ROENTGEN DIAGNOSIS OF CONGENITAL ABSENCE OF PECTORALIS MUSCLES.

Abstract

Congenital absence of the pectoralis muscles is unusual, although it is the most common of all congenital muscle anomalies (1, 15). Roentgenographically there is a complete or incomplete unilateral hyperlucent lung field. Other lesions of the thorax, extrinsic and intrinsic, which simulate the condition must be differentiated. Case Reports Case I: A 65-year-old man was admitted on May 7, 1961, for repair of a left direct inguinal hernia. Physical examination revealed a flattened right anterior chest wall and sternum. Amazia, scarcity of hair and fat on the anterior chest wall and axilla, and a rudimentary nipple which was displaced medially and cephalad were noted. A roentgenogram of the chest revealed congenital absence of the sternocostal portion of the right pectoralis muscle (Fig. 1). Case II: A white female infant one and a half years old was admitted on July 8, 1956, for correction of syndactyly of the right hand. Brachydactyly and hypoplasia of the hand were also present. X-ray examination of the hand disclosed absence of all the distal phalanges of all the fingers but not of the thumb, as well as absence of the right pectoralis major muscle (Fig. 2). Case III: A 16-month-old white male infant was admitted because of a congenital defect of the right anterior chest wall. Roentgen examination of the chest showed hyperlucency of the lower two-thirds of the right hemithorax and a pectoral shelf. In addition, the right third and fourth ribs were small and malformed (Fig. 3, A and B). Case IV: A 42-year-old Negress was admitted for correction of syndactyly of the index and middle fingers of the left hand. X-ray examination revealed a short middle phalanx of the middle finger and absence of the middle phalanx of the index finger. A roentgenogram of the chest revealed absence of the left pectoralis major muscle (Fig. 4). Discussion Various theories have been advanced concerning the etiology of pectoral muscle defects. It is congenital, yet heredity apparently does not play an important role. Lewis (5) attributes the anomaly to an embryologic defect. The anlage of the pectoralis muscles is situated above the first rib in the ventral lateral extensions of the cervical myotomes (9). As the embryo develops the premuscle mass enlarges and moves downward, becoming attached first to the clavicle, then to ribs, sternum, and abdominal fascia and at the same time becoming differentiated into the major and minor pectoral muscles. Lewis believes that the defect results from failure of attachment and subsequent atrophy of the pars sternocostalis of the pectoralis maj or and minor. In support of his argument is the fact that in most of the reported cases, the clavicular portion of the muscle is present while the sternocostal portion of pectoralis m ajor and minor is absent. Indeed, absence of the pectoralis major without associated defect in the pectoralis minor apparently does not occur (7).