Archives 2012

Median N Compression

 

 

Compression Sites of the Median Nerve

 

The Median Nerve is derived from the nerve roots of (C5), C6, C7, C8 and T1. The nerve itself is formed in the axilla when the medial and lateral cords of the brachial plexus come together at the level of the subscapularis muscle. The lateral cord contributes primarily sensory axons from C6 and C7, while the medial cord provides most of the motor input via C8 and T1. 

Compression of the neurovascular bundle sub-pectoralis minor can affect the median nerve. As we discussed in the window on Thoracic Outlet Syndrome, this is an uncommon area of entrapment and will most likely involve vascular structures. Repetitive or prolonged shoulder abduction are common causative factors. 

Thickening of the deltopectoral fascia usually via fibrosis of its distal edge can compress the median nerve. Trauma and repetitive use of the pectoral muscles can be causative. 

Langer’s (aka axillopectoral) muscle runs from the latissumus dorsi insertion anteriorly to the tendon of pectoralis major. As such it can compress the neurovascular bundle at this location, however it is an anomalous muscle with a 7% unilateral occurence. Repetitive shoulder abduction is the probable etiology.

 

The Ligament of Struthers is another anomalous tissue (<1% of the population) that connects the supracondylar process of the humerus to the medial epicondyle. This band may show on radiographs and may also compress the brachial artery and vein which pass through the same space. 

The median nerve passes between the humeral and ulnar heads of the pronator teres muscle. Compression here may occur via muscular hypertrophy or from fibrous bands within. 

One reliable functional test is the reproduction of symptoms within one minute of compression applied by the clinician’s thumb over the proximal pronator teres (4 cm. distal to the antebrachial crease). 

The Lacertus fibrosus is a thick fascial band from the biceps brachii to the forearm fascia. This fascia also connects with the pronator teres. One of the signs of lacertus syndrome is a dimpling or indentation of the pronator teres muscle due to a constriction of the lacertus fibrosus. Therefore for both these potential compression zones the clinician should test resisted forearm pronation and supination as well as resisted elbow flexion. 

The anterior interosseus nerve is efferent to flexor pollicics longus, flexor digitorum profundus (2 and 3 fingers) and the pronator quadratus. Note the anatomic location of this compression is near the origin of the pronator muscle, therefore requiring differential diagnosis from the previous two conditions. There will be no sensory deficits with this condition. Weakness of pinch strength between thumb and forefinger is one of the clinical signs. Compression to this nerve branch may occur with either overuse or trauma. 

Carpal tunnel syndrome is the most common compression neuropathy to the median nerve. The median nerve is located in an osteofibrous tunnel bounded by the transverse carpal ligament and the carpal bones. Sharing this space are 9 flexor tendons. Just about anything intrinsic or extrinsic to the carpal tunnel can cause compression in this region. There are a number of functional tests including Phalen’s, Tinel, sensory loss in median distribution of the hand (the “eyes of the hand”) and weakness of abductor pollicis brevis (patient holds the pads of the thumb and fifth finger together against resistance). One key to carpal tunnel syndrome is to look for proximal causes of compression that may contribute (double crush syndrome) or mimic the carpal tunnel presentation.