Abnormal Insertion of Latissimus Dorsi — an Incidental Finding During Modified Radical Mastectomy

Abstract

Dear editor, Latissimus dorsi is the large flat dorsolateral muscle on the trunk which originates from the spinous process of T7–T12, thoracolumbar fascia, iliac crest, posterior surface of sacrum, inferior three to four ribs and inferior angle of the scapula. It inserts on the floor of the intertubercular groove of the humerus. Variations in the muscle are common. A rare muscular anomaly of the axilla, the axillary arch muscle (AAM) [1, 2] also called Langer’s axillary arch (LAA) [7], Axillo-pectoral muscle or “Achselbogen muskel” [1] is sometimes seen in around 7% of the population. It is more often found in cadaveric dissections rather than in surgical patients. Although AAM is not very rare, it is generally neglected and not explored or described well. It has immense clinical and morphologic importance for surgical operations performed in the axillary region; thus, surgeons should well be aware of its possible existence [5]. AAM is a thin muscular strip extending from latissimus dorsi to the pectoralis major. Variations in the anomaly are its adherence to the coracoid process of scapula, teres major, long head of triceps, coracobrachialis, biceps brachii and pectoralis minor [1, 3]. A typical muscle variation of latissimus dorsi, the axillary arch is represented by the muscular or fibromuscular slip detached from the anteroinferior border of the latissimus dorsi passing over the axilla under the axillary fascia crossing the medial side of the brachial plexus to continue as a septum intermusculare mediale brachii distally to the medial epicondyle of humerus [2]. Initially reported as a muscle variation in the axillary fossa by Bugnone in 1783 [7], the “axillary arch” was identified by Ramsay in 1795. However, it was Langer in 1846 who explained the axillary muscle in a greater detail. It was called the musculus dorsoepitrochlearis, costoepitrochlearis, chondroepitrochlearis, chondrohumeralis, and pectorodorsalis by Bergman et al. in 1988 [4]. The full extent of the muscle is rarely present. The most common form extends from latissimus dorsi to short head of biceps, pectoralis minor or coracoid process. The anomaly relates to its embryological origin and evolution. The axillary arch muscle usually receives its nerve supply from the medial pectoral nerve, suggesting that it is derived from the pectoral muscles. When closely associated with the latissimus dorsi, the AAM may be supplied by the thoracodorsal nerve [5]. Arch-shaped variations in the axilla could be considered in two groups, muscular form (type I) and tendinous form (type II), accompanying different subtypes based on their nerve supplies and site of their attachment points [2]. However, clinical classification of the axillary arches could be defined as superficial and deep arch groups. Superficial group arches cross in front of the vessels and nerves, and the veins, that may play a role in intermittent obstruction of the axillary vein. Deep group arches occur deeply on the posterior or lateral walls of the axilla. These arches usually cross only parts of the neurovascular bundle and axillary or radial nerves could be affected [6] Fig. 1. AAM may not cause any symptoms, while in some a tight cord can cause shoulder pain or neurovascular compression, costoclavicular compression syndrome, axillary vein entrapment, median nerve entrapment, hyperabduction syndrome, thoracic outlet syndrome and shoulder instability syndrome [2]. This muscle can obscure palpability of axillary lymph nodes [2]. In axillary lymph node clearance, it can obscure nodes and lead to incomplete clearance. Sometimes, the arch may be mistaken for latissimus dorsi and the dissection may proceed laterally during level I dissection, causing damage to lateral lymphatics and increase chances of lymphoedema. During sentinel lymph node biopsy, difficulty may arise in node pick-up especially if only methylene blue dye is used. In the patient undergoing a latissimus dorsi flap for coverage after breast surgery, failure to recognise this variant will result in compression of the thoraco-dorsal pedicle and subsequent loss of the flap [1, 2]. Quite often, it is mistaken for a * Naaz Jahan Shaikh [email protected]; [email protected]