[MR neurography reveals fascicular constriction of the median nerve in a patient with neuralgic amyotrophy].

Diagnosing neuralgic amyotrophy can be challenging in clinical practice. Here, we report the case of a 37-years old Japanese woman who suddenly developed neuropathic pain in the right upper limb after influenza vaccination. The pain, especially at night, was severe and unrelenting, which disturbed her sleep. However, X-ray and MRI did not reveal any fractures or muscle injuries, and brain MRI did not reveal any abnormalities. During neurological consultation, she was in a posture of flexion at the elbow and adduction at the shoulder. Manual muscle testing suggested weakness of the flexor pollicis longus, pronator quadratus, flexor carpi radialis (FCR), and pronator teres (PT), while the flexor digitorum profundus was intact. Medical history and neurological examination suggested neuralgic amyotrophy, particularly anterior interosseous nerve syndrome (AINS) with PT/FCR involvement. Innervation patterns on muscle MRI were compatible with the clinical findings. Conservative treatment with pain medication and oral corticosteroids relieved the pain to minimum discomfort, whereas weakness remained for approximately 3 months. For surgical exploration, lesions above the elbow and fascicles of the median nerve before branching to the PT/FCR were indicated on neurological examinations; thus, we performed high-resolution imaging to detect possible pathognomonic fascicular constrictions. While fascicular constrictions were not evident on ultrasonography, MR neurography indicated fascicular constriction proximal to the elbow joint line, of which the medial topographical regions of the median nerve were abnormally enlarged and showed marked hyperintensity on short-tau inversion recovery. In patients with AINS, when spontaneous regeneration cannot be expected, timely surgical exploration should be considered for a good outcome. In our case, MR neurography was a useful modality for assessing fascicular constrictions when the imaging protocols were appropriately optimized based on clinical assessment.