Obstructive apneas, hypopneas and snoring may be caused by blocked nasal passages and/or excessive tissue in the airway, such as tonsils or adenoids. Surgery can help clear these passages and restore night-time breathing. Surgery may completely treat the breathing problems or make nasal CPAP/BiLevel more tolerable. Obstructive events may also be caused by relaxation of the muscles that normally hold the airway open. In this case, nasal CPAP/BiLevel may be needed even after surgery to splint the airway open. Once the airway is cleared, nasal CPAP/BiLevel may work where it did not before.