0227: The Posterior Interosseous Nerve - Where Have You Been All My Life?

Abstract

Ultrasound is a highly sensitive and specific modality for the diagnosis of lateral epicondylitis, and sonographic evaluation of the lateral elbow is a commonly requested examination. Sonographic musculoskeletal examinations have their best utility when targeted to the area of clinical suspicion. While the lateral epicondyle and CEO are routinely the target of sonographic examination, lateral elbow pain can also result from neuropathy affecting the nerves of the arm - in particular, the posterior interosseous nerve (PIN). The PIN may be affected by direct trauma or idiopathic compression. Termed Posterior Interosseous Nerve Entrapment or Radial Tunnel Syndrome, compression of the PIN can result in focal pain that mimics the clinical picture of lateral epicondylitis. For this reason, sonographic examination for lateral elbow pain where lateral epicondylitis is suspected should include examination of the PIN. The Posterior Interosseous Nerve (PIN), or deep branch of the radial nerve branches from the radial nerve just distal to the elbow joint. The PIN then passes through the radial tunnel. The course of the PIN through the radial tunnel takes it past a number of structures where compression of the nerve can occur. The most significant site is the superior arcade of the superficial layer of the supinator muscle (The Arcade of Frohse). Radial tunnel syndrome is characterized by pain located in the region of the radial tunnel - approximately 2.5cm distal to the elbow crease. Pain may be brought on by direct pressure or supination of the forearm and may also be exacerbated by resisted extension of the index finger. Radial tunnel syndrome should also be suspected where treatment of lateral epicondylitis has failed. Assessment of the nerve should focus on identification of anatomy. The PIN should be assessed for any contour deformation, mass effect or swelling. The nerve should follow a smooth path through the radial tunnel. While a dramatic change in nerve diameter is diagnostic for nerve compression, sharp contour changes should also be viewed with suspicion. Additionally, masses such as ganglia may compress the nerve. The CEO should also be assessed. Assessment of the contralateral nerve and CEO is mandatory. Sonography has established a role in assessment of the lateral elbow for lateral epicondylitis. Radial tunnel syndrome may however mimic this condition clinically. Sonography is an ideal imaging modality to assess the PIN and assessment of this structure should form a routine part of elbow examination for lateral elbow pain. Sonographers should also conduct careful clinical examination of patients with lateral elbow pain, as pain identified distal to the lateral epicondyle raises the suspicion of radial tunnel syndrome.