Anterior Interosseous Syndrome

Excerpt

The anterior interosseous nerve serves as the terminal motor branch of the median nerve. Originating approximately 5 to 8 cm distal to the lateral epicondyle, this nerve emerges in the proximal forearm between the 2 heads of the pronator teres muscle and courses deep along the interosseous membrane. As a purely motor nerve, the anterior interosseous nerve innervates, in proximal-to-distal order, the flexor pollicis longus (FPL) of the thumb, the flexor digitorum profundus (FDP) of the index and middle fingers, and the pronator quadratus of the forearm (see Images. Anterior Interosseous Nerve and Forearm Innervations).

Anterior interosseous nerve syndrome refers to an isolated palsy affecting these 3 muscles, without any associated sensory loss. The condition typically presents with forearm pain and a distinctive weakness in the pincer movement of the index finger and thumb. The underlying pathophysiology remains uncertain and continues to prompt debate. Many cases appear secondary to transient inflammatory neuritis, though nerve compression and trauma represent established causes. Current theories often cite either an idiopathic immune-mediated neuritis or an intrinsic compressive lesion within the forearm.

Multiple hypotheses have emerged regarding the etiology of the condition. Despite ongoing discussion among upper extremity surgeons, the prevailing view classifies the syndrome as a form of neuritis. Clinicians should note that direct external trauma to the anterior interosseous nerve, which can produce similar muscle weakness, does not qualify as true anterior interosseous nerve syndrome due to its distinct pathophysiological mechanism.

Parsonage and Turner first identified the syndrome in 1948, followed by Leslie Gordon Kiloh and Samuel Nevin in 1952, who characterized it as an isolated lesion of the anterior interosseous nerve. Historically, the condition was known as Kiloh-Nevin syndrome. Various treatment approaches have demonstrated reasonable outcomes. Most cases resolve spontaneously, and conservative management, including symptomatic treatment over a period of 3 to 6 months, remains the preferred initial strategy. Both medical and surgical interventions have been explored, with variable timing and results.