Imaging Studies
Radiography
In a suspected entrapment of the radial nerve in the arm, radiography should be performed to detect or rule out a fracture, healing callus, or tumor as the cause of entrapment. In radial tunnel syndrome and posterior interosseous nerve syndrome, radiographs should be obtained to detect or rule out elbow or forearm fractures, dislocations or instabilities, tumors, and arthrosis.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) is useful in detecting tumors such as lipomas and ganglions, as well as aneurysms and rheumatoid synovitis.
A study by Jengojan et al found that magnetic resonance neurography based on diffusion tensor imaging was capable of detecting nerve changes during acute entrapment of the radial nerve at the spiral groove. [25] Compression of the nerve resulted in a transient increase in local radial nerve fractional anisotropy values.
Ultrasonography
Ultrasonography (US) is capable of providing useful information in the evaluation of the upper-extremity nerves, including the radial nerve; in addition to entrapment neuropathies, it can demonstrate pathologies such as nerve tumors and traumatic nerve injuries. [26]
Toros et al, in a study of 26 patients with sensory or motor deficiency of a peripheral nerve from trauma or entrapment, confirmed that high-resolution US could show the exact location, extent, and type of lesion, yielding important information that might not be obtained by other modalities. [27] US provided reliable visualization of injured nerves in all 26 patients. Axonal swelling and hypoechogenicity of the nerve was diagnosed in 15 cases, loss of continuity of a nerve bundle in 17, formation of a neuroma of a stump in 6, and partial laceration of a nerve with loss of the normal fascicular pattern in 5. US findings were confirmed at operation in the 19 patients who underwent surgery. [27]
Lo et al compared US with electrophysiologic testing in the diagnosis of entrapment neuropathy of the radial nerve at the spiral groove, which is relatively common but technically challenging to localize. The study included 32 normal controls, to obtain ultrasound parameters of the radial nerve, and 10 patients with suspected radial neuropathy, in whom US and electrophysiologic techniques were used. US correctly identified all six patients with radial neuropathy; the four patients with other diagnoses did not show ultrasound abnormalities. US also required significantly less time than electrophysiologic testing did. [28]
Electrodiagnostic Testing
Electromyography (EMG) and nerve conduction studies yield abnormal results in radial nerve injuries in the middle and distal third of the humerus. EMG helps locate the site of injury and helps the clinician monitor the nerve recovery over time. EMG may not be positive for 3-6 weeks following injury. By 4 months after the injury, nerve recovery should be demonstrable. EMG may be performed initially to provide a baseline, but unless the nerve is severed, no changes will be observed for 3-6 weeks.
All electrodiagnostic test results are within normal limits in radial tunnel syndrome.
In posterior interosseous nerve syndrome, nerve conduction studies and EMG usually yield abnormal results and help determine the site of compression. These findings may not become abnormal for 3-6 weeks after injury. Serial EMG usually is not necessary once the diagnosis is established but can be used to document improved or worsening nerve function.
If the medial branch of the posterior interosseous nerve is compressed, EMG and conduction studies reveal abnormal function of the extensor carpi ulnaris, the extensor digitorum communis, and the extensor digiti quinti. If only the lateral branch is compressed, then abnormal function of the abductor pollicis longus, the extensor pollicis longus and brevis, and the extensor indicis proprius is revealed. The site of compression is localized by all function proximal to the compression being normal and all function distal to the compression being abnormal.
Diagnostic Procedures
To help differentiate lateral epicondylitis from radial tunnel syndrome, injection of cortisone and lidocaine injection may be performed. An injection into the area of the lateral epicondyle should resolve almost all of the symptoms of lateral epicondylitis, at least temporarily. Injection of cortisone and lidocaine into the area of the radial tunnel usually fails to relieve the discomfort or relieves it only incompletely.
In de Quervain disease, injection of cortisone and lidocaine into the tendon sheath of the extensor pollicis brevis and the abductor pollicis longus relieves the symptoms immediately and sometimes permanently. An injection of cortisone and lidocaine into the area of compression of the superficial radial nerve causes the symptoms to subside; however, numbness in the nerve distribution follows, and when the injection wears off, the symptoms return.
A Tinel sign is present in patients with Wartenberg syndrome but usually not in those with de Quervain disease. The two conditions may be related. The inflammation from de Quervain disease causes an inflammation of the nerve. With resolution of one condition, the other may subside or may be adequately treated with an injection. [29]
