Patient With Rigidity and Fasciculations: Steroid-Responsive Presentation of Anti-IgLON5 Disease.

An 82-year-old man presented with 2-year lasting widespread muscular fasciculations, cramps, and limb stiffness, with spontaneous movements in the right lower limb, unsteady gait (Video 1), and falls. Neurophysiologic studies disclosed signs of neuromuscular hyperexcitability. CSF analysis showed high tau protein concentration (543 pg/mL; reference values, <404) and unique-to-CSF oligoclonal bands. Serum and CSF anti-IgLON5 antibodies were positive (Figure 1). He carried the anti-IgLON5 disease-associated HLA-DRB1*10:01 allele. Brain MRI, thoracoabdominal CT, whole-body FDG-PET, and video-polysomnography were unremarkable. No sleep disturbances, bulbar symptoms, parkinsonism, or dementia were detected. Intravenous methylprednisolone (500 mg/d for 5 days), followed by oral benzodiazepines, prompted rapid functional recovery, with limb stiffness and gait improvement (Video 1), which persisted at 6-month follow-up. Anti-IgLON5 disease has progressive course and protean clinical presentations, representative, in our patient, for overlapping signs and symptoms of neuromuscular hyperexcitability and rigidity. Identification of rare phenotypes is important because prompt recognition and treatment can improve prognosis.