Anterior Interosseous Syndrome

By admin On Nov 27, 2023

Ovid: 5-Minute Sports Medicine Consult, The

David Z. Frankel

Basics

The anterior interosseous nerve (AIN) is a motor branch of the median nerve that innervates the pronator quadratus, flexor pollicis longus, and flexor digitorum profundus serving the index finger.
Anterior interosseous syndrome is a catch-all term for neuropathies that result in paralysis of these muscles secondary to a number of causes.

Etiology

The most frequent causes of anterior interosseous syndrome are direct traumatic nerve damage and external compression.

Traumatic causes:
- Penetrating trauma
- Blunt injury
- Traction injury
- Fracture
- Surgery
- Venipuncture
- Injection
- Cast pressure
External compression:
- Bulky tendinous origin of the deep head of the pronator teres
- Soft tissue mass such as lipoma, ganglion, or tumor
- Accessory muscle
- Fibrous band originating from the superficial flexor
- Vascular abnormality

Diagnosis

History

Pain in the forearm
Sensory loss not noted
Weakness noted as difficulty with writing or with fine-pinch activities

Physical Exam

The signature finding is weakness of the flexor pollicis longus, flexor digitorum profundus indicis, and pronator quadratus.
Weakness of the flexor pollicis longus and flexor digitorum profundus to the index finger is indicated by an inability to make the “OK sign.” Rather the distal interphalangeal (DIP) joint of the index finger and interphalangeal (IP) joint of the thumb are hyperextended during attempted tip-to-tip pinch.
The pronator quadratus is difficult to isolate clinically. Weakness may be detected by asking the patient to forcibly pronate the forearm against resistance (resist supination) with the elbow flexed at 90 degrees.
The AIN provides no sensory fibers to the skin; therefore, abnormal sensation testing rules out anterior interosseous neuropathy.

Diagnostic Tests & Interpretation

Electrodiagnostic testing:
- Should include electromyography (EMG) of the flexor pollicis longus, pronator quadratus, and flexor digitorum profundus indicis
- The latency and amplitude of compound muscle action potential are compared with those of the unaffected side.
- Sensory nerve action potentials are normal.
Imaging modalities:
- Both MRI and US may be helpful in confirming the clinical diagnosis of anterior interosseous syndrome.
- MRI findings of edema in the muscles innervated by the AIN on T₂-weighted fat-suppressed images
- The AIN may appear swollen on US when compared with the normal contralateral nerve.
- US of the AIN-innervated muscles may show a loss of bulk, increased reflectivity, reduced perfusion on Doppler sonography, and lack of active contraction of the affected muscles.

Differential Diagnosis

Tendon rupture
Brachial plexus neuritis (Parsonage-Turner syndrome)
Congenital absence of the flexor digitorum longus and flexor pollicis longus
Partial lesion of the median nerve

P.31

Treatment

Once an accurate diagnosis of anterior interosseous syndrome is made, treatment must be guided by the underlying etiology.
Penetrating trauma suggests nerve disruption or compression and is best treated with surgical exploration and nerve decompression or repair.
With blunt trauma, an EMG suggestive of a complete lesion may mandate early surgical exploration. Partial injuries may be given an opportunity to recover spontaneously. If no improvement is noted after 6–12 wks of conservative management, surgical exploration is necessary.
The typical nonsurgical treatment regimen includes rest, splinting, and observation.
Spontaneous paralysis of the AIN may be treated nonsurgically for 12 wks, with surgical exploration if no clinical or EMG improvement is evident.
Spontaneous recovery after 12 mos is documented, and some advocate nonsurgical management for at least this long before proceeding with surgical exploration.
Tendon transfers allow functional reconstruction of the thumb and index finger should the AIN fail to recover, be unreconstructable, or irreversible muscle atrophy is present after prolonged muscle denervation.

Additional Reading

Chin DH, Meals RA. Anterior interosseous nerve syndrome. J Am Soc Surg Hand. 2001;1:249–257.

Dang AC, Rodner CM. Unusual compression neuropathies of the forearm, part II: median nerve. J Hand Surg Am. 2009;34:1915–1920.

Dunn AJ, Salonen DC, Anastakis DJ. MR imaging findings of anterior interosseous nerve lesions. Skeletal Radiol. 2007;36:1155–1162.

Hide IG, Grainger AJ, Naisby GP, et al. Sonographic findings in the anterior interosseous nerve syndrome. J Clin Ultrasound. 1999;27:459–464.

Martinoli C, Bianchi S, Pugliese F, et al. Sonography of entrapment neuropathies in the upper limb (wrist excluded). J Clin Ultrasound. 2004;32:438–450.

Codes

ICD9

354.1 Other lesion of median nerve
354.9 Mononeuritis of upper limb, unspecified

Clinical Pearls

Dysfunction of the AIN is characterized by weakness of the pronator quadratus, flexor pollicis longus, and flexor digitorum profundus serving the index finger.
On examination, the patient cannot make the “OK sign” with the thumb and index finger.
The AIN does not provide sensory nerves to the skin; therefore, abnormal sensation rules out anterior interosseus syndrome.
Penetrating trauma or blunt trauma with an EMG suggestive of a complete AIN lesion mandates surgical exploration.