Having surgery to remove the tonsils and adenoids significantly reduces sleepiness and behavioral problems in kids who snore, but it does not improve their performance on attention and memory tests. That’s the finding from a long-awaited clinical trial conducted by researchers at Brigham and Women’s Hospital, Boston Children’s Hospital, and elsewhere.

Doctors perform nearly half a million of these surgeries every year In the United States on children under age 15, in the belief that the procedure will fix the chronic snoring problem, called sleep apnea, and all of the symptoms that go with it: sleepiness, irritability, and distracted or hyperactive behaviors.

To test whether that was the case, the researchers recruited 464 children ages 5 to 9 with sleep apnea in Boston and other cities. They randomly assigned them to have either the surgery, called adenotonsillectomy, or nothing but routine check-ups with a sleep specialist. Seven months later, they asked teachers and parents how the children were doing and found that those who had the surgery had much more of an improvement in their behavioral issues compared with those who didn’t have it.

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On the other hand, the study, which was published online Tuesday by the New England Journal of Medicine, didn’t find that surgery resulted in any significant improvement in how children performed on cognitive tests to assess how well they could focus, analyze and solve problems, and recall what they had just learned.

“It’s not that uncommon to see this type of disconnect,” said study leader Dr. Susan Redline, associate clinic director of the Brigham’s Division of Sleep Medicine. “In a structured environment, kids may perform adequately on tests, but in day to day activities there were problems previously identified that got better after the surgery.”

She acknowledged, though, that it’s possible parents of children who had surgery to fix their sleep issues may have perceived a greater improvement in their child’s behaviors due to wishful thinking. But teachers also reported the same progress, and they may not have known that the student they were evaluating had sleep apnea or a procedure to treat it.

Such positive assessments will likely be taken as a sign by pediatricians to keep recommending adenotonsillectomies to their young patients with sleep apnea—especially if they have behavioral problems at home and school.

“Our study supports the use of surgery in these children,” Redline said, “but it also suggests that there really doesn’t seem to be any harm in watchful waiting in those children” who aren’t experiencing problems maintaining self-control or focus.

Sleep apnea occurs in about 2 to 4 percent of children because of structural defects that make it difficult for oxygen to pass through airways during sleep, often causing snoring, gasping for breath, or pauses in breathing. Sleep specialists have speculated that a lack of oxygen getting to the brain during sleep might lead to cognitive deficits and behavioral issues.

Removing the tonsils and adenoids that sit near these airways allows for air to pass more freely with less obstruction. It usually reduces snoring, improves sleep, and is thought to enable more oxygen to reach the brain.

In an editorial that accompanied the study, Dr. Robert Brouillette, a McGill University pediatrician, wrote that “it remains possible that months to years of untreated obstructive sleep apnea can impair attention and executive function in children.” A greater improvement on cognitive tests might have been seen in children younger than five or in those with more severe forms of the condition.