This week’s episode on wrist drop strives to complement episode 5 on foot drop and aid in the localization of a neuromuscular lesion. Dr. Laura Mainardi Villarino reviews with me several ways on how to conceptualize wrist extension weakness.
To briefly review the anatomy, the radial nerve is supplied by all three trunks of the brachial plexus: the upper (C5, C6), middle (C7), and lower (C8, T1) trunks. Together, these three trunks merge to form the posterior cord, a large bundle of axons that sprout 3 branches: the upper subscapular nerve which innervates the subscapularis, the lower subscapular nerve which innervates the teres major, and the thoracodorsal nerve which innervates the latissimus dorsi. Eventually the posterior cord divides into its final two tributaries, the axillary nerve (known for its innervation of the deltoid and teres minor) and the radial nerve, which does a whole bunch of cool stuff that we talk about in our show, but importantly can produce wrist drop.
A rule of thumb on peripheral upper extremity neuroanatomy you definitely want to take away from this episode is that in general the more proximal muscle groups are innervated by roots higher up in the brachial plexus whereas more distal muscle groups are innervated by roots lower down in the brachial plexus. Consider the deltoid for example, an axillary nerve-innervated proximal muscle (which like the radial nerve, originates from the posterior cord), but the axillary nerve originates from the C5/C6 roots and upper trunk. Then consider the abductor pollicis brevis, a median nerve-innervated distal muscle (which like the radial nerve, is ultimately supplied by C8/T1 nerves of the lower trunk).
When you’re seeing a patient with isolated wrist extension weakness, using your exam to confirm that the wrist extensor muscles (namely the extensor carpi radialis brevis and longus) are weak is probably necessary to confirm what the patient has told you already, but this has almost zero localizing value. Conceptually, you want to distinguish whether the lesion (which we will assume is peripheral for the purpose of this episode) is due to a C6 or C7 radiculopathy, upper or middle trunk problem, a posterior cord lesion, or a peripheral radial neuropathy. In the episode, we start by excluding more proximal lesions and work our way distally.
Is the lesion due to a C6 or C7 radiculopathy? You will want to isolate a muscle group that is innervated by a nerve that originates at the C6 root but not the posterior cord or radial nerve. If you look at the chart by Preston and Shapiro (below), you will find that the deltoid is not a great muscle to test in this circumstance because it does not distinguish C6 from posterior cord here since the deltoid is innervated by the axillary nerve. The biceps brachii, however, is innervated by the musculocutaneous nerve which comes off C5/C6 root and the lateral cord. Therefore, if biceps strength is preserved, the patient will not have a C6 or upper trunk process.
Distinguishing a C7 radiculopathy is difficult clinically (easier electrographically). To do this, you must find a muscle innervated by a nerve originating from the C7 root and not the posterior cord or radial nerve: for instance, the pronator teres. If this is normal, it’s not a C7 radiculopathy.
Could wrist drop be due to a lesion of the upper trunk? To exclude an upper trunk lesion, we can isolate a nerve coming from the upper trunk, like the suprascapular nerve which innervates the supraspinatus and infraspinatus. As you know, the supraspinatus provides the first 15 degrees of shoulder abduction, followed by the deltoid and lastly the trapezius. The infraspinatus, on the other hand, is the principal external rotator of the shoulder. So you can test external rotation here to exclude an upper trunk lesion.
Can it be due to a lesion of the middle trunk? Since the middle trunk is supplied solely by the C7 root, and we excluded C7 radiculopathy, it can’t be a middle trunk problem.
Can wrist drop be attributed to a posterior cord lesion? While I told you earlier that testing deltoid function does not offer much localizing value for distinguishing a C6 radiculopathy from a posterior cord lesion, it is the best way to distinguish a posterior cord lesion from a peripheral radial neuropathy.
So where in the radial nerve are we left? As the nerve dives into the axilla, it gives 2 branches to the triceps brachii and a sensory branch called the posterior cutaneous nerve to the back of the upper arm. Axillary compression, usually in the setting of inappropriate use of crutches or more rarely, tumors, will cause weakness of all muscles innervated by the radial nerve (including the triceps) and sensory loss to much of the posterior upper extremity.
More distally, as everyone remembers from medical school, a lesion of the spiral groove of the humerus will obviously preserve strength at the triceps but will cause weakness of all other radial nerve innervated muscles. The spiral groove is the most common site of injury of the radial nerve. Patients are left with weakness of wrist extension in addition to finger extension due to paralysis of the extensor indicis proprius, extensor digitorum communis, extensor carpi ulnaris, and long head of extensor carpi radialis; they have mild weakness of forearm supination from supinator weakness; and they have mild weakness of elbow flexion from weakness of the brachioradialis.
Even more distally, a lesion of the posterior interrosseous nerve, also called the deep radial motor nerve, can cause similar dysfunction to that of a lesion at the spiral groove. But there is one muscle you can test here that may help you distinguish a neuropathy of the PIN from that of the radial nerve at the spiral groove: the brachioradialis. So put your patient’s arm in a partially pronated position and have her flex at the elbow. You may either see or feel the brachioradialis muscle contract with elbow flexion, and that will confirm the lesion to the radial nerve is distal to the spiral groove and will be found at the PIN.
For more information about the case, check out our episode this week. Definitely worth your time!
Preston DC & Shapiro BE. Electromyography and neuromuscular disorders, 3rd ed. Saunders (2013).