This disorder is extremely variable in the extent and severity of the symptoms and in the age when symptoms appear. Symptoms often do not appear until ages 10 to 26, but it is not uncommon for symptoms to appear much later. In some cases, symptoms never develop.
Symptoms are usually mild and very slowly progressive.
Facial muscle weakness is common. Shoulder muscle weakness causes deformities such as scapular winging and sloping shoulders. There is difficulty raising the arms because of shoulder and arm muscle weakness. Weakness of the lower legs is possible as the disorder progresses and can be severe enough to interfere with walking. Other body systems are usually not affected, and intellectual function is normal.