Early Smell Exposure May Be Critical for Sensory Development

Many parents try to offer their healthy babies a rich sensory environment, full of new sights, sounds, smells, and tastes, to stimulate the developing brain to appreciate life’s delicious complexities to the fullest. And for vision and hearing, there is a solid foundation of research showing that there is a critical period early in life when adequate stimulation of those senses is essential for their healthy development.

Now, a pilot study conducted at The Children’s Hospital of Philadelphia and published in the journal JAMA Otolaryngology provides some of the first-ever evidence for a critical period in developing the sense of smell, or olfaction, too. The findings have particular implications for rehabilitating young patients, including severely premature infants, who receive lifesaving medical interventions that temporarily prevent airflow through the nasal passages during this potentially critical period.

A group of such patients were the participants in the CHOP study. A third of the 18 participating children had undergone tracheostomy at or before age 4. At the time of testing they were still using that artificial airway, meaning that air did not flow past their nasal passages to expose them to scent stimuli as they breathed. Another third of the children had a past tracheostomy at or before age 4 for a period of at least six months, but at the time of testing they were breathing through their nasal passages. The remaining children were age-matched controls who were recruited from the otolaryngology clinic but never had a tracheostomy.

In smell tests, the children who had a current or past tracheostomy both performed poorly compared to controls — supporting the idea that the tracheostomy groups had missed a critical period for sensory development, and that later removal of the tracheostomy alone was not enough for a full recovery of olfaction.

“This was a little bit surprising,” said Will Kennedy, a medical student at the Perelman School of Medicine at the University of Pennsylvania, who was first author of the study. “There have been studies in the past in adults that show when you give adults a tracheostomy, they do recover the sense of smell and have robust smell abilities. If there was not a critical or sensitive period, kids should also regain a robust sense of smell.”

Without early stimulation of the neurologic pathways in the olfactory system, there seems to be a permanent and persistent deficit in smelling ability that persists later in life, noted Steven Sobol, MD, MSc, a CHOP pediatric otolaryngologist and surgeon and associate professor at Penn, and the study’s senior author.

“When dealing with critically ill children early in life, quality-of-life senses such as smell and taste are understandably not considered,” Dr. Sobol said. “One of the take home messages for us was that physicians should be aware there is this potential for smell dysfunction later on in life.”

Decreased sense of smell also has important safety implications, Dr. Sobol noted. Children and adults with limited olfaction may face dangers from consuming spoiled food and drink or from failing to detect the smells of smoke or gas.

For that reason, the team is next working on developing protocols for a type of smell rehabilitation that they plan to test in future studies. The process will involve exposure to malodorous compounds, such as those in gas and rotten food, that would be most essential to detect for safety. However, if their preliminary evidence suggesting that some children may have missed out on a critical period for olfactory development is correct, then rehabilitation may not work. Prevention strategies may be a better future approach.

“Being aware this is something that needs to be addressed, even though we don’t have the solution at this point, will eventually lead to therapies that will improve the eventual quality of life in these patients once they get through the early challenges,” Dr. Sobol said.

The study team noted that while their findings are intriguing, additional studies that follow children longitudinally would help confirm the existence of a critical period for olfactory development.

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