Is age-at-onset criterion relevant for the response to methylphenidate in attention-deficit/hyperactivity disorder?

Since DSM-IV criteria for attention-deficit/hyperactivity disorder (ADHD) require that some symptoms causing impairment must be present before 7 years of age, clinicians are faced with a diagnostic and treatment dilemma on how to proceed with late-onset ADHD patients. We aimed to compare the response to methylphenidate between a group of patients fulfilling all DSM-IV ADHD criteria (full ADHD diagnosis) and a group of patients fulfilling all DSM-IV criteria except the age-at-onset criterion (late-onset ADHD).

We evaluated 180 children and adolescents (4-17 years old) and 111 adults from our ADHD unit. All ADHD diagnoses were assessed using DSM-IV criteria. Methylphenidate was administered twice daily (8 a.m. and noon), but an extra dose was allowed between 5 and 6 p.m. for children and adolescents needing extra coverage in the evening. The minimum dose was 0.30 mg/kg/day. Response to treatment was assessed in methylphenidatenaive subjects using the Swanson, Nolan, and Pelham Scale-version IV (SNAP-IV) at baseline and after 1 month of treatment. Data were collected from January 2000 to January 2006.

In both samples, subjects with the full ADHD diagnosis did not have a better response to methylphenidate at doses around 0.5 mg/kg/day than the late-onset ADHD subjects. In fact, adults with late-onset ADHD had a better response to methylphenidate than adults with the full diagnosis, even after adjustment for confounders (baseline SNAP-IV total score and ADHD types) (children and adolescents: F = 0.865, p = .354; adults: F = 5.760, p = .018).

These results concur with recent literature questioning the validity of the DSM-IV age-at-onset criterion for the diagnosis of ADHD and suggest that clinicians should consider implementing methylphenidate treatment for subjects with late-onset ADHD.