Clinical case: Pectoralis minor syndrome

Objectives

• The venous drainage pattern of the upper limb.
• Adson’s test, how the test is done and the anatomical basis of this test.
• The functional anatomy of the pectoral muscles.
• Why did the surgeons wish to avoid injuring the intercostobrachial and pectoral nerves?

Venous drainage pattern of the upper limb

Veins of the upper limb are divisible into two sets, superficial and deep, both of which eventually open into a common trunk, the axillary vein, which then becomes the subclavian vein after passing under the clavicle (Figures 7&8). The deep veins - with the exception of the axillary - are arranged in pairs that accompany the arteries and are similarly named (venae comitantes; accompanying or companion veins). The superficial veins - veins of the digits and palm-predominantly drain to the dorsal aspect of the hand (form dorsal venous network). The veins leaving the dorsum unite to form two channels, those from the radial side form the cephalic vein, whereas those from the ulnar side form the basilic.

The cephalic vein runs along the radial side of limb, communicates with the basilic at the elbow (median cubital, often used for venipuncture) and continues along the lateral side of the anterior surface of the arm, finally entering the deltopectoral triangle to join the axillary. The basilic vein traverses the posterior surface of the ulnar side of forearm, then inclines anteriorly passing anteromedially around elbow and about 1/3 up the arm penetrates the deep fascia, paralleling the brachial artery helping to form the brachial veins (accompanying veins of the brachial artery, the medial and lateral brachial veins). At the inferior border of the teres major the brachial veins become the axillary vein.

Adson's test

Adson's test is a provocative test for TOS. The test checks for compression of the subclavian artery by a cervical rib or spastic anterior and middle scalene muscle. The patient sits in an upright position; the examiner passively extends, abducts and externally rotates the affected arm while palpating the radial pulse; the patient is asked to take a deep breath and hold it in while the neck is extended and rotated toward the affected side. The test is positive if there is a marked decrease, or disappearance, of the radial pulse.

Although this test is commonly used, its sensitivity and specificity are unclear with some reports considering the test to be virtually meaningless. Part of the issue is that TOS is not really a single entity in that it can be associated with neurological and/or vascular deficits. Neurological TOS is abbreviated as NTOS and it is far more common than vascular TOS. Yet Adson’s maneuver only tests for vascular, specifically arterial TOS, and thus its value appears limited for diagnosing TOS. For the patient in this case, the test was negative despite the venous obstruction.

Functional anatomy of the pectoral muscles

The pectoralis major is a large and fan-shaped muscle of the chest wall that is composed of a sternal and a clavicular head. (Figure 1) The distal attachment of both heads is into the intertubercular sulcus (bicipital groove) of the humerus. The clavicular head arises from the anterior surface of the clavicle; the sternocostal head arises from the sternum, the superior six costal cartilages and the aponeurosis of the external oblique muscle. The pectoralis major functions to adduct and medially rotates the upper limb; the clavicular head also acts to flex the upper limb, whereas the sternocostal head can extend the upper limb from a flexed position (such as when using the crawl in swimming).

The pectoralis minor lies deep to the major (Figures 1&6). It originates from the 3rd-5th ribs and inserts into the coracoid process of the scapula. The pectoralis minor acts to stabilize the scapula by drawing it anteroinferiorly on the thoracic wall. The pectoralis minor is also an accessory respiratory muscle that can act to elevate the upper ribs during periods of respiratory distress. Both muscles receive innervation from the medial and lateral pectoral nerves.

Intercostobrachial and pectoral nerves

The lateral cutaneous branch of the second intercostal nerve is named the intercostobrachial nerve (Figure 2). It pierces the external intercostal muscle and the serratus anterior, traverses the axilla to the medial side of the arm, and anastomoses with a fiber from the medial brachial cutaneous nerve. The intercostobrachial nerve then continues along the arm to supply the skin of the superior half of the medial and posterior part of the arm. The size of the intercostobrachial nerve is inversely related to that of the medial brachial cutaneous nerve.

In addition to its role in supplying cutaneous sensation to the upper arm, the intercostobrachial nerve is important relative to referred pain from a myocardial infarction. Often, especially in men, pain from a heart attack is referred to the area of distribution of this nerve because of the sympathetic afferent nerves that, similar to the intercostobrachial nerve, enter the spinal cord at the T2 level. The surgeons also strove to avoid injuring the medial and lateral pectoral nerves (Figures 1&2) so that the pectoralis muscles were not denervated.