Palatal Prolapse[1], also called Expiratory Palatal Obstruction (EPO)[2], Expiratory Regrograde Palatal Prolapse[8], Retropalatal airway closure[5] or Expiratory Collapse[22], is a common[20]  sleep disorder where the soft palate swings (prolapses) towards the nose during expiration. In this position the soft palate limits or even blocks the expiratory air flow. The palate functions as an one-way valve[4][5] and after the expiratory phase ends, gravity and inspiratory flow usually return the palate to a downward-hanged position. Similar phenomenon was observed during Bag-Mask Ventilation[6] and in sedated patients[7] as early as 1953[13]. Palatal prolapse during sleep may cause expiratory sleep apnea if the soft palete adheres to the velopharynx, predicting inspiratory palatal collapse[15]. Palatal prolapse is more common in patients with obstructive sleep-disordered breathing[5]. Rarely recognized and treated, palatal prolapse is not responsive to standard sleep apnea treatments. Other devices or palate surgery[3][8] but not oral appliance therapy[14] may be helpful for some patients. Myasthenia gravis patients may exhibit palatal prolapse even while awake [23].