RESUMO
Stiff-person syndrome is characterized by persistent muscle spasms, involving agonist and antagonist muscles simultaneously, starting in the lower limbs and trunk. It tends to occur in the fourth to sixth decade of life, presenting with intermittent spasms that later become continuous and usually painful. Minor sensory stimuli, such as noise or light touch, precipitate severe spasms. Spasms do not occur during sleep and only rarely involve cranial muscles. We present a case that for two years was diagnosed and treated as a conversion disorder associated with depression. After two years she was admitted to another hospital with an unmistakable picture of stiff-person syndrome with hypertrophy and rigidity of lower limb muscles, compatible electrophysiology and positive anti-GAD antibodies. She had autoimmune hypothyroidism, that should have raised the suspicion of stiff-person syndrome earlier. She responded to intravenous immunoglobulin and mycophenolate mofetil and and to tranquilizers that have muscle relaxant properties.