This case was reported in a literature article and described the occurrence of radial nerve palsy in a 26-year-old male patient who received Influenza vaccine. On an unknown date, the patient received Influenza vaccine (intramuscular). On an unknown date, 16 hrs after receiving Influenza vaccine, the patient experienced radial nerve palsy, weakness of arms, strength loss of, demyelination (serious criteria GSK medically significant), neuritis, nerve compression and vaccine administered at inappropriate site. The patient was treated with Steroid (Unspecified). On an unknown date, the outcome of the radial nerve palsy, weakness of arms, demyelination, neuritis and nerve compression were resolved with sequelae and the outcome of the strength loss of was recovered/resolved and the outcome of the vaccine administered at inappropriate site was unknown. The reporter considered the radial nerve palsy, weakness of arms, strength loss of, demyelination, neuritis and nerve compression to be related to Influenza vaccine. Additional information received: This case was reported in a literature article and it described the occurrence of radial nerve palsy in a 26-year-old male patient who had received an unspecified influenza vaccine (manufacturer unknown). The patient had no previous history according to the authors and was working as a doctor at the time of the event. No further information on the patient's concurrent medical conditions, medical and family history or concomitant medication was provided by the authors. On an unspecified date, the patient an unspecified influenza vaccine (dosage unknown; intramuscular, site: posterolateral portion of his left midarm, approximately 2.5 cm distal to the clinically identifiable deltoid tuberosity; batch number not provided). On an unspecified date, 16 hours after the administration of the vaccine, the patient began to develop progressively worsening weakness over his distal left upper extremity. After 20 hours, the patient had total weakness in the affected muscles. He presented to an outpatient hand surgery clinic the day after the vaccination complaining of left wrist drop. Upon medical examination he was found to have a small puncture wound in the posterolateral portion of his midarm, approximately 2.5 cm distal to the clinically identifiable deltoid tuberosity. His motor examination revealed a 4/5 strength in his triceps, a 2/5 strength in his brachioradialis and 0/5 strength in wrist extension, finger extension and thumb extension. No sensory abnormalities were noted. The rest of the neurovascular examination was within normal limits according to the authors. Based on the results of the magnetic resonance imaging and the ultrasound scan, the nerve palsy was classified as secondary to a demyelinating process related to inflammation around the nerve and not to axonal damage. Magnetic resonance imaging of the left upper arm performed on the day after the injection, revealed oedema along the course of the radial nerve without focal collection. The nerve itself was not grossly enlarged or damaged. Ultrasound 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. Nerve conduction studies and electromyography study were performed a month after the vaccination to assess the patient's peripheral nerve function. They revealed a 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 evaluation on the day after the injury. Treatment consisted of a splint and an unspecified steroid taper treatment. The patient declined neurolysis. Outcome of the event was recovery with sequelae as the patient reported minimal functional limitations and being able to resume work one year after the vaccination with a 5/5 strength in his triceps, a 5/5 strength in his brachioradialis and 4/5 strength in wrist extension, finger extension and thumb extension. He had reported progressive improvement in his motor function 6 months after the injury that had started with the brachioradialis and progressed inferiorly to his thumb extensor over time. The authors classified the event as secondary to inappropriate placement of the Influenza vaccination. The authors concluded that "we present the first case of an injection injury to the radial nerve caused by inappropriately placed intramuscular Influenza vaccination with advanced imaging. The aetiology of injury-direct trauma to the nerve versus indirect injury via an inflammatory process-remains unclear. Routine vaccination against the Influenza vaccine is warranted for healthcare personnel. With training in the appropriate administration of an intramuscular injection and a basic understanding of anatomy, iatrogenic injury to nearby peripheral nerves can be avoided. With the large number of vaccinations given annually, inadvertent injury may be inevitable."