Children with different types of ADHD have physiological differences, researchers at The University of Texas at Austin find
Oct. 12, 2005
AUSTIN, Texas—Both the ears and fingers of children diagnosed with one type of attention-deficit/hyperactivity disorder (ADHD) differ from those of children diagnosed with another type of ADHD, according to researchers at The University of Texas at Austin.
These differences may result from differences in exposure to sex hormones early in life, perhaps during prenatal development, the research team said.
“Some children diagnosed with ADHD have symptoms of both hyperactivity and inattention,” said one author of the report, graduate student Greg Westhafer. “This category is officially called the ADHD/Combined type. Other children exhibit the symptoms of inattention but not the hyperactivity, and this category is called the ADHD/Inattentive type. Both categories of ADHD are seen more commonly in boys than in girls.”
One of the measurements made by the research team was otoacoustic emissions, which are echo-like sounds produced by the inner ear, or cochlea, after clicks are presented to the ear. These otoacoustic emissions are measured using a sensitive microphone placed in the outer ear canal.
“Otoacoustic emissions are typically stronger in girls than in boys,” said Dr. Dennis McFadden, another author of the report. “The new research shows that the otoacoustic emissions of boys diagnosed as ADHD/Inattentive are weaker than those of boys diagnosed as ADHD/Combined and also of control boys not having ADHD.”
The research team calls the emissions in the ADHD/Inattentive boys hyper-masculinized because they were so weak compared to the emissions of both girls and boys not diagnosed with ADHD.
The research team also measured the lengths of the fingers. Westhafer said, “Although the absolute lengths did not differ across the different categories of children tested, certain length ratios did. For example, when the length of the second finger was divided by the length of the fifth finger, the resulting ratio was smaller in boys diagnosed as ADHD/Inattentive than in boys diagnosed as ADHD/Combined or in control boys not having ADHD. A similar result was observed for other finger-length ratios on both hands.”
Because the length ratios in the ADHD/Inattentive boys were so much smaller than the finger-length ratios of both girls and boys not diagnosed with ADHD, the research team calls this measure hyper-masculinized also.
“To our knowledge, otoacoustic emissions and finger-length ratios are among the first physiological measures to show differences between children diagnosed as ADHD/Inattentive and ADHD/Combined,” Westhafer said. “And the fact that both measures were hyper-masculinized surely is an important clue about underlying causes. The implication is that these two subtypes of ADHD children may result from different sequences of events during prenatal development.”
“Our interpretation of these results is that the boys diagnosed as ADHD/Inattentive were exposed to higher-than-normal levels of male hormones, called androgens, during prenatal development,” McFadden said. “We are conducting additional research to see if other measures also are hyper-masculinized in ADHD/Inattentive boys.”
“We believe that these findings support a contention that many of us have been making for years,” said Dr. Caryn Carlson, another member of the research team. “The current categories of ADHD/Inattentive and ADHD/Combined may constitute different disorders, not different subtypes of a single disorder.”
The new findings are reported in an upcoming issue of the journal Clinical Neuroscience Research.