Incidence of Neuroleptic Malignant Syndrome During Antipsychotic Treatment in Children and Youth: A National Cohort Study

<b><i>Objective:</i></b> The incidence of neuroleptic malignant syndrome (NMS), a rare, potentially fatal adverse effect of antipsychotics, among children and youth is unknown. This cohort study estimated NMS incidence in antipsychotic users age 5-24 years and described its variation according to patient and antipsychotic characteristics. <b><i>Methods:</i></b> We used national Medicaid data (2004-2013) to identify patients beginning antipsychotic treatment and calculated the incidence of NMS during antipsychotic current use. Adjusted hazard ratios (HRs) assessed the independent contribution of patient and antipsychotic characteristics to NMS risk. <b><i>Results:</i></b> The 1,032,084 patients had 131 NMS cases during 1,472,558 person-years of antipsychotic current use, or 8.9 per 100,000 person-years. The following five factors independently predicted increased incidence: age 18-24 years (HR [95% CI] = 2.45 [1.65-3.63]), schizophrenia spectrum and other psychotic disorders (HR = 5.86 [3.16-10.88]), neurodevelopmental disorders (HR = 7.11 [4.02-12.56]), antipsychotic dose >200mg chlorpromazine-equivalents (HR = 1.71 [1.15-2.54]), and first-generation antipsychotics (HR = 4.32 [2.74-6.82]). NMS incidence per 100,000 person-years increased from 1.8 (1.1-3.0) for those with none of these factors to 198.1 (132.8-295.6) for those with 4 or 5 factors. Findings were essentially unchanged in sensitivity analyses that restricted the study data to second-generation antipsychotics, children age 5-17 years, and the 5 most recent calendar years. <b><i>Conclusion:</i></b> In children and youth treated with antipsychotics, five factors independently identified patients with increased NMS incidence: age 18-24 years, schizophrenia spectrum and other psychotic disorders, neurodevelopmental disorders, first-generation drugs, and antipsychotic doses greater than 200 mg chlorpromazine-equivalents. Patients with 4 or 5 of these factors had more than 100 times the incidence of those with none. These findings could improve early identification of children and youth with elevated NMS risk, potentially leading to earlier detection and improved outcomes.