Infantile onset has been described, but is rare. Initial weakness is seen in facial muscles, starting in orbicularis oculi, orbicularis oris, and zygomaticus. Patients may have difficulty with labial sounds, whistling, or drinking through the straw. Weakness may be asymmetric. Extraocular and pharyngeal muscles are spared.
Scapular fixation is weak from the onset.
Winging of the scapula is the most characteristic sign. The scapula is more laterally placed than normal. It moves upwards in shoulder abduction. The deltoid muscle is usually spared and shoulder abduction weakness is predominantly due to weak scapula fixation. If the scapula is manually stabilized against the chest wall, patient may experience improved movement.