Peripheral nerve injuries are diagnosed by mapping cutaneous sensory loss, and detecting weakness in specific muscles or groups of muscles. The cutaneous nerves and dermatomes are shown in figures 26a,b.

The skin of the upper limb is supplied by the brachial plexus (anterior roots of C5–T1), with contributions from C4, over the tip of the shoulder and the clavicle, and T2 and 3, along the inside of the upper arm and axilla. T6 supplies the skin over the thumb and C8 the little finger; C7 has a variable supply over the central fingers. As there is marked overlap between dermatomes, sensory loss is most easily defined across axial lines, where separated dermatomes lie adjacent to each other. The muscle innervation of the limb differs from that of sensation, in that the proximal muscles are supplied by the upper nerve roots, and T1 supplies the small muscles of the hand. The biceps reflex is supplied by C5/6, supinator C6 and triceps 6/7.

The brachial plexus may be damaged by penetrating injuries and by severe traction injuries at birth, pulling either downwards, avulsing the upper roots (Erb’s palsy), or upwards, pulling on the C8 and T1 roots (Klumpke’s palsy). Motorcycle accidents are also notorious for dragging the shoulder downwards, while the helmet sustains an upward pull. Pressure around the thoracic inlet, such as from a cervical rib, damages T1. C5/6 injuries produce weakness of deltoid, supraspinatus, infraspinatus, teres minor, biceps and brachialis. The limb hangs limp by the side, medially rotated and fully pronated in the “waiter’s tip” position. Sensory loss is over the outer aspect of the arm and forearm. T1 injuries produce weakness of the hand and sensory loss over the inside of the arm.

The long thoracic nerve, to serratus anterior, may be damaged during axillary dissection; paralysis of this muscle produces a winged scapular. The thorocodorsal nerve may also be damaged in radical axillary dissection; weakness of latissimus dorsi may produce remarkably few symptoms. The axillary nerve passes around the surgical neck of the humerus, and is easily damaged in fractures at this site and in shoulder dislocation. In these conditions, the resultant paralysis of deltoid and teres minor is painful to demonstrate, therefore look for the associated sensory loss, over a small area of skin, near the humeral attachment of the deltoid.

The radial nerve may be damaged in fractures of the shaft of the humerus, as it passes along the radial groove. This produces a disabling paralysis of wrist extension (wrist drop) but minimal sensory loss, over the first dorsal interossei.

Median nerve injuries are usually associated with carpal tunnel compression, producing weakness of the thenar muscles, and sensory loss over the palmar aspect of the lateral three and a half fingers. Night pain is a disturbing symptom, and wasting gradually becomes obvious. Injury at the level of the elbow, may accompany elbow dislocation, fractures and misplaced injections. There is loss of flexion of the thumb, and index and middle fingers. Retained adductor pollicis (ulnar) and extensor pollicis muscles (radial), turn the thumb into the plane of the palm (simian palm). This, together with the outstretched (extended) index finger, produces the ‘benediction sign’, when the hand is raised and facing forward.

Ulnar nerve damage is classically at the elbow, where the nerve is superficial, as it passes behind the medial epicondyle; it may also be trapped as it passes through the cubital tunnel, between the heads of flexor digitorum profundus. There is weakness of this muscle, all the interossei, the medial two lumbricals and the adductor pollicis: Froment’s sign is diagnostic for the latter (page 141). Interossei weakness may be demonstrated by failure to grip paper between the fingers, and weakness of the pinch grip between the fingers and the thumb. With more distal ulnar nerve lesions the loss of wrist and finger flexion is less marked.

Figure 26
a. Cutaneous nerves and dermatomes, anterior.

1. Supraclavicular
2. Upper lateral cutaneous of arm
3. Anterior cutaneous (branch of)
4. Intercostobrachial
5. Medial cutaneous of arm
6. Lateral cutaneous of forearm
7. Medial cutaneous of forearm
8. Palmar cutaneous of ulnar
9. Palmar cutaneous of median
10. Radial digital
11. Median digital
12. Ulnar digital
13. Subcostal
14. Lateral cutaneous of thigh
15. Genitofemoral
16. Ilioinguinal
17/19. Medial/intermediate cutaneous of thigh
18. Obturator
20. Saphenous
21/22. Lateral cutaneous of calf
23. Superficial peroneal
24. Sural
25. Deep peroneal

Figure 26
b. Cutaneous nerves and dermatomes, posterior.

