Stiff-person syndrome is a rare progressive neurological disorder characterized by constant painful contractions and spasms of voluntary muscles, particularly the muscles of the back and upper legs. Symptoms may occur gradually, spreading from the back and legs to involve the arms and neck. Symptoms may worsen when the affected individual is anxious or exposed to sudden motion or noise. Affected muscles may become twisted and contracted, resulting in bone fractures in the most severe cases. Individuals with stiff-person syndrome may have difficulty making sudden movements and may have a stiff-legged, unsteady gait. Sleep usually suppresses frequency of contractions. Stiffness may increase and patients may develop a hunched posture (kyphosis) or a swayback (lordosis). Researchers theorize that stiff-person syndrome may be an autoimmune disorder. Other autoimmune disorders such as diabetes, pernicious anemia (a chronic, progressive blood disorder), and thyroiditis (inflammation of the thyroid gland) may occur more frequently in patients with stiff-person syndrome.