The use of pharmacogenetics in differential diagnosis. A case report

We present the clinical case of an 8 year old child with attention deficit hyperactivity disorder (ADHD), who developed a psychopathological disorder compatible with acute psychosis after two years of using therapeutic doses of methylphenidate. We performed a pharmacogenetic study in order to find the cause of the serious adverse reaction. Biochemical amplification by PCR of CYP2D6 and its alleles, 3*, 4*, 5* and 6* was done to find if the reaction was toxic, if the patient had a poor metabolizer genotype. Clinical case: an 8 year old male patient, without antecedents, who was being successfully treated for two years with methylphenidate at 5 mg/day orally for ADHD, with goodscholastic results as well. The patient went to ambulatory emergency complaining of hypotonia, bradypsychia and difficulty articulating words with intervals of verbosity, speech disconnected with reality and having periods of complete verbal absence. The patient was admitted in the pediatric unit. After suspending medication, the clinical symptoms improved until they disappeared. Blood profile as well as biochemical complementary tests were normal. A nuclear magnetic resonance (NMR) did not reveal organic alteration which could have explained the symptoms. EEG was also normal. After rechallenge, one month after the first event, the boy developed the same symptoms. A CYP2D6 genotype analysis was done (3*, 4*, 5* y 6* alleles) where we found him to be a normal metabolizer. This discards a pharmacokinetic alteration of genetic origin, which could explain an abnormal reaction to methylphenidate. This adverse reaction could be due to the prolonged use of this drug and the action of CYP2D6 in the levels of dopamine.