By Michael Rubin, MD

Professor of Clinical Neurology, Weill Cornell Medical College

Dr. Rubin reports no financial relationships relevant to this field of study.

SYNOPSIS: High-resolution MRI of peripheral nerves can help make an accurate diagnosis of the Parsonage-Turner syndrome by revealing a bull’s-eye sign in cross-sectional images of the nerve.

SOURCE: Sneag DB, Saltzman EB, Meister DW, et al. The MRI bullseye sign: An indicator of peripheral nerve constriction in Parsonage-Turner syndrome. Muscle Nerve DOI: 10.1002/mus.25480. [Epub ahead of print].

Localization of the lesion in neuralgic amyotrophy, also known as Parsonage-Turner syndrome (PTS), can be challenging. Differentiating upper trunk plexopathy from C5-6 radiculopathy, or lower trunk plexopathy from C8-T1 radiculopathy, and particularly identifying mononeuropathy as due to PTS, is difficult electrodiagnostically when sensory nerve conduction studies or needle electromyography are normal. In addition, cervical magnetic resonance imaging (MRI) may cloud the diagnosis by demonstrating multilevel foraminal stenosis. Torsion, or hourglass constrictions (HGCs), of a peripheral nerve have been found during surgery in PTS patients with mononeuropathy, but have been rarely identified by MRI. Recent technological advances, including software and surface coil developments, have enabled high-resolution peripheral nerve MRI to detect focal fascicular abnormalities of nerves both at the plexus and extra-plexus levels. By identifying hourglass constrictions in PTS, MRI may be rendered more sensitive in diagnosing this disorder, and differentiating it from radiculopathy or mononeuropathy from other causes.

Six patients (four men and two women), mean age 43.3 years, with PTS based on history, clinical examination, and electrodiagnostic findings underwent 3.0 Tesla MRI of the brachial plexus, arm, elbow, and/or forearm, as indicated by the clinical findings, followed by surgical exploration. MRI studies were interpreted prior to surgery by a single radiologist specialized in peripheral nerve MRI, and nerve constriction was defined as a > 75% focal caliber change of the nerve trunk and/or individual nerve fascicle. All patients were found to have HGCs of affected nerves, and thus surgery focused on the site of identified constriction, with all patients undergoing neurolysis, and in one patient, nerve transfer. Surgery was performed a mean 12.4 months after symptom onset, and in all cases, only after electrodiagnostic studies confirmed absent or minimal recovery in these patients, affecting the suprascapular, radial, and axillary nerves, and the anterior interosseous nerve and pronator teres fascicles of the median nerve trunk.

Among 11 affected nerves, 23 constriction sites were identified. Within each nerve or fascicle, an average of 2.3 sites of constriction were seen, and all but two were associated with a bull’s-eye sign within 2 cm proximal to the narrowing, manifested as a central hypointensity, encircled by peripheral signal hyperintensity, on intermediate-weighted fast spin echo or fat-suppressed imaging, or both, orthogonal to the nerve’s longitudinal axis. Surgery confirmed the site of nerve constriction, but no definitive extrinsic cause was found to explain the narrowing. High-resolution MRI may play a more pivotal role in PTS diagnosis by identifying HGCs of affected nerves.

COMMENTARY

With an incidence as high as 1/1000, affecting men twice as often as women of all ages, and recurring in at least 25% of idiopathic cases, the pathophysiology of neuralgic amyotrophy, or PTS, remains unknown, encompassing infectious or immune triggers (10% may have concomitant hepatitis E virus infection in the acute phase of PTS), mechanical causes (10% are reportedly preceded by excessive arm exercise), and genetic factors. When presenting with the typical history of extreme shoulder or arm pain followed hours to days later by muscle weakness and appropriate focal clinical findings, diagnosis may be made in the office. Often, neurologists rely on electrodiagnostic studies (NCS/EMG) to confirm the diagnosis but low sensitivity and sampling error diminish the diagnostic value. MRI demonstration of the bull’s-eye sign appears to accurately localize hourglass constrictions and, although the cause is unknown, is supportive of a diagnosis of PTS.