(238) Influence of relative age on initiation of methylphenidate and speech therapy in children aged 5 to 10 years: a French nationwide cohort study

Background: The age difference between children of the same schooling level can be as much as 12 months, which represents a very significant relative difference, especially in the early years. The relative age effect has been extensively described for school and sports performance, but is rarely reported in the medical field, except for Attention Deficit Hyperactivity Disorder.

Objectives: To quantify the effect of relative age within the same schooling level on methylphenidate (MPH) and speech therapy (ST) initiation.

Methods: A cohort study based on the National Health Data System-SNDS (EPI-MERES mother-child register) included all children born in France between 2010 and 2016, from September of the year in which they turned five. Children with chromosomal abnormalities, congenital malformations, mental or behavioral disorders identified prior to study entry were excluded. The children were followed until July of the year in which they turned 10, or until July 2022 (end of study), or until the occurrence of an outcome, whichever came first. In France, children born in January are the oldest and those born in December the youngest within the same school year. Adjusted Cox models were used to estimate associations between month of birth and initiation of MPH and ST.

Results: A total of 38,794 children began treatment with MPH (incidence rate 2.3/1000 person-years (pa) among 4,769,837 children included) and 692,086 with ST (53.1/1000 pa among 4,188,985 children included). Among children of the same schooling level, the risk of initiating MPH or ST increased steadily with the month of birth from January (reference group) to December (MPH: adjusted hazard ratio 1.07 (1.02-1.14) for February to 1.55 (1.47-1.62) for December; speech therapy: 1.03 (1.02-1.04) for February to 1.64 (1.62-1.66) for December). Trends were similar in subgroup analyses by sex, gestational age, birth rank, socioeconomic level and entry year. Other risk factors associated with MPH and ST initiation were male sex, prematurity, low birth weight for gestational age and in utero exposures to tobacco, alcohol, psychotropic drugs and valproic acid.

Conclusions: Among children with the same schooling level in France, an age difference of a few months (relative age effect) has a strong impact on the frequency of initiation of MPH (despite a low level of prescription in France) and ST. Our results confirm the effect of birth month on MPH prescription, and are the first, to our knowledge, to highlight this effect on ST. Two hypotheses could be put forward to explain this result: a mismatch between the educational expectations of the school system and the relative age and maturity level of the children, or an earlier identification of neurodevelopmental disorders in the youngest children in each age group.