Pectoralis quartus muscle: A rare anatomical variant

Abstract

Mammographic screening in a 60-year-old female patient showed anomalous muscular branch, parallel to the pectoral major muscle bilateral and symmetrical, interspersing the retromammary fat (Figure 1), identified as pectoralis quartus muscles. Complementary ultrasound assessment demonstrated a hypoechogenic fibrillar bundle with oblique disposition extending from the inferior parasternal region to the humeral head bilaterally (Figure 2). Computed tomography assessment confirmed the findings (Figures 3 and 4), also showing two muscular bellies located in the parasternal regions, consistent with sternalis muscles. Although often overlooked in breast image evaluation, the pectoral musculature is included in the radiologic exams for breast cancer screening, and it is important to know its variations to avoid diagnostic errors and problems with possible future surgical procedures, such as breast reconstruction. The sternalis muscle is a common variant of the pectoral musculature, and it is found in about 7.8% of cadaveric studies, 0.2% of mammography, and 6.4% of computed tomography. Its proximal insertion may vary among sternum, clavicle, sternocleidomastoid fascia, pectoralis major muscle, ribs and superior costal cartilages, distal insertion of ribs, inferior costal cartilages, inferior fibers of the pectoralis major muscle, abdominal rectus sheath, and external oblique aponeurosis. It is considered a normal anatomic variant of the pectoral musculature without functional implications for the patient, however, acquires importance for eventually mimicking deep hyperdense breast lesions in the inner quadrants seen on the mammographic craniocaudal view.Pectoralis quartus muscle is a less prevalent and less known variant; it is a segmented portion of the pectoralis major muscle that runs below or beside the inferior fibers of the pectoralis major muscle. Its origin is in lower costal arches, lateral edge of the pectoralis major F I G U R E 1 Bilateral mammography at craniocraudal (A and B) and medio-lateraloblique (C and D) views showing muscle branch parallel to the pectoralis major pectoral muscle bilateral and symmetrical, interspersing retromammary fat [Colour figure can be viewed at wileyonlinelibrary. com]

Unilateral Pectoralis Quartus Muscle – A Case Report

“…To the best of our knowledge, there has been a single report of the variant muscle intraoperatively, and even though the PQ limited the surgical field, the result was satisfactory [12]. A PQ has also been reported during a mammographic screening, and although the authors suggest that it would not raise suspicions about neoplasia, they state that knowledge of such anatomical variations is important to avoid additional examinations [7]. It could be possible that due to PQ's course through the axilla, the muscle may compress the neurovascular structures of the brachial region, thus leading to the corresponding pathology.…”

The pectoralis quartus and chondro-epitrochlearis combined muscle variation: description and surgical relevance

“…14 The preoperative identification of these muscles by imaging is difficult, and few cases have been reported in the literature. 15,16 If identified during the surgical procedure, Natsis et al 4 propose that the pectoralis quartus is bent towards the pectoralis major. In the case where chondro-epitrochlearis or atypical forms of pectoralis quartus prevents surgical access, it is proposed that the muscle be divided since its removal does not imply functional impairment.…”