A 52-year-old woman underwent a C5/6 ACDF for cervical radiculopathy through a left-sided approach two years ago. Radiographs are shown in Figure A. She has had an altered voice since this operation. Recently, the patient has developed myelopathic symptoms including gait instability and dexterity problems with her hands. An MRI shows a fusion at C5/6, and an adjacent-level midline disc herniation at C4/5 with cord compression and myelomalacia. Laryngoscopy of the vocal cords demonstrates abnormal function of the vocal cords on the left hand side. Which of the following is the most appropriate treatment for this patient?