Dysfunction of the inner ear may be a source of hyperactivity disorder, according to research reported in the journal Science.
The recently published study provides some of the first evidence that a sensory impairment, such as an inner ear disorder, could actually cause molecular changes in the brain itself. These alterations to the brain can then lead to behavioral changes such as hyperactivity, researchers suggest.
Attention-deficit hyperactivity disorder, or ADHD, is a condition that makes it unusually difficult for kids to concentrate on tasks, to pay attention, to sit still, and to control impulsive behavior. While some children exhibit mostly inattentive behaviors and others predominantly hyperactive and impulsive, the majority of those with ADHD have a combination of both, which may make it very difficult for them to function in school, and create a lot of conflict at home.
Symptoms of hyperactive or impulsive ADHD:
Fidgeting or squirming, trouble staying in one place or waiting his turn
Excessive running and climbing
Trouble playing quietly
Always seems to be “on the go” or “driven by a motor”
Excessive talking or interrupting, blurting out answers
Symptoms of inattentive ADHD:
Makes careless mistakes
Is easily distracted
Has difficulty following instructions
Doesn’t seem to be listening when spoken to directly
Has trouble organizing
Avoids or dislikes sustained effort
Is forgetful, always losing things
— from childmind.org
The findings could help explain why children with auditory and vestibular (balance) impairments are often found to have higher rates of attention deficit hyperactivity disorder.
Dr. Michelle Antoine and investigators from the Albert Einstein College of Medicine of Yeshiva University in New York studied a group of mice with an abnormally high level of activity, including the unusual behavior of continuously chasing their tails.
The particular group of mice had a gene mutation that results in profound deafness and impairment of the balance mechanism of the inner ear. The affected gene, known as Slc12a2, is responsible for mediating the transport of sodium, chloride and potassium molecules from tissues within inner ear and central nervous system.
Researchers found that blocking the activity of, or “deleting,” the Slc12a2 gene in the inner ears of healthy mice (a gene also found in humans) caused the mice to become more active.
The findings led researchers to suspect abnormal function in the stratum, a portion of the brain responsible for motor control. Further observation confirmed their hypothesis, indicating abnormally high levels of two key proteins involved in neurotransmitter action within the stratum.
To see if the effects could be reversed, mice with the genetic mutation were given injections of haloperidol (a drug commonly used to control tics associated with Tourette’s syndrome) to counteract the high protein levels found within the stratum. The injections had the desired effect and the activity levels of the mice returned to normal.
The study certainly raises a number of questions, particularly the possibility that other sensory impairments unrelated to the inner ear could cause or contribute to psychiatric or motor disorders.
While the results are intriguing, caution must always be exercised when extrapolating animal data to humans. Furthermore, genetically-linked medical disorders are often not the result of a single genetic mutation.
The researchers are optimistic at the possibility of a more effective medication being developed for conditions such as ADHD as a result of their study.
“Our research shows that severe inner ear defects can lead to long term changes in the brain that in turn cause behaviors such as hyperactivity,” professor Jean Herbert, a co-author in the study, was quoted in Science. “ In a broader sense, our studies suggest that the effect sensory deficits have on brain development may be vastly underestimated.”
A full text of the Science article, “A Causative Link Between Inner Ear Defects and Long-Term Striatal Dysfunction,” can be found online at bit.ly/1jcEHr0 .
Dr. Kevin Liebe is an audiologist at Columbia Basin Hearing Center and currently serves as president-elect of the Washington State Academy of Audiology. He has been a contributor to various news publications in print and online on topics relating to hearing loss and public health.