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Kiloh-Nevin Syndrome (Anterior Interosseous Nerve Syndrome)
Presentation Review - Dr Amorgianos

Indroduction

Kiloh-Nevin Syndrome, more commonly known as anterior interosseous nerve (AIN) syndrome, is a rare, purely motor neuropathy of a branch of the median nerve. The AIN supplies the flexor pollicis longus (FPL), the radial half of the flexor digitorum profundus (FDP) (primarily to the index finger), and the pronator quadratus. Patients typically present in early to mid-adulthood with subtle motor deficits affecting pinch and fine motor tasks. Unlike most median nerve compressions, sensation remains intact. The syndrome is thought to result from either a compressive or inflammatory insult—often at the level of the pronator teres or in the proximal forearm—leading to selective axonal loss. Early recognition and accurate diagnosis are crucial, as misdiagnosis may delay proper treatment and result in persistent functional impairment.

Eight Clinical Signs

  • Impaired “OK” Sign Formation: Inability to form a proper “OK” sign, due to simultaneous weakness of FPL and FDP muscles.

  • Weakness of Flexor Pollicis Longus: Noticeable difficulty in flexing the thumb’s distal joint.

  • Weakness in Flexor Digitorum Profundus to the Index Finger: Difficulty flexing the distal phalanx of the index finger, specifically the radial portion of FDP.

  • Normal Sensory Examination: Preserved sensation (touch, pain, proprioception), distinguishing it from other median nerve conditions.

  • Forearm Discomfort or Mild Pain: Occasional vague discomfort or mild aching along the forearm, particularly with activity.

  • Difficulty with Precision Grip: Clumsiness or difficulty performing fine motor tasks like buttoning or writing due to impaired pinch grip.

  • Positive “Pinch” Test Abnormality: Patients show abnormal pinch grip without proper rounded contour of a normal “OK” sign.

  • Electromyographic Evidence of Denervation: EMG tests typically indicate axonal loss in muscles innervated by AIN.

Essential Diagnostic Studies

  • Detailed Clinical Examination: Central to the diagnosis, primarily identified through the inability to form an “OK” sign and weakness in FPL and FDP muscles. Provocative maneuvers may help highlight symptoms.

  • Electromyography (EMG) and Nerve Conduction Studies (NCS): EMG shows denervation and reduced recruitment in affected muscles. Sensory nerve conduction remains normal.

  • Magnetic Resonance Imaging (MRI): Used to rule out compressive lesions or muscle edema/atrophy.

  • High-Resolution Ultrasound: Dynamic imaging to detect structural anomalies, extrinsic compressions, or focal nerve swelling.

  • Provocative Clinical Testing: Clinical tests such as the pinch grip test further substantiate diagnosis.

Latest Treatment Options

Management is tailored to severity and chronicity:

  1. Conservative Management: Initial rest, activity modification, NSAIDs, and avoidance of aggravating activities.

  2. Physical Therapy: Rehabilitation programs including nerve gliding, muscular strengthening, and biomechanical corrections.

  3. Corticosteroid Injections: Ultrasound-guided injections around suspected inflammation sites to reduce edema and enhance nerve recovery.

  4. Surgical Decompression: Indicated for persistent symptoms or identifiable compressive lesions, typically involving open or endoscopic nerve decompression and neurolysis.

  5. Tendon Transfer Procedures: For chronic, refractory cases where nerve recovery is incomplete, tendon transfers (e.g., brachioradialis to replace weak FPL) may restore functional pinch and grip.

Conclusion

Kiloh-Nevin Syndrome (AIN syndrome) is a distinctive motor neuropathy characterized by specific motor deficits (weakness in FPL and index FDP, inability to form an “OK” sign) and intact sensation. Accurate and prompt diagnosis, supported by clinical examination, EMG/NCS, and advanced imaging, is essential. Treatment ranges from conservative care and physical therapy to injections and surgical interventions, guided by symptom severity and progression. A multidisciplinary approach ensures optimal recovery of hand function.

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Dr. Christodoulos Amorgianos Orthopedic Trauma Surgeon – Specialist Hand Microsurgery & Interventional Pain Specialist
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