Anti-IgLON5 disease presenting with myokymia and favourable response to FcRN-antagonist combined with oral glucocorticoids: case report.

Anti-IgLON5 disease is a rare autoimmune neurological disorder characterized by anti- neuronal surface autoantibodies targeting the Immunoglobulin-Like Cell Adhesion Molecule 5 (IgLON5) protein. We report a 53-year-old male patient diagnosed with anti-IgLON5 disease who presented with the uncommon manifestation of generalized multi-site myokymia. Laboratory tests showed positive anti-IgLON5 IgG antibodies in both serum (titer 1:100) and cerebrospinal fluid (CSF) (titer 1:1), with no specific abnormalities detected on brain magnetic resonance imaging (MRI) and whole-exome sequencing. Considering patients' refusal to use high-dose glucocorticoids due to concerns about drug side effects, as well as the uncertainty surrounding the efficacy of intravenous immunoglobulin in treating IgLON5 disease, we selected a combined immunosuppressive therapy regimen consisting of intravenous neonatal fragment crystallizable receptor (FcRn) antagonist (efgartigimod) and sequential oral prednisone. The patient's myokymia were rapidly alleviated after combined treatment. During an 18-month follow-up, symptoms completely abated within the first 6 months, and antibody titers declined gradually to negative. Mild symptom recurred at 7 months post-initial treatment. Serum anti-IgLON5 IgG antibodies became positive (titer 1:10) at 14 months, however, these antibodies had been negative at 3 months after the start of treatment. Symptomatology was well-controlled after adjusting the oral prednisone dose. This case expands the clinical spectrum of anti-IgLON5 disease, highlights the necessity of screening for neuronal autoantibodies in patients with myokymia, and suggests that the combination of FcRn antagonists and glucocorticoids may emerge as a new immunotherapeutic option for this condition.