Multifocal Motor Neuropathy; Right greater than left upper extremity paresthesia/ Tingling in fingertips/ sensations of pins and needles in his hands; Bilateral lower extremity weakness (right greater than left); Numbness; Sensation of blurred vision in the left eye towards with some drooping; Sensation of blurred vision in the left eye towards with some drooping; Decreased overall muscle bulk; Absent tendon reflexes bilaterally; Fasciculations noted more prominent over the right lower extremity; Diminished vibration sense over toes bilaterally; Demyelinating motor neuropathy; Conduction block; Unable to ambulate independently; motor impairment; This case was reported in a literature article and described the occurrence of multifocal motor neuropathy in a 66-year-old male patient who received Flu unspecified (Influenza vaccine) for prophylaxis.   The patient's past medical history included tobacco user.   On an unknown date, the patient received Influenza vaccine. On an unknown date, less than 2 weeks after receiving Influenza vaccine, the patient experienced multifocal motor neuropathy (serious criteria hospitalization and GSK medically significant), paresthesia (serious criteria hospitalization), lower extremities weakness of (serious criteria hospitalization), numbness (serious criteria hospitalization), ptosis (serious criteria hospitalization), blurred vision (serious criteria hospitalization), muscle atrophy (serious criteria hospitalization), tendon reflex absent (serious criteria hospitalization), fasciculation (serious criteria hospitalization), decreased vibratory sense (serious criteria hospitalization), demyelinating polyneuropathy (serious criteria hospitalization and GSK medically significant), conduction disorder (serious criteria hospitalization), unable to walk (serious criteria hospitalization) and motor dysfunction (serious criteria hospitalization). The patient was treated with immunoglobulins nos (Intravenous Immunoglobulin), non-drug therapy (Physical Therapy) and operations and procedures (Plasmapheresis (Nos)). On an unknown date, the outcome of the multifocal motor neuropathy, paresthesia, lower extremities weakness of, numbness, ptosis, blurred vision, muscle atrophy, tendon reflex absent, fasciculation, decreased vibratory sense, demyelinating polyneuropathy, conduction disorder and motor dysfunction were recovering/resolving and the outcome of the unable to walk was recovered/resolved.   The reporter considered the multifocal motor neuropathy, paresthesia, lower extremities weakness of, numbness, ptosis, blurred vision, muscle atrophy, tendon reflex absent, fasciculation, decreased vibratory sense, demyelinating polyneuropathy, conduction disorder, unable to walk and motor dysfunction to be related to Influenza vaccine.     Additional Information: GSK Receipt Date: 13-APR-2022 Reporter's Comments: The patient had a past medical history of tobacco use. The patient presented with right greater than left upper extremity paresthesia as well as bilateral lower extremity weakness (right greater than left), numbness, and tingling in his fingertips. He also reported a sensation of blurred vision in the left eye towards the evening on the day of presentation with some drooping.  The patient had received a flu shot about 10 days prior to the presentation. He denied any urinary symptoms, dysphagia, shortness of breath, fever, chills, chest pain, or facial droop. Initial vitals recorded on presentation were stable. Physical exam was remarkable for decreased overall muscle bulk, motor exam showed intact strength 5 by 5 bilateral upper and lower extremities. Upper extremity reflex testing including biceps, triceps, and brachioradialis were 1 by 4 bilaterally; patellar and Achilles' tendon reflexes were absent bilaterally with fasciculations noted more prominent over the right lower extremity. Sensory exam was remarkable for diminished vibration sense over toes bilaterally and was intact at the knees and fingertips bilaterally. Light touch was preserved throughout the lower extremities. Lab workup including complete blood count (CBC), comprehensive metabolic panel (CMP), Lyme titers, creatine kinase (CK), aldolase, vitamin D, and B12 levels were within normal range. Myasthenia gravis antibody panel was negative. MRI head without contrast showed no acute intracranial abnormality. As the patient was afebrile and CBC was normal and no evidence of mental status changes cerebrospinal fluid (CSF) analysis was not done.  EMG was interpreted as abnormal due to prolonged latency of right median compound muscle action potential (CMAP) and right peroneal CMAP, low conduction velocity (C.V.) of right ulnar CMAP, and low C.V. right peroneal CMAP. Sensory nerve action potential (SNAP) was largely normal. It was concluded that there was electrophysiologic evidence of a demyelinating motor neuropathy with evidence of conduction block, raising the possibility of atypical Guillain-Barre syndrome (GBS) or other acute inflammatory demyelinating polyneuropathy (AIDP) without evidence of sensory involvement. However, due to his motor involvement as evidenced by EMG studies that were more prominent in the upper than lower extremities, there was also a concern for multifocal motor neuropathy (MMN). He was tested for the anti-GM1 antibody and his anti-GM1 IgM was positive with greater than100% (reference range 0-30) and negative for the anti-GM1 IgG antibody. He was treated with intravenous immune globulin (IVIG) at 0.4 g/kg daily for five days. He completed the course of IVIG with marked improvement of symptoms. He was discharged to acute inpatient rehabilitation with Physical Therapy (PT) and Occupational Therapy (OT) and was also recommended to follow up as an outpatient. He was scheduled to receive 0.5 g/kg IVIG every 28 days following discharge. About a month later, prior to his scheduled IVIG infusion, the patient's clinical condition deteriorated at acute inpatient rehabilitation necessitating him to be readmitted to the hospital. During that hospitalization, he was treated with plasmapheresis (five rounds of 3000 mL exchanges with 5% albumin) and subsequently discharged to acute inpatient rehabilitation. It was noted then that he was unable to ambulate independently following the second hospitalization. However, he was continued on monthly plasmapheresis (one round of 3000 mL exchanges with 5% albumin every 28 days) and participated in aggressive acute inpatient physical and occupational therapy rehabilitation. Two months later, he was able to ambulate without assisted devices. Three months later, he was able to ambulate normally with only sensations of pins and needles in his hands, but with a normal neurologic examination. He has been continued on monthly plasmapheresis (one round of 3000 mL exchanges with 5% albumin every 28 days) and his disease has remained clinically quiescent for over a year, from the time of initial presentation. MMN is a rare and disabling disease and several studies suggest strong immune-mediated pathogenesis mainly because of the presence of IgM anti-GM1 serum antibodies and the positive response noted to treatment with IVIG. IVIG remains the treatment of choice for MMN and is recommended as first-line therapy based on multiple studies and randomized trials. IVIG is commonly used in the treatment of immune-mediated disorders. While patients do remain stable with maintenance IVIG treatment, most patients will slowly deteriorate over many years. The present case was rare, as it was found that the patient developed MMN with high anti-GM1 IgM antibody levels a few days after influenza vaccination. The patient also had motor impairment as evidenced by EMG studies. He was treated with IVIG with an improvement of his symptoms initially; however, later deteriorated and required plasmapheresis which was continued for maintenance therapy. Although the literature review showed IVIG as a mainstay of treatment for MMN, this patient seemed to have a positive response while on plasmapheresis, after failing IVIG and has put his disease currently into remission. Conclusion: Post-influenza vaccination MMN is a rare entity, which should be a differential in patients presenting with rapidly progressive neurological symptoms and features of GBS as rapid diagnosis and appropriate treatment can have better clinical outcomes. A thorough neurological examination and testing with EMG could aid in prompt diagnosis and management with anti-GM1 antibodies where appropriate. Additional Supportive Information: This article is not available for regulatory reporting due to copyright restriction