Case Report: Efgartigimod in refractory non-tumor Lambert-Eaton myasthenic syndrome.

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder, associated with neoplastic disease in 50-60% of cases. In severe cases, currently available immunomodulatory therapies frequently fail to achieve adequate control. We report a 20-year-old woman with non-tumor LEMS refractory to 3,4-diaminopyridine, sequential immunosuppression with prednisone, azathioprine, mycophenolate mofetil and rituximab and plasma exchange, who remained severely disabled after two years of progressive disease. Off-label efgartigimod (1000mg subcutaneously weekly for four weeks, with the interval between cycles initially adjusted according to symptom recurrence), produced rapid and sustained clinical improvement, including recovery from a respiratory decompensation episode. The Myasthenia Gravis Activities of Daily Living (MG-ADL) score decreased from a baseline of 5-9 to a minimum of 2, and the Quantitative Myasthenia Gravis (QMG) score improved from 13-18 to 8-11. Symptoms consistently recurred toward the end of the third week of each treatment-free interval, prompting a fixed two-week interval, which stabilized clinical response and reduced fluctuation severity. This is, to our knowledge, the first report of sustained clinical benefit from FcRn inhibition in non-paraneoplastic, long-standing refractory LEMS. As in myasthenia gravis, LEMS is mediated by pathogenic circulating IgG antibodies, and these findings support FcRn inhibition as a rational treatment strategy in refractory LEMS regardless of tumor association.