 1: RIGHT UNILATERAL RECTO STERNALIS

FIG 2: SCHEMATIC DIAGRAM SHOWING

RIGHT UNILATERAL RECTO STERNALIS

Type A: Unilateral on the right; lateral to sternum; left side absent.

Type B: Right-sided on the body of sternum; lower fibers on xiphoid; left side absent.

Type C: Unilateral on the left; runs medially; right side absent.

Type D: Unilateral but crosses midline.

Type E: Bilateral on either side of the sternum.

Type F: Right muscle longer than left; lies closer to midline.

Type G: Bilateral; merges with both pectoralis majors.

Type H: Crossed fibers; right merges with left pectoralis major and vice versa.

Note:

• Types A–D are unilateral
• Types E–H are bilateral.
• Types D, G, and H involve merging with pectoralis major.

In this study both cases reported comes under Type-A according to Jelev et al.(5).

The rectus sternalis muscle, an infrequent anatomical variant, was identified in 11.1% of the cadavers examined in this study, with a slightly higher prevalence in females (16.7%) compared to males (8.3%). This incidence aligns closely with broader estimates for Asian populations, where rates can reach up to 11.5%, as opposed to lower figures in Europeans (4.4%) and Africans (8.4%) [8]. Comparatively, earlier global reviews have reported an average prevalence of 3-6%, with variations influenced by sex, ethnicity, and methodology [6]. For instance, Scott-Conner and Al-Jurf noted a general male prevalence of 6.4% and female of 8.7%, which mirrors the sex-based trend observed here, though our female rate is notably higher [6]. In Indian contexts, incidences range from 0.69% in surgical settings like modified radical mastectomy to 4-8% in cadaveric dissections [8, 24]. Our findings exceed some Indian reports, such as Parmar and Gupta's 5% (including both unilateral and bilateral cases) and Shah's 4-8%, but are consistent with the upper end of Asian data [24, 8]. Recent studies further support this variability; a 2024 cadaveric report from Egypt described a rare branched variant without specifying population incidence, while a meta-analysis indicated an overall cadaveric prevalence of 7.8% [25, 8]. Notably, unilateral presentations predominated in our sample, exclusively on the right side, echoing the literature's emphasis on unilateral over bilateral occurrences [4, 5]. Pichler's historical observation of no occurrences in a year's dissections contrasts with reports of multiple muscles in single cadavers, such as Sarikcioglu et al.'s three (one right, two left) or Ramachandran and Kothandaraman's three on the right, highlighting the spectrum of rarity and multiplicity [26, 27, 28]. Contemporary cases, like Bahgat et al.'s bilateral branched sternalis in 2024, underscore ongoing discoveries of novel patterns [25].

Morphologically, the rectus sternalis in our cases exhibited parasternal positioning, with origins from the external oblique aponeurosis and pectoral fascia, and insertions blending with the sternocleidomastoid tendons, classifying both as Type A per Jelev et al.'s system (unilateral, right-sided, lateral to sternum) [5]. This configuration concurs with descriptions by Pillay et al., who reported origins from pectoral fascia and insertions into the sternocleidomastoid, and Amrutha et al., noting bilateral origins from the external oblique aponeurosis continuing to the sternocleidomastoid [29, 14]. Parmar and Gupta similarly observed insertions into the sternocleidomastoid, reinforcing this pattern [24]. Variations in origin and insertion are well-documented; Jelev et al. proposed origins from the sternum or infraclavicular region inserting into the rectus sheath or lower ribs, while Clemente described ascents from lower costal cartilages blending with the sternocleidomastoid [5, 30]. Turner reported early observations of the muscle's variability, and Novakov et al. linked it to nearby muscles like the pectoralis major or rectus abdominis [31, 32]. Recent reports expand this; Wang et al. described an unusual origin from the rectus abdominis superior portion with branching, and Katara et al. detailed a unilateral variant superficial to the pectoralis [33, 34]. Our measurements (12.5 cm x 3.5 cm in male, 8.5 cm x 2.5 cm in female) fall within reported ranges, though smaller than some bilateral cases, suggesting sex or individual differences.

