A 48-year-old man with schizophrenia has been stable on haloperidol 10 mg daily for 6 years. Over the past 48 hours, he develops high fever (40.2°C), severe muscle rigidity, altered mental status, and profuse diaphoresis. Laboratory findings: CK 3800 U/L, WBC 14,500/μL, urine myoglobin positive. He was last given haloperidol 12 hours ago. Which of the following best distinguishes this presentation from serotonin syndrome?