This medically confirmed literature report (initial receipt 13-Oct-2015) concerns a 26 year old male patient. The patient has no relevant past medical history. On an unknown date, the patient received an influenza virus vaccine (brand and batch number unspecified) to the left arm. The patient did not experience any paresthesias or other symptoms at the time of injection. Sixteen hours post influenza vaccination, the patient developed progressively worsening weakness over his distal left upper extremity.  After 20 hours, the patient experienced total weakness in the affected muscles.  The patient had no past medical history and the review of the systems were unremarkable. Physical examination revealed a small puncture wound in the posterolateral portion of his mid-arm approximately 2.5cm distal to the clinically identifiable deltoid tuberosity.  A motor exam demonstrated 4/5 strength in his triceps, 2/5 strength in his brachioradialis and 0/5 strength in wrist extension, finger extension and thumb extension.  No sensory abnormalities were noted. The remainder of the neurovascular exam was within normal limits. The day after influenza vaccination, the patient presented to an outpatient hand surgery clinic complaining of left wrist drop.  An magnetic resonance (MR) imaging of the left upper arm on the day after the injection revealed edema along the course of the radial nerve without focal collection. The nerve itself was not grossly enlarged or damaged. Ultrasound (US) examination of the patient's arm on the day after the injection demonstrated swelling of the radial nerve in the distal third of the arm starting approximately 3 cm distal to the injection site. At the center of the swollen segment, an adhesive band was identified as compressing the nerve, likely an incidental finding since the band was not near the site of injection and the nerve was found to be swollen proximally and distally to the band. Given the results of the MR and the US, the impression was that the nerve palsy was secondary to a demyelinating process related to inflammation around the nerve and not to axonal damage. A splint and a steroid taper were prescribed. Surgery in the form of neurolysis was offered but the patient elected non-operative management. One month following the injury, nerve conduction studies and electromyography (EMG) demonstrated severe left radial neuropathy with total motor axonal loss with sparing of the radial sensory nerve.  A follow-up ultrasound obtained at the same time revealed progression of the swelling of the radial nerve in both size and craniocaudal extent, with involvement of the posterior interosseous nerve and signs of supinator atrophy.  This swelling and atrophy were absent on the ultrasound taken the day after injury. Approximately six months post initial injury, the patient reported progressive improvement in motor symptoms beginning with the brachioradialis and progressive inferiorly to his thumb extensor over time. At one year post injury, physical examination revealed 5/5 strength in his triceps, 5/5 strength in his brachioradialis and 4/5 strength in wrist extension, finger extension and thumb extension.  The patient reported minimal functional limitations and was able to resume work in full. The outcome was recovered. Reporter's Comments: Reporter's comments: Imaging suggests that the nerve injury was caused by injection of the vaccination into the nerve sheath and not the nerve itself.  It is hypothesised that the injectate dissected down the tissue planes along the radial nerve causing a chemical neuritis, leading to axonal ischemia and axon loss.  The progressive development of symptoms over time supports the notion that the palsy developed secondary to an inflammatory process.  Alternatively, it is also possible that the total axonal loss was due to ischemia from a chemical neuritis induced by the injectate.  The authors believe the reason for the palsy was inappropriate location of the injection. Sender's Comments: Company comments: Seriousness: Serious. Medically significant. Listedness: Unlisted. Vaccine brand unspecified. Assessment according to the CCSI for CSL Inactivated Influenza Virus Vaccine and the Afluria Product Information all events are unlisted. Causality: Probable. The progression of events had a reasonable time to onset 16 hours post vaccination. The events occurred on the limb that received vaccination with no other history of trauma, concomitantly administered vaccines, or ongoing infectious process. Improper technique in vaccination could possibly lead to injection of the vaccine into the nerve sheath that triggers local inflammatory processes. The compartmentalization into the same nerve sheath may explain the isolated damage to the same nerve with neuromuscular dysfunction observed distally to the injection site. The inflammation could lead to possible nerve damage resulting in the clinical signs and symptoms exhibited by the patient. The physical findings of muscular weakness was supported by diagnostic studies (ultrasound, EMG, nerve conduction, and imaging studies) indicating injury to the radial nerve secondary to an inflammatory process. For temporal relationship, plausible mechanism of nerve injury, and supporting evidence, the causality is assessed as probable.