A 38-year-old male with a 12-year history of schizoaffective disorder has been maintained on haloperidol decanoate for the past 6 years with good symptom control. He presents to clinic with a sustained involuntary leftward rotation and lateral flexion of his neck (torticollis) that developed gradually over the past 4 months. The abnormal posturing is present throughout the day, worsens with stress and fatigue, and partially improves with a sensory trick of touching the right side of his jaw. There has been no recent change in his medication regimen, and he denies any new medications or substance use. On examination, the sustained neck deviation is accompanied by mild hypertrophy of the left sternocleidomastoid muscle. Which of the following best characterizes this movement disorder?