Embryologically, the rectus sternalis's origins remain contentious, with theories viewing it as a vestigial remnant or developmental anomaly. Ruge considered it a subcutaneous trunk muscle akin to mammalian cuticular layers, while Clemente labeled it a misplaced pectoralis major portion [22, 30]. Barlow proposed it as a panniculus carnosus remnant, and Sadler linked it to the ventral longitudinal muscle column, explaining continuities with infrahyoid or rectus abdominis muscles [7, 11]. Hung and Lucaciu suggested formation via pectoral muscle cleavage or migration, and Snosek et al. attributed it to disturbances in pectoralis major development, potentially yielding related variants like pectoralis minimus [12, 23]. Harish and Gopinath posited separated pectoral fibers, supported by shared pectoral innervation, while Cunningham described it as an aberrant, displaced segment [8, 13]. Connections with the pectoralis major, varying from fibers to fascicles, may arise from incomplete rotation, leading to migrations and links with the rectus abdominis or sternocleidomastoid [5, 31]. Recent reviews, such as Snosek et al.'s comprehensive analysis, affirm these theories without resolving the debate, emphasizing the need for molecular studies [23].

Regarding neurovascular supply, tracing the rectus sternalis's innervation is challenging due to fine twigs prone to damage during dissection, as noted by Cunningham [13]. O'Neill and Folan-Curran found 55% supplied by external/internal thoracic nerves, 43% by intercostal nerves, and 2% by both [9]. Harish, Shah, Jelev, and Barlow correlated supply with derivation: pectoral nerves if from pectoralis major, intercostal if from rectus abdominis [8, 5, 7]. Barlow's review of 147 cases showed 68.5% pectoral, 26.7% intercostal, and 4.8% combined [7]. A recent meta-analysis by Arango-Toro et al. reported 44% pectoral innervation across 369 specimens [35]. Arterial supply derives from internal mammary perforators, per Motabagani et al. [10].

Clinically, the rectus sternalis associates with anomalies like anencephaly (present in half of cases per Eisler) and variations in kidneys, suprarenals, or testes [22, 23]. It mimics conditions such as abscesses, lipomas, hematomas, or breast tumours, and may cause chest asymmetry or nipple deviation [15, 18, 19]. ECG alterations are possible [17], and detection via arm manoeuvres aids identification [20]. In imaging, it appears as an unusual finding in mammography (4/32,000 cases) [16]. Surgically, it complicates mastectomy by harbouring residual tissue but serves as a flap in reconstructions [20, 21]. Recent reports include incidental discoveries during breast surgery (8% African, 11% Asian descent) [4], hypertrophic variants in males [3], and use in mammoplasty [19]. Sternalis syndrome, involving exercise-induced pain, warrants differential consideration.

Limitations include the small sample size, restricting generalizability, and absence of innervation tracing or advanced imaging. Future research should employ larger cohorts and techniques like MRI for in vivo detection.

In summary, our findings contribute to understanding rectus sternalis in South Indian populations, emphasizing its diagnostic and surgical relevance.

This study presents with two cases of unilateral recto sternalis with an incidence of 11.11%. Sternalis syndrome is characterized clinically by severe deep sternal chest discomfort that is generally exacerbated by vigorous exercise or strenuous physical activity. Sternalis syndrome needs to be included in the differential diagnosis of exercise induced chest pain, particularly when standard diagnostic methods fail to provide relief. Rectus sternalis muscle can be misdiagnosed as the breast mass on a routine cranio- caudal projection in mammography. During mastectomy whenever this muscle is encountered care should be taken not to leave behind any breast tissue under this muscle. The rectus sternalis though a vestigial muscle plays an indispensable role in flap reconstruction surgeries of the head and neck, anterior chest wall and breast. With advancement in imaging techniques such as multidetector CT and MRI, presence of sternalis can be detected easily.

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