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Handedness Linked to ADHD

MedpageToday

Ambidextrous 5-year-olds are more likely to develop attention deficit-hyperactivity disorder symptoms later on as compared with right- and left-handed children, according to a study by a team of Swedish researchers.

The findings are an indication that brain morphology and neural circuitry associated with handedness also affects cognitive and behavioral function, according to Alina Rodriguez, PhD, of Uppsala University in Sweden, and colleagues in an online report in Pediatrics.

Action Points

  • Explain to interested patients that ADHD and other learning disorders have previously been linked to particular brain abnormalities.
  • Explain that hand preference is also believed to be determined by brain organization established early in development, perhaps before birth. However, a relationship between handedness and ADHD and related disorders was not previously known.

The conclusions emerged from a longitudinal study of 7,871 children in northern Finland born in 1986, of whom 87 were mixed-handed, 632 showed a left-hand preference, and the remainder were right-handed at age 5.

Rodriguez and colleagues found that when these children were tested at age 8, teachers were about twice as likely to report hyperactivity (odds ratio 2.06, 95% CI 1.09 to 3.91) or a combination of inattention and hyperactivity (OR 2.16, 95% CI 1.21 to 3.86) in the mixed-handed children compared with the right-handed ones.

Similar increases in rates of teacher assessments of probable psychiatric disturbance and in overall school performance, as well as in parental reports of language problems, were seen in the ambidextrous children.

Moreover, these learning and behavioral problems persisted into adolescence.

Cohort members were evaluated at age 16 by parents on ADHD symptoms and provided self-reports of language and math performance in school.

Those classed as mixed-handed were more than three times as likely to suffer inattention (OR 3.53, 95% CI 1.65 to 7.53) or a combination of inattention and hyperactivity-impulsivity (OR 3.16, 95% CI 1.42 to 7.08) relative to right-handed 16-year-olds, according to their parents.

Self-reported problems in Finnish language and math classes were also significantly more common in the mixed-handed youths (OR 1.85, 95% CI 1.00 to 3.43 for language; OR 2.55, 95% CI 1.48 to 4.37 for math).

These effects were not seen in left-handed children, Rodriguez and colleagues reported.

"Mixed-handedness can be used as a marker of risk for difficulties and warrants additional evaluation," they wrote.

Adjusting for gender and birth weight and gestational age reduced the strength of the association between mixed-handedness and behavioral and learning problems. The point estimates of odds ratios for inattention and/or hyperactivity declined 1.4 to 1.6 and statistical significance was lost.

But that actually supports the hypothesis that brain abnormalities stemming from before birth underlie what they called "atypical lateralization" and subsequent learning and behavior problems, Rodriguez and colleagues indicated.

"Recent neuropsychological work related to patterns of brain organization and function corroborates our findings," they added.

They cited research indicating that ADHD is associated with left-side motor deficits, apart from hand preference, as well as reduced attention to visual stimuli on the left versus right side. These observations suggest weaker right hemisphere function, Rodriguez and colleagues argued.

Other studies have suggested that neural transmission can be asymmetric between hemispheres, they said.

"These studies together highlight the possible interconnection among mixed-handedness, neurotransmitter dysfunction in the right hemisphere, and ADHD symptoms," they wrote.

The authors pointed out that the "impreciseness of handedness as a proxy for atypical lateralization is a limitation." Yet, the cost of direct assessment of the brain by imaging or electrical activity is "prohibitive" so handedness was a "feasible alternative for large epidemiologic studies."

And they said that even though the number of prevalence of mixed-handedness was low, "it seems unlikely that those with right- or left-handedness were misclassified as mixed."

Disclosures

The study was funded by the Academy of Finland, Sigrid Juselius Foundation, Thule Institute, University of Oulu, Finland, and the National Institute of Mental Health. Rodriguez also was supported by VINNMER.

No potential conflicts of interest were reported.

Primary Source

Pediatrics

Rodriguez A, et al "Mixed-handedness is linked to mental health problems in children and adolescents" Pediatrics 2010; DOI: 10.1542/peds.2009-1165.