Which of the following motor deficits is characteristic of anterior interosseous nerve (AIN) syndrome, a branch of the median nerve?

Explanation

## Anterior Interosseous Nerve (AIN) Syndrome ### Definition and Anatomy The anterior interosseous nerve is a motor branch of the median nerve that arises in the proximal forearm, typically just distal to the pronator teres. AIN syndrome is a pure motor neuropathy with no sensory involvement. ### Motor Innervation of AIN **Mnemonic:** **LOAF** (though this applies to lumbricals innervated by median proper, the key AIN muscles are): - **Flexor Pollicis Longus (FPL)** — flexes the interphalangeal (IP) joint of the thumb - **Flexor Digitorum Profundus (FDP) to index and middle fingers** — flexes the distal interphalangeal (DIP) joints of digits 2 and 3 - **Pronator Quadratus** — pronates the forearm (deep forearm) ### Clinical Presentation of AIN Syndrome | Finding | Present? | Explanation | |---------|----------|-------------| | Loss of thumb IP flexion | **YES** | FPL paralysis | | Loss of index/middle DIP flexion | **YES** | FDP paralysis | | Loss of forearm pronation | **YES** | Pronator quadratus paralysis | | Loss of sensation | **NO** | AIN is purely motor | | Loss of thumb opposition | **NO** | Opponens pollicis (median proper) is spared | | Loss of wrist flexion | **NO** | Flexor carpi radialis (median proper) is spared | **Key Point:** AIN syndrome presents with the **"OK sign" loss** — the patient cannot make an "OK" gesture because they cannot flex the thumb IP joint and the index DIP joint simultaneously. ### Clinical Causes - Compression by pronator teres, fibrous bands, or anomalous muscles - Trauma or fracture near the elbow - Idiopathic (most common) - Rarely: tumors or inflammatory conditions **High-Yield:** AIN syndrome is a pure motor syndrome with NO sensory loss. The classic sign is inability to flex the thumb IP joint and index DIP joint (loss of the "OK sign"). **Clinical Pearl:** Patients with AIN syndrome often present with weakness of thumb flexion and are initially thought to have a more proximal (higher) median nerve lesion, but the preservation of sensation and other median-innervated motor functions distinguish it.