Case report: Atypical anti-GlyR antibody-positive pediatric progressive encephalomyelitis with rigidity and myoclonus presenting as chronic joint pain.

Progressive encephalomyelitis with rigidity and myoclonus (PERM) represents the most severe subtype of stiff-person spectrum disorders and is exceedingly rare in children. We report a 14-year-old female patient who presented with chronic joint pain and axial rigidity as the initial manifestations, with a disease course of two years, during which she had been diagnosed with juvenile idiopathic arthritis but lacked typical peripheral arthritis signs and showed poor response to therapy. The patient exhibited paroxysmal axial rigidity, autonomic dysfunction, and sleep disturbances. Although typical brainstem signs were absent, serum anti-glycine receptor (GlyR) antibodies were positive at a titer of 1:100, and brain magnetic resonance imaging revealed a hyperintense lesion on T2-weighted imaging in the right brainstem, leading to a clinical diagnosis of anti-GlyR antibody-positive PERM. Following treatment with intravenous immunoglobulin (IVIG) combined with methylprednisolone, her symptoms resolved, and she remained relapse-free during one year of follow-up. This case highlights that PERM can present with chronic joint pain and axial rigidity as prominent initial manifestations in children. For patients with unexplained axial rigidity accompanied by autonomic symptoms, even in the absence of typical brainstem signs, early serum anti-GlyR antibody screening should be considered. Delayed diagnosis does not necessarily predict poor prognosis, and prompt immunomodulatory therapy can still achieve favorable long-term outcomes.