2.2. Segmental distribution:

Where you suspect involvement of spinal nerves rather than peripheral nerves (e.g. injuries to the spine or brachial plexus, cervical spondylosis etc.) you must examine myotomes and dermatomes. These are the muscle masses and areas of skin supplied by single spinal nerves (no matter how the nerve fibres within these spinal nerves are finally distributed via the limb plexuses and peripheral nerves).

2.3. Myotomes (1):

Normally two roots produce movement of a joint in one direction, and two in another. This is true at the elbow, where weakness of elbow flexion and an absent biceps tendon jerk indicate C5,6 involvement; and where weakness of extension and an absent triceps jerk suggest a C7,8 lesion. This general rule is followed throughout the lower limb, but is modified in the highly specialised upper limb.

2.4. Myotomes (2):

In a distal or proximal joint the four spinal segments involved differ by plus or minus one, so that theoretically the shoulder should be controlled by C4,5,6,7. However, C4 has been suppressed, with the result that abduction is mediated through C5 alone (deltoid, supraspinatus etc). Adduction (involving principally pectoralis major) is controlled by C6,7.

2.5. Myotomes (3):

At the wrist, where C6,7 would have been expected to control palmar flexion only, it is the case that these two segments control dorsiflexion as well.

2.6. Myotomes (4):

Both flexion and extension of the fingers are controlled by C7,8.

2.7. Myotomes (5):

In the case of pronation and supination a single spinal segment is involved, namely C6.

2.8. Myotomes (6):

A single segment again, namely T1, is involved in producing abduction and adduction of the fingers: these movements are carried out by the small muscles (intrinsics) of the hand. Note: In testing for myotomes the ability to perform the above movements should be assessed by MRC grading, and note made of the segments affected. Often the defect can be localised to a single segment.

2.9. Dermatomes:

Note that the middle finger is supplied by C7, and that there is a regular, easily remembered sequence of sensory distribution round the preaxial line of the limb.

2.10. Types of brachial plexus injury (1):

Lesions in continuity: more than half of plexus injuries are of this pattern. Traction is the commonest cause, and the nerve roots are affected between the intervertebral foramina and the clavipectoral fascia (postganglionic). The lesions may be transient (neuropraxia). If the axons degenerate (axonotmesis), regeneration occurs at the rate of 1 mm/day, provided the axons can penetrate the intraneural scar.

2.11. Lesions with ruptured nerve roots (2):

In more severe injuries the nerves are disrupted at the same level. Only surgical intervention can offer any hope of recovery, but repair, even with nerve grafting, may be impossible because of extensive intraneural damage. It is important to differentiate between lesions of this type, lesions in continuity (where the treatment is expectant), and cord avulsion lesions (where the prognosis is hopeless).

2.12. Complete avulsion lesions (3):

The nerve is avulsed from the cord and surgical repair is impossible. Motor axons degenerate and the paralysed muscles show denervation fibrillation potentials on the electromyograph (EMG). The cells of the sensory nerves in the dorsal root ganglion remain intact (preganglionic lesion); although sensation is lost, conduction within the distal nerve remains and may be detected through externally applied electrodes.

2.13. Partial avulsion lesions (4):

Rarely, the posterior roots are spared, so that there may be the paradox of muscle paralysis accompanied by preservation of sensation. The prognosis for the motor loss is in these circumstances also hopeless. Although the exact nature of the lesion may seem fairly clear after clinical examination and further investigation, in many cases the picture is complex owing to the fact that one or more of these injuries may be combined.

2.14. Long-standing plexus lesions (1):

In Erb’s palsy (upper obstetrical palsy), which affects the upper trunk of the brachial plexus (and hence C5,6), there is deformity of the limb, which is held in a characteristic position: the wrist is flexed and pronated, and the fingers flexed. The elbow is extended and the shoulder internally rotated (waiter’s tip deformity). The nerve to rhomboids and the long thoracic nerve are usually spared.

