Anterior interosseous nerve (AIN) syndrome

1. AIN syndrome presents with combined motor and sensory deficits.

False

The AIN is a purely motor branch of the median nerve, supplying flexor pollicis longus, flexor digitorum profundus to the index and variably to the long finger, and pronator quadratus. It carries no cutaneous sensory fibres, and sensation in the hand is unaffected in classical AIN syndrome. Kiloh and Nevin's 1952 description in the British Medical Journal defined the syndrome as an isolated motor neuritis of the AIN.

• Kiloh LG, Nevin S. Isolated neuritis of the anterior interosseous nerve. Br Med J. 1952;1(4763):850-851. PMID 14916168

2. The Kiloh–Nevin sign — inability to form a normal 'O' between thumb and index finger — is a clinical feature of AIN syndrome.

True

The eponymous Kiloh–Nevin sign reflects paralysis of flexor pollicis longus (loss of thumb interphalangeal flexion) and flexor digitorum profundus to the index finger (loss of distal interphalangeal flexion); the patient pinches in a triangle configuration rather than forming a true rounded 'O'. The sign was implicit in Kiloh and Nevin's original description and remains the classical clinical screening manoeuvre.

• Kiloh LG, Nevin S. Isolated neuritis of the anterior interosseous nerve. Br Med J. 1952;1(4763):850-851. PMID 14916168

3. The pronator quadratus is spared in AIN syndrome.

False

Pronator quadratus is innervated by the most distal motor branch of the AIN and is typically affected in AIN syndrome. Dunn et al. reviewed MR imaging in surgically and clinically confirmed AIN cases and identified oedema-like signal change within the pronator quadratus in every case in their series. PQ denervation oedema is now considered a hallmark MRI feature of the condition.

• Dunn AJ, Salonen DC, Anastakis DJ. MR imaging findings of anterior interosseous nerve lesions. Skeletal Radiol. 2007;36(12):1155-1162. PMID 17938918

4. Many cases of nontraumatic 'isolated' AIN palsy are now thought to represent a non-compressive inflammatory neuropathy (a forme fruste of Parsonage–Turner / neuralgic amyotrophy) rather than mechanical compression at the elbow.

True

Maldonado et al. (2016) reinterpreted electrodiagnostic and MRI data in a Mayo Clinic cohort of patients with nontraumatic 'isolated' AIN palsy and concluded that the imaging and electrophysiological findings were more consistent with an inflammatory neuropathy (Parsonage–Turner syndrome) than with mechanical compression. Sneag et al. (2020) subsequently demonstrated, on high-resolution MR neurography and ultrasound in 45 patients, fascicular constrictions of the median nerve proximal to the elbow joint (mean MR-neurography 5.4 cm; mean ultrasound 7.5 cm above the elbow), with no extrinsic forearm compression identified — supporting AIN syndrome as an intrinsic, non-compressive, fascicular neuropathy.

• Maldonado AA, Amrami KK, Mauermann ML, Spinner RJ. Reinterpretation of Electrodiagnostic Studies and Magnetic Resonance Imaging Scans in Patients with Nontraumatic 'Isolated' Anterior Interosseous Nerve Palsy. Plast Reconstr Surg. 2016;138(5):1033-1039. PMID 27391838
• Sneag DB, Arányi Z, Zusstone EM, Feinberg JH, Queler SC, Nwawka OK, et al.. Fascicular constrictions above elbow typify anterior interosseous nerve syndrome. Muscle Nerve. 2020;61(3):301-310. PMID 31773766

5. MRI may demonstrate denervation oedema within the pronator quadratus in AIN syndrome.

True

Dunn et al. (2007) reported that oedema-like T2-weighted signal change within the pronator quadratus muscle was identified in every case in their MR imaging series of AIN lesions. This finding is the canonical imaging hallmark of AIN syndrome and is widely cited in contemporary reviews.

• Dunn AJ, Salonen DC, Anastakis DJ. MR imaging findings of anterior interosseous nerve lesions. Skeletal Radiol. 2007;36(12):1155-1162. PMID 17938918