Part eleven. Summary of upper limb nerve injuries
In order to obtain a quick appraisal of the integrity of a major limb nerve it is not necessary to test every muscle supplied. Usually a key muscle and action can be selected that will indicate whether or not the nerve is intact. The following summary includes notes on selected nerve injuries and methods for exposing nerves if exploration or repair is required.
Brachial plexus
Damage to the whole plexus is rare but devastating. The most common cause is a motorbike accident, landing on the shoulder with the neck being forced in the opposite direction, so avulsing the nerve roots. If all the roots are damaged the whole limb is immobile and anaesthetic, and Horner's syndrome (see p. 408) may be present, on account of the connections between nerve roots and the sympathetic trunk. If serratus anterior and the rhomboids are still in action, the damage is distal to the root origins of the dorsal scapular and long thoracic nerves; if supraspinatus and infraspinatus escape, the damage is distal to the upper trunk.
The most common traction injury to the plexus is to the upper roots and trunk (C5 and 6—Erb's paralysis) and includes birth injury (Erb–Duchenne paralysis). The abductors and lateral rotators of the shoulder and the supinators are paralysed so that the arm hangs by the side, medially rotated, extended at the elbow and pronated, with loss of sensation on the lateral side of the arm and forearm.
Damage to the lowest roots (C8 and T1) is unusual as with a cervical rib or Klumpke's paralysis due to birth injury during a breech delivery where the arm remains above the head. The small muscles of the hand are those most obviously affected, leading to ‘claw hand’ with inability to extend the fingers, and sensory loss on the ulnar side of the forearm.
Pectoralis major, being the only muscle supplied by all five segments of the plexus, may be a useful guide to the extent of a plexus injury.
Surgical approach. The supraclavicular part of the plexus can be exposed in the angle between sternocleidomastoid and the clavicle. The inferior belly of omohyoid and the lateral branches of the thyrocervical trunk are divided and the prevertebral fascia incised to display the trunks of the plexus. Sternocleidomastoid and the underlying scalenus anterior are retracted medially to display the roots of the plexus. Scalenus anterior may need to be detached from the first rib (carefully avoiding damage to the phrenic nerve) to expose the lower roots and trunk. To expose the infraclavicular part, the deltopectoral groove is opened up and pectoralis minor detached from the coracoid process so that the plexus cords and their branches around the axillary artery can be dissected out from the axillary sheath. The middle part of the clavicle may have to be removed to expose the divisions of the plexus.
Axillary nerve
The nerve may be damaged in 5% of dislocations of the shoulder, in fractures of the upper end of the humerus or by misplaced injections into deltoid; shoulder abduction is weak and there is a small area of anaesthesia over the lower part of the muscle. Complete division of the nerve is unlikely and surgical exposure is rarely indicated.
Musculocutaneous nerve
This nerve is rarely injured. Its function may be assessed by testing for elbow flexion by biceps, while palpating the muscle.
Surgical approach. Exposure of the nerve involves opening up the deltopectoral groove and identifying the nerve as it enters coracobrachialis from the lateral cord of the plexus.
Radial nerve
The nerve is most commonly injured high up, by fractures of the shaft of the humerus. The characteristic lesion is ‘wrist drop’ with inability to extend the wrist and metacarpophalangeal joints (but the interphalangeal joints can still be straightened by the action of the interossei and lumbricals). Sensory loss is minimal and usually confined to a small area overlying the first dorsal interosseous, on account of overlap from the median and ulnar nerves. Transient paralysis may be due to improper use of a crutch pressing on the nerve in the axilla, or ‘Saturday night palsy’ from draping the arm over a chair when in a state of diminished consciousness. With such high injuries, triceps paralysis can be detected by testing elbow extension. As branches to the long and medial heads of triceps arise in the axilla, elbow extension is not lost after nerve injury following humeral shaft fracture.
