Progressive Muscle Rigidity and Spasms in an Older Adult: A Diagnostic Challenge.

Stiff-person syndrome (SPS) is a rare autoimmune neurologic disorder characterized by progressive rigidity, painful spasms, and heightened sensitivity to stimuli. We present the case of a 73-year-old woman initially diagnosed with immune-mediated myopathy attributed to statin use. Although her symptoms temporarily improved with corticosteroid therapy, she later developed severe rigidity and spasms triggered by touch and emotional stress, with preserved sensorium, findings that were inconsistent with typical myositis. Further evaluation revealed continuous motor unit activity on electromyography and the presence of high-titer anti-glutamic acid decarboxylase 65 (anti-GAD65) antibodies, supporting a diagnosis of SPS. This case highlights the importance of considering autoimmune neurologic disorders in patients with unexplained muscle stiffness, particularly when symptoms are stimulus-induced and unresponsive to standard therapies.