Radial Nerve Block at the Humerus

Indications and Clinical Considerations

Injury of the proximal portion of the radial nerve at the level of the humerus is commonly encountered in clinical practice. It is frequently seen following antecedent trauma including humeral shaft fractures and following stab and gunshot wounds of the upper extremity ( Fig. 54.1 ). It can also occur following strenuous physical activity including overhead throwing. The mechanism for thrower’s palsy of the radial nerve is thought to be due to abnormal torque forces applied to the muscles of the upper extremity used to throw. These forces can be severe enough to occasionally result in the spontaneous occurrence of a thrower’s fracture of the humerus. Rarely, isolated proximal radial nerve palsy is seen in patients suffering from acute herpes zoster or Guillain-Barré syndrome. Compromise of the radial nerve at the level of the humerus presents clinically as weakness or paralysis of all of the wrist and finger extensors as well as the forearm supinators. If the lesion is proximal, weakness of the triceps muscle will also be present. Numbness of the dorsum of the wrist and the dorsal aspects of a portion of the thumb and index and middle fingers will be present.

Radial tunnel syndrome is an entrapment neuropathy of the radial nerve that is often clinically misdiagnosed as resistant tennis elbow. In radial tunnel syndrome, the posterior interosseous branch of the radial nerve is entrapped by a variety of mechanisms that have in common a similar clinical presentation. These mechanisms include aberrant fibrous bands in front of the radial head, anomalous blood vessels that compress the nerve, ganglion cysts, or a sharp tendinous margin of the extensor carpi radialis brevis (see Fig. 54.1 ). These entrapments may exist alone or in combination.

Regardless of the mechanism of entrapment of the radial nerve, the common clinical feature of radial tunnel syndrome is pain just below the lateral epicondyle of the humerus. The pain of radial tunnel syndrome may develop after an acute twisting injury or direct trauma to the soft tissues overlying the posterior interosseous branch of the radial nerve, or the onset may be more insidious, without an obvious inciting factor. The pain is constant and is made worse with active supination of the wrist. Patients often note the inability to hold a coffee cup or hammer. Sleep disturbance is common. On physical examination, there is tenderness to palpation of the posterior interosseous branch of the radial nerve just below the lateral epicondyle. Elbow range of motion is normal. Grip strength on the affected side may be diminished. Patients with radial tunnel syndrome exhibit pain on active resisted supination of the forearm.

Cervical radiculopathy and tennis elbow can mimic radial tunnel syndrome. Radial tunnel syndrome can be distinguished from tennis elbow in that, in radial tunnel syndrome, the maximal tenderness to palpation is distal to the lateral epicondyle over the posterior interosseous branch of the radial nerve, whereas in tennis elbow the maximal tenderness to palpation is over the lateral epicondyle ( Fig. 54.2 ). Electromyography helps distinguish cervical radiculopathy and radial tunnel syndrome from tennis elbow. Plain radiographs are indicated for all patients with radial tunnel syndrome to rule out occult bony disease. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, uric acid, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging scans and/or ultrasound imaging of the elbow and forearm is indicated if joint instability or occult tumor or mass is suspected ( Fig. 54.3 ). The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

Clinically Relevant Anatomy

The radial nerve is made up of fibers from C5-T1 spinal roots. The nerve lies posterior and inferior to the axillary artery. Exiting the axilla, the radial nerve passes between the medial and long heads of the triceps muscle. As the nerve curves across the posterior aspect of the humerus, it supplies a motor branch to the triceps. Continuing its downward path, it gives off a number of sensory branches to the upper arm. At a point between the lateral epicondyle of the humerus and the musculospiral groove, the radial nerve divides into its 2 terminal branches (see Fig. 54.1 ). The superficial branch continues down the arm along with the radial artery and provides sensory innervation to the dorsum of the wrist and the dorsal aspects of a portion of the thumb, index, and middle fingers. The deep posterior interosseous branch provides the majority of the motor innervation to the extensors of the forearm.