What is the main symptom of AIN syndrome?

The main symptom of Anterior Interosseous Nerve (AIN) syndrome is weakness in finger flexion, particularly affecting the flexor pollicis longus and the lateral half of the flexor digitorum profundus. This results in the inability to flex the thumb and the index finger at the distal interphalangeal joints. Patients may struggle with tasks that require pinching or gripping, as the affected muscles are crucial for grasping.

The condition is caused by compression of the AIN, often due to trauma, which affects the motor function without sensory loss, distinguishing it from other neuropathies. While weakness in thumb opposition and loss of grasp strength might seem relevant, they stem from the weakness in finger flexion, making finger flexion the primary concern in AIN syndrome. Numbness in the thumb and first two fingers relates more to dysfunction of the median nerve at larger levels, rather than being specific to AIN syndrome, which purely involves motor deficits.