1. Supraclavicular
2. Upper lateral cutaneous of arm
3. Posterior cutaneous of arm
4. Intercostobrachial
5. Medial cutaneous of arm
6. Lower lateral cutaneous of arm
7. Posterior cutaneous of forearm
8. Medial cutaneous of forearm
9. Lateral cutaneous of forearm
10. Superfical radial
11. Dorsal branch of ulnar
12. Iliohypogastric
13. 15. 16. Lateral cutaneous of thigh
14. Posterior cutaneous of thigh
17. Obturator
18. Lateral cutaneous of calf
19. Medial cutaneous of thigh
20. Sural
21. Sural communicating
22. Saphenous

In long-standing median and ulnar nerve lesions, due to ischemia and other chronic conditions, such as leprosy, there is also marked muscle contracture, and this may produce a claw hand (main-en-griffe). The claw appearance is accentuated by interosseous muscle wasting. Clawing of the little and ring fingers is most marked with ulnar nerve injuries at the wrist. In lesions of the ulnar nerve at the elbow joint, additional paralysis of the medial finger tendons of the flexor digitorum profundus, prevents flexion deformity of the interphalanageal joints; this produces the ulnar nerve paradox: the higher the lesion the less the deformity.

The lower limb is supplied by the sacral plexus (L4–S3), the distribution of the sensory dermatomes correspond to those of the arm – along the outer surface to the periphery, and then proximally along the medial aspect of the limb (figure 26a,b). The groin and anterior thigh gain additional sensation from L1 – 3; thus you kneel on L3/4, walk on S1 and sit on S3. The knee jerk is supplied by L3/4, the ankle by S1 and the plantar reflex by S1/2: L5 is not represented in these reflexes, but can be specifically tested in dorsiflexion of the great toe.

The roots of the sacral plexus are well protected within the pelvis. The sciatic nerve may be damaged as it passes outwards through the greater sciatic notch over the ischium, posterior to the head of the femur. In posterior dislocation of the hip, there is usually an associated fracture of the acetabular rim. The injury typically occurs in motorcycle accidents, when force applied to the bent knee is transmitted along the femur to the flexed hip. Sciatic palsy produces a flail limb.

Nerve injuries around the knee joint spare the hamstring muscles, but damage to the tibial nerve, produces loss of plantar flexion and sensory denervation of the sole and lateral aspect of the foot (sural branch). Injury below this level produces paralysis of the small muscles of the sole; the unopposed action of the long flexor and extensor muscles produces a high arched sole. If the sural nerve is spared, there is still sensory loss over the sole, but sensation over the lateral aspect of the foot is preserved.

Injuries to the common peroneal nerve usually occur as it passes over the lateral aspect of the neck of the fibula, through direct trauma and fractures at this site (bumper fracture). The paralysis of the peroneal and anterior muscles, produces a characteristic foot drop, with flopping of the foot during the gait. Sensory loss is over the anterolateral aspect of the lower leg and foot. The foot becomes inverted by the unopposed action of the tibialis posterior muscle. Sensory loss from damage to the deep peroneal nerve is limited to the dorsal skin of the first web.

The femoral nerve (L2,3,4) can be damaged by surgery around the psoas muscle and its sheath. A dramatic temporary paralysis may be produced by a spontaneous retroperitoneal bleed in patients on anticoagulants; there is usually a mass in the loin. The resultant disability is marked, with sensory loss over the anterior and medial thigh, and along the course of the saphenous nerve, on the medial aspect of the leg and foot. There may be paralysis of iliopsoas (L1,2) and pectineus, with loss of hip flexion, as well as paralysis of quadriceps, with loss of knee extension and instability of the knee joint. The patient is unable to extend the knee when sitting on the edge of an examination couch. The femoral nerve can also be the site of diabetic neuropathy.

Damage to the obturator nerve (L2,3,4) may occur in femoral hernia repair, particularly if the nerve takes an aberrant course, over the posterior aspect of the body of the pubic bone. Paralysis of the adductors is not total, since the adductor magnus receives part of its nerve supply from the sciatic nerve, together with the remainder of the hamstring muscles.

The lateral cutaneous nerve of the thigh (L2,3) may be compressed within the lateral fibres of the inguinal ligament, producing pain, hyperaesthesia and sensory loss over the lateral aspect of the upper thigh (meralgia parasthetica).