What is the anterior interosseous syndrome?
Anterior interosseous syndrome is caused by entrapment of the anterior interosseous branch of the median nerve.
It can be provoked by blunt trauma or repetitive flexion at elbow and wrist such as using an ice pick.
The site of entrapment is between the tendinous origins of the pronator teres and the flexor digitorum superficialis.
On occasion, it is caused by either a viral or inflammatory process.
Symptoms include pain in the forearm and wrist. Easy fatigability and weakness in the distal flexors of the thumb and index finger are often noted.
Creating a tight O-ring sign with the thumb and index finger is not possible; instead a flattened, weak pinch is made.
Anterior interosseous syndrome is an uncommon cause of forearm and wrist pain. The onset of symptoms in patients with anterior interosseous syndrome is usually after acute trauma to the forearm or after repetitive forearm and elbow motions, such as using an ice pick.
In this setting, the pain and muscle weakness of anterior interosseous syndrome are thought to be secondary to compression of the anterior interosseous nerve just below the elbow by the tendinous origins of the pronator teres muscle and flexor digitorum superficialis muscle of the long finger or by aberrant blood vessels.
In some patients, no antecedent trauma is identified, and an inflammatory cause analogous to Parsonage-Turner syndrome has been suggested as the cause of anterior interosseous syndrome in the absence of trauma.
Clinically, anterior interosseous syndrome manifests as acute pain in the proximal forearm and deep in the wrist. As the syndrome progresses, patients with anterior interosseous syndrome may report a tired or heavy sensation in the forearm with minimal activity and the inability to pinch items between the thumb and index finger because of paralysis of the flexor pollicis longus and the flexor digitorum profundus.
What are the Symptoms of Anterior interosseous syndrome
Physical findings include the inability to flex the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger resulting from paralysis of the flexor pollicis longus and the flexor digitorum profundus. A positive Playboy and Spinner sign may also be present.
Tenderness over the forearm in the region of the pronator teres muscle is seen in some patients with anterior interosseous syndrome. A positive Tinel sign over the anterior interosseous branch of the median nerve approximately 6 to 8 cm below the elbow also may be present.
How is Anterior interosseous syndrome diagnosed?
Electromyography helps distinguish cervical radiculopathy, thoracic outlet syndrome, and carpal tunnel syndrome from anterior interosseous syndrome. Plain radiographs are indicated in all patients who present with anterior interosseous syndrome to rule out occult bony pathology.
Based on the patient’s clinical presentation, additional tests, including complete blood cell count, uric acid level, erythrocyte sedimentation rate, and antinuclear antibody testing, may be indicated.
Magnetic resonance imaging (MRI) and ultrasound imaging of the forearm is indicated to help clarify the diagnosis and if a primary elbow pathological process or a space-occupying lesion is suspected. Injection of the median nerve at the elbow serves as a diagnostic and therapeutic maneuver.
How is anterior interosseous syndrome treated?
Antiinflammatory medications, local steroid/anesthetic injection, pregabalin, gabapentin, duloxetine, or tricyclic antidepressants (TCAs) such as nortriptyline can be tried. If these fail, surgical decompression may be necessary.
The anterior interosseous syndrome also should be differentiated from cervical radiculopathy involving the C6 or C7 roots, which sometimes may mimic median nerve compression. Cervical radiculopathy and median nerve entrapment may coexist as the double crush syndrome. The double crush syndrome is seen most commonly with median nerve entrapment at the wrist or carpal tunnel syndrome. Anterior interosseous syndrome can be distinguished from pronator syndrome and median nerve compression by the ligament of Struthers, because the pain of anterior interosseous syndrome occurs more distally and is accompanied by the characteristic loss of ability to pinch items between the thumb and index finger.
Nonsteroidal anti inflammatory drugs (NSAIDs) or cyclooxygenase-2 (COX-2) inhibitors represent a reasonable first step in the treatment of anterior interosseous syndrome. The use of the tricyclic antidepressants, such as nortriptyline, at a single bedtime dose of 25 mg, titrating upward as side effects allow, also is useful, especially if sleep disturbance is present. It is important for the patient to avoid repetitive trauma thought to be contributing to this entrapment neuropathy. If these maneuvers fail to produce rapid symptomatic relief, injection of the median nerve at the elbow with a local anesthetic and steroid is a reasonable next step. If symptoms persist, surgical exploration and release of the anterior interosseous branch of the median nerve are indicated.
Median nerve block below the elbow is a relatively safe block, with major complications being inadvertent intravascular injection and persistent paresthesia secondary to needle trauma to the nerve.
This technique can be performed safely in the presence of anticoagulation by using a 25- or 27-gauge needle, albeit at increased risk for hematoma, if the clinical situation dictates a favorable risk-to-benefit ratio.
These complications can be decreased if manual pressure is applied to the area of the block immediately after injection.
Application of cold packs for 20-minute periods after the block also decreases the amount of post-procedure pain and bleeding the patient may experience.
Avoidance techniques for the repetitive movements responsible for anterior interosseous syndrome are often forgotten in the rush to treatment. Median nerve block at the elbow is a simple and safe technique in the evaluation and treatment of the aforementioned painful conditions. Careful neurological examination to identify preexisting neurological deficits that may later be attributed to the nerve block should be performed in all patients before beginning median nerve block at the elbow.