2.15. Long-standing plexus lesions (2):

In Klumpke’s paralysis the small (intrinsic) muscles, including the hypothenar and thenar groups, are wasted and there is a claw hand deformity. There is sensory loss on the medial side of the forearm and wrist. In many cases there is an associated Horner syndrome. (Note also that 38% of patients who receive radiotherapy for breast carcinoma develop a brachial neuropathy.)

2.16. Long-standing plexus lessons (3):

The T1 root alone may be involved, the sole signs being wasting of the small muscles of the hand, including the thenar group, along with sensory loss on the medial side of the hand only. Lesions of this type are seen in incomplete lower obstetrical palsy, cervical spondylosis, cervical rib syndrome, neurofibromatosis, and apical and metastatic carcinoma.

2.17. Acute traumatic lesions of the brachial plexus:

The commonest mechanisms of injury involve depression of the shoulder combined with lateral flexion of the neck to the opposite side, or traction on the arm. Motorcycle accidents are the single most common cause. On inspection, look for the presence of telltale bruising over the shoulder or at the root of the neck. In the more severe cases the arm hangs flaily at the side.

2.18. Assessment (1):

Begin by determining the extent of the lesion, i.e. which segments are involved, and whether the involvement is partial or complete. Start by testing for active movements in the shoulder, elbow, wrist and fingers, relating your findings to the myotomes responsible for these movements. Then check for sensation to pinprick and light touch, again noting the dermatomes involved (which normally correspond with the previously determined myotomes).

2.19. Assessment (2):

After determining which segments are affected, you should try to form an opinion as to the type of injury. This may be difficult to clarify, but the more evidence there is of proximal damage, the greater the chance of cord avulsion and a poor prognosis. Horner’s syndrome, characterised by (A) pseudoptosis, (B) smallness of the pupil on the affected side, and (C) dryness of the hand from absence of sweating, occurs when the T1 root is involved close to the canal.

2.20. Assessment (3):

Look for sensory loss above the clavicle. This area is normally supplied by C3,4, and if this is affected it generally indicates that the injury has been so severe that it has not only involved the plexus but the roots above; it is usually indicative of a proximal injury with a poor prognosis. Deep bruising in the posterior triangle is also strongly suggestive of a preganglionic lesion.

2.21. Assessment (4):

Now test the first nerves that come off the plexus. Nerve to rhomboids (C5): Ask the patient to place the hand on the hip and to resist the elbow being pushed forwards; feel for contraction in the rhomboid muscles. Absence of activity is indicative of a lesion proximal to the formation of the upper trunk of the plexus (and suggestive of cord avulsion). Presence of activity means a lesion distal to the intervertebral foramen.

2.22. Assessment (5):

Nerve to serratus anterior (C5,6,7): Damage to this nerve produces winging of the scapula, which is normally demonstrated by asking the patient to lean with both hands against a wall, but this test may have to be abandoned in the presence of an extensive plexus lesion. (Note that the nerve to serratus anterior may be damaged in isolation through lifting very heavy weights.)

2.23. Assessment (6):

Suprascapular nerve (C5,6): This nerve arises from the upper trunk of the plexus and supplies the supraspinatus and infraspinatus. To test for activity in the supraspinatus, ask the patient to try to abduct the arm against resistance; feel for muscle contraction above the spine of the scapula. (The infraspinatus may be tested by feeling for muscle contraction below the spine of the scapula while the patient attempts to rotate the shoulder externally.)

2.24. Assessment (7):

Other tests, observation and investigations. (1) Tinel’s sign: Tap vigorously at the side of the neck, working from above downwards in the line of the nerve roots as they emerge from the spine. The test is positive if there is marked, painful paraesthesia in the corresponding dermatomes: for example, if tapping over the C6 root produces severe pain and tingling in the thumb. A positive test generally indicates a ruptured nerve root and a postganglionic lesion. (It is said, however, that the test may also be positive in the presence of an avulsed posterior root ganglion.)

(2) X-ray and MRI scans: