This recommendation is based on mounting evidence in animals and in humans that general anesthesia might damage developing young brains.
Researchers at Mayo Clinic compared the learning skills of 350 kids who underwent 1 or more general anesthesia before age 2 to 700 kids who have never been under sedated (general) anesthesia. What they found was that before the age of 19 (after statistical normalization):
No significant difference in learning disability between kids who have been under anesthesia once (23 out of 100) with kids who have never been under anesthesia (21 out of 100).
However, those kids who have been under anesthesia 2 or more times had increased rate of learning disability (36 out of 100).Based on animal studies, anesthetics are known to cause accelerated loss of brain cells during development which leads to learning and behavior problems later in life. Whether that's also true for humans has not been studied.
However, further study is required as the results were obtained based on observation and did not account for other variables (type of surgery for example).
This information, though not definitive, does warrant extra circumspection by parents and surgeons when deciding to pursue surgery in kids under age 3 years, no matter how minor the surgery including ear tubes, tonsillectomy, adenoidectomy, and turbinate reduction. That said, studies currently support that there is NO difference in learning and cognition among healthy children with a single short anesthesia exposure before age 36 months compared with healthy siblings with no anesthesia exposure.
If surgery pursued, one should perform as much as required to minimize need for a 2nd procedure at a later date. That means multiple procedures under one anesthesia would be preferable than multiple procedures at different times.
Of course, surgery should only be done if the benefits outweigh the risks including anesthetic risks.
Cognitive and Behavioral Outcomes After Early Exposure to Anesthesia and Surgery. Published online in Pediatrics October 3, 2011. doi: 10.1542/peds.2011-0351
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Motor and cognitive outcome at school age of children with surgically treated intestinal obstructions in the neonatal period. Early Hum Dev 2013;89:181-5.
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Behavior and development in children and age at the time of first anesthetic exposure. Anesthesiology 2009;110:805-12
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Association between a single general anesthesia exposure before age 36 months and neurocognitive outcomes in later childhood. JAMA 2016;315:2312-20.
General Anesthetic and Sedation Drugs: Drug Safety Communication - New Warnings for Young Children and Pregnant Women. FDA 12/14/16