A 72-year-old woman is evaluated for a 6-month history of gradually worsening, nonsuppressible involuntary movements. The patient first noticed twitching movements of the lower part of the face, with occasional unintended thrusting of the tongue through the lips and biting of the cheeks inside the mouth. She further reports pressure and a pulling sensation at the back of the neck that causes her head to pull backward suddenly and the occasional tendency to drop objects from her hands secondary to uncontrollable jerking movements. The patient has chronic gastroparesis that is treated with metoclopramide and bipolar disorder that is well controlled with lamotrigine and quetiapine. She has no family history of a movement disorder or any neurologic disease.
On physical examination, temperature is 37.5 °C (99.5 °F), blood pressure is 145/76 mm Hg, and pulse rate is 80/min. Frequent stereotyped pursing movements of the lips, occasional tongue protrusion and forceful jaw closure, continual slow and nonrhythmic movements of the fingers, and recurrent cervical retrocollis are noted. Occasional rapid jerking movements of the arms and infrequent facial grimacing also are present. Gait is slow and marked by short steps and reduced arm swing.
Which of the following is the most appropriate next step in management?