(Kiosk 4) The Shark Attack Sign: A Novel Neuromuscular Sonographic Finding to Localize Entrapment Mononeuropathies - A Case Report

Case Diagnosis: Right ulnar nerve mononeuropathy

Case Description or Program Description: A 22-year-old female was referred by Orthopedic Surgery 6 months s/p right ulnar nerve transposition for weakness and numbness of the right forearm and hand. The surgery had provided initial relief, but the patient’s condition had been progressively worsening since then. Physical examination revealed weakness along the right ulnar innervated finger flexors and abductors, diminished sensation in the volar and palmar ulnar aspect of the hand and a positive Tinel’s sign over the ulnar elbow.

Setting: Outpatient General Physical Medicine & Rehabilitation Clinic

Assessment/Results: Nerve Conduction Studies of the right ulnar nerve showed a reduced amplitude SNAP and CMAP, as well as reduced motor conduction velocity across the elbow and a prolonged distal motor latency. Neuromuscular ultrasound (NMUS) of the right ulnar nerve revealed borderline cross-sectional area and increased hypoechogenicity at the medial epicondyle. On dynamic scanning, a hypoechoic triangular tissue plane encroached from the left to the right of the screen, resembling a “shark attack” deep to the nerve, corresponding to the area where the nerve lost its fascicular architecture.

Discussion (relevance): Electrodiagnostic (EDX) studies have for decades been the mainstay diagnostic tools to diagnose entrapment mononeuropathies. They help clinicians localize the disorders but don’t always elucidate the etiology of the entrapment. Recently, a growing body of evidence has shown NMUS to be effective in augmenting EDX studies by providing anatomical details and more precisely identifying the etiology.

Conclusions: The “shark attack sign” may represent a novel sonographic finding where a new muscle or myofascial plane intersects with a nerve at a site of entrapment. This may allow clinicians to localize entrapment neuropathies more precisely, especially in challenging cases where electrodiagnostic findings are ambiguous. This facilitates collaboration with surgeons to achieve the best outcomes for these patients. Further case series with surgical findings are needed to confirm its clinical utility.