There are multiple compression neuropathies of the upper extremity. Some neuropathies, like carpal tunnel, are quite common; others like posterior interosseous nerve (PIN) syndrome are not. Knowledge of anatomy and function of each nerve is essential to diagnose which nerve and compression site is involved correctly. The posterior interosseous nerve is a branch of the radial nerve, which comes off the posterior cord of the brachial plexus. The radial nerve, with nerve roots C5 to T1, travels down the arm and divides into superficial and deep branches in the proximal forearm. Normally the deep branch of the radial nerve dives into the posterior forearm through the heads of supinator to emerge as the posterior interosseous nerve. Anatomical variants include the deep radial nerve passing through the Arcade of Frohse to become the posterior interosseous nerve. This variant can increase susceptibility to impingement. The posterior interosseous nerve supplies motor innervation to the posterior forearm. The terminal branch of the posterior interosseous nerve travels distally into the floor of the 4th dorsal compartment of the wrist to innervate the dorsal wrist capsule.
Compression neuropathies of the radial nerve distal to the elbow include radial tunnel syndrome, posterior interosseous nerve syndrome, and Wartenberg syndrome. Each of these has distinct symptoms which can help with identifying the correct diagnosis. Posterior interosseous nerve syndrome is a compressive neuropathy of the posterior interosseous nerve which innervates the extensor compartment of the forearm. It usually has an insidious onset, often presenting with weakness in finger and thumb extension. However, there should be preservation in wrist extension due to the radial nerve innervated extensor carpi radialis longus. It is often self-limiting and resolves with conservative measures. However, symptoms that are refractory to nonoperative treatment may require surgical decompression.
Posterior interosseous nerve syndrome can result from trauma, space-occupying lesions like in rheumatoid arthritis, brachial neuritis, and spontaneous compression. The most common site of compression is at the arcade of Frohse (the proximal edge of the supinator). Repetitive pronation/supination activities can also can posterior interosseous nerve syndrome.
Posterior interosseous nerve syndrome occurs more often in males than females in a 2 to 1 ratio. It also occurs about twice as often in the right arm versus the left arm.
The pathophysiological basis of nerve injury depends on the severity of nerve compression. Nerve injury can subdivide into three categories: neuropraxia, axonotmesis, and neurotmesis. Neuropraxia is the mildest form and is demyelination at the site of injury. This injury is usually from compression or traction and can result in slowed conduction velocities. Depending on the severity, it can cause muscle weakness but should have a negative Tinel sign at the site of injury. The prognosis for recovery is excellent, ranging from a few days up to 12 weeks. Axonotmesis involves demyelination as well as damage to the axons, resulting in muscle weakness and may have a positive Tinel sign at the site of injury. Neurotmesis is the last and most severe nerve injury in which the nerve is completely transected, resulting in no nerve conduction. Surgical correction is necessary for recovery.
The patient may possess a history of trauma or fracture of that extremity. Posterior interosseous nerve syndrome can be present in Monteggia fractures or radial head fracture-dislocations. The patient may exhibit weakness with finger extension. When asked to make a fist, the wrist may deviate radially due to extensor carpi ulnaris weakness. Depending on injury severity there may be a positive Tinel sign at the site of injury.
The evaluation may include an electromyography (EMG) and nerve conduction study (NCS). This study may show denervation changes in the muscles innervated by the posterior interosseous nerve. There will be sparing of muscles innervated by the radial nerve, including triceps, anconeus, brachioradialis, and extensor carpi radialis longus (ECRL). There will also be normal sensory nerve action potential of the superficial radial nerve.
Treatment of posterior interosseous nerve syndrome starts with non-surgical management, which can include splinting, NSAIDs, physical therapy, activity modification. Surgical treatment is reserved for those refractory to conservative management for at least 3 months. Surgical decompression focuses on releasing areas of compression. Areas that may be decompressed include releasing fibrous bands superficial to the radiocapitellar joint, the fibrous edge of extensor carpi radialis brevis (ECRB), ligating the leash of Henry (radial recurrent artery), releasing the arcade of Frohse, and the distal edge of the supinator. After surgery, the patient should start early active range of motion. The patient may continue to see improvements in symptoms for months after surgery.
The differential diagnosis for posterior interosseous nerve syndrome includes radial tunnel syndrome and Wartenberg syndrome.
Radial tunnel syndrome may involve the same sites of compression as posterior interosseous nerve syndrome. However, it presents with forearm pain without motor weakness. Pain usually starts in the dorsoradial forearm and may radiate down the lateral forearm to the dorsal radial hand. There may be tenderness over the mobile wad, which is often confused with lateral epicondylitis. The tenderness in radial tunnel syndrome is usually more distal then lateral epicondylitis; the maximal tenderness is approximately 3 to 5 cm distal to the lateral epicondyle.
Wartenberg syndrome, also known as "cheiralgia paresthetica," is a compression of the superficial sensory radial nerve. This nerve is usually compressed between the brachioradialis and extensor carpi radialis longus tendons. Because the superficial sensory radial nerve has no motor innervation, the symptoms are purely sensory. There will be no motor weakness. Patients may present with paresthesias, numbness, or ill-defined pain over the dorsal radial aspect of the hand. There can also be an association with Wartenberg syndrome and De Quervain tenosynovitis.
Complications can include an incomplete decompression, continuation of symptoms, inability to return to work at their preoperative level, as well as inability to do physically demanding jobs.
Patients should receive counsel that non-operative management should be trialed excessively before exploring operative treatment. PIN decompression is not as successful as carpal tunnel or cubital tunnel release.
Interprofessional communication can help in the treatment and rehab for posterior interosseous nerve syndrome. It is crucial for the primary care physician, orthopedic specialty-trained nurse, emergency department physician, and the physical therapist to communicate on types of rehab needed as well as create clear expectations to enhance patient outcomes. (level V). The key in many cases is to avoid repetitive activity. The results for patients who comply with therapy are good, but unfortunately, relapses are frequent in people who have jobs that require repetitive movements.
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