Stiff-person syndrome mimicking ankylosing spondylitis

Abstract

Stiff-person syndrome (SPS) is a rare autoimmune neurological disorder characterized by progressive muscle rigidity and episodic spasms, predominantly affecting the axial and proximal limb muscles. Its atypical presentation can lead to diagnostic delays and misdiagnosis, particularly as musculoskeletal disorders such as ankylosing spondylitis (AS) in patients presenting with persistent back stiffness and reduced spinal mobility. The present study reports the case of a 35-year-old male with no significant comorbidities who presented with a 4-month history of backache and progressive stiffness involving the axial skeleton and upper limbs. He was initially evaluated for AS; however, normal inflammatory markers, negative HLA-B27 status, and normal spinal magnetic resonance imaging findings raised diagnostic uncertainty. Electromyography demonstrated continuous motor activity in the paraspinal muscles, while markedly elevated anti-glutamic acid decarboxylase (GAD) antibody titers (1716.05 IU/mL) supported the diagnosis of SPS. Treatment with benzodiazepines, baclofen, and immunomodulatory therapy resulted in substantial clinical improvement. This case highlights the importance of considering SPS in patients with persistent axial stiffness despite normal inflammatory markers and imaging findings. Early recognition and appropriate GABAergic and immunomodulatory therapy may prevent unnecessary investigations and interventions and improve long-term outcomes. Greater awareness of SPS among rheumatologists and neurologists is essential for timely diagnosis and management.