Surgical approach. The radial nerve in the arm may be exposed from the back by developing the interval between the long and lateral heads of triceps to reveal the nerve as it crosses the upper part of the medial head before coming to lie in the radial groove (Fig. 2.17). At the elbow brachioradialis and extensor carpi radialis longus are retracted laterally to show the nerve dividing into its superficial and deep (posterior interosseous) branches. The superficial part of supinator can be incised if the deep branch has to be followed downwards.
Ulnar nerve
This is most commonly injured behind the elbow or at the wrist. The classical sign of a low lesion is ‘claw hand’ ( Fig. 2.50), with hyperextension of the metacarpophalangeal joints of the ring and little fingers and flexion of the interphalangeal joints because their interossei and lumbricals are paralysed and so cannot flex the metacarpophalangeal joints or extend the interphalangeal joints. The claw is produced by the unopposed action of the finger extensors and of flexor digitorum profundus. Injury at the elbow or above gives straighter fingers (‘ulnar paradox’) because the ulnar half of flexor digitorum profundus is now out of action and cannot flex the distal interphalangeal joints of the ring and little fingers. Wasting of interossei eventually becomes obvious on the dorsum of the hand, giving the appearance of ‘guttering’ between the metacarpals. There is variable sensory loss on the ulnar side of the hand and on the little and ring fingers but often less than might be expected.
Figure 2.50 ‘Claw hand’ due to a lesion of the ulnar nerve at the wrist.
Testing for abduction of the index finger by the first dorsal interosseous assesses small muscle function in the hand that is dependent on an intact ulnar nerve supply. Paralysis of the ulnar half of flexor digitorum profundus by a high lesion can be detected by the inability to flex the distal interphalangeal joint of the little finger.
Surgical approach. Exposure of the ulnar nerve in the arm is along the medial border of biceps, where the nerve is medial to the brachial artery. At the elbow it is easily approached behind the medial epicondyle, and in the forearm it can be followed upwards from the pisiform, where it lies between the bone and ulnar artery, by displacing flexor carpi ulnaris medially.
Median nerve
This is most commonly injured at the wrist—by cuts, or compression in the carpal tunnel. Theoretically there is sensory loss over the radial three fingers and radial side of the palm, but the only autonomous areas of median nerve supply are over the pulp pads of the index and middle fingers. With high lesions of long duration, there is wasting of the front of the forearm because the long flexors (except flexor carpi ulnaris and half of flexor digitorum profundus) and the pronators are paralysed. Typically the hand is held with the index finger straight, in the ‘pointing finger’ position, often with all other fingers flexed, including the middle finger. Although the part of flexor digitorum profundus to the middle finger tendon usually has a median supply (like the whole of superficialis), its close connection with the part supplied by the ulnar nerve can lead to middle finger flexion, and this part of the muscle may even be supplied by the ulnar nerve. Furthermore the branch to the index finger part of the flexor digitorum superficialis arises near the mid-forearm, rather than in the cubital fossa. For high lesions, test flexor pollicis longus and finger flexors by pinching together the pads of thumb and index finger. Following lesions at wrist level, abduction of the thumb is not possible, and in longstanding cases there is wasting of the thenar eminence (especially abductor pollicis brevis).
Surgical approach. In the mid-arm the median nerve is easily exposed by incision along the medial border of biceps, where the nerve is anterior to the brachial artery, and in the cubital fossa they lie medial to the biceps tendon. In the forearm it is displayed by detaching the radial head of flexor
digitorum superficialis from the radius and turning the muscle medially to show the nerve adhering to its deep surface. Relief of compression in the carpal tunnel involves dividing the flexor retinaculum longitudinally on the ulnar side of the nerve, to avoid damage to the muscular (recurrent) branch which usually arises immediately distal to the retinaculum and curves radially into the thenar muscles. The incision is sited just medial to the prominent skin crease at the base of the thenar eminence to avoid damage to the palmar cutaneous branches of the median and ulnar nerves.