305 - Posterior Interosseus Nerve Syndrome Confirmed and Treated Utilizing Neuromuscular Ultrasound/Sonography: A Case Report

Case Diagnosis: Right Posterior Interosseus Nerve Syndrome

Case Description: A 63-year-old male presented with right sided wrist drop, and right upper extremity weakness following a fall on outstretched hand. Cervical MRI displayed concern for right C4 foraminal narrowing. Physical exam was notable weakness with right sided wrist extension, metacarpophalangeal extension, supination and elbow flexion. Patient was referred following lack of improvement with OT, and continual wrist and finger extension.

Discussions: PINS is a compressive neuropathy of the PIN proximal to the Arcade of Frohse. PINS most commonly occurs spontaneously but can also occur following trauma. PINS typically results in difficulty with supination and finger extension, resulting in finger drop. Treatment of PINS ranges from conservative measures such as physiotherapy, to corticosteroid injections or surgical management.

Electromyography of the patient’s right wrist/forearm revealed posterior interosseus neuropathy, with sparing of function in the superficial radial sensory nerve. Electrodiagnostic findings correlated with neuromuscular ultrasound of right wrist/forearm showing hypoechogenicity and marked focal enlargement of the posterior interosseous nerve. Given the EMG findings and US imaging, the patient was diagnosed with posterior interosseus nerve syndrome. The patient was treated with US-guided corticosteroid injection and anesthetic hydrodissection around the PIN. At 2-3 month follow-up imaging of the forearm decreased inflammation and echogenicity of the PIN was seen. Additionally, the patient demonstrated functional improvement on exam.

Conclusions: This case displays an atypical presentation of PINS, exhibiting wrist drop, which is often not seen due to separate innervation of the extensor carpi radialis longus. Neuromuscular ultrasound can be utilized to visualize focal nerve swelling, abnormal architecture, specify location, severity of injury, and administer guided interventional treatment.