Second-hand smoke (SHS) exposure was associated with myopia in children, according to a cross-sectional study from Hong Kong.
The study, led by Associate Professor Jason Yam from the Chinese University of Hong Kong, found exposure to SHS was associated with greater myopic refraction, longer axial length, and earlier onset of myopia.1
Every increase in units of 10 cigarettes per day was associated with 0.07D more myopia
The statistically significant effect sizes were small, but consistent, and showed a dose-response association with severity of exposure and younger age.
Every increase in units of 10 cigarettes per day was associated with 0.07D more myopia, and overall exposure to SHS was associated with 1.8 months earlier onset of myopia, the study found.
“Younger children may be potentially exposed to more SHS chemical components, as they have faster respiratory rates and breathe more air relative to their body mass,” the study authors wrote.
“Moreover, younger children have limited control over their living environment and less awareness of the risks of SHS for self-protection. Regarding an SHS-related myopic shift, a possible mechanism is the activation of nicotinic acetylcholine receptors localised at the retina and other ocular tissues by nicotine, which may have dose-dependent effects.
“Furthermore, younger children with a less developed visual system are more sensitive to conditions, such as chemical components in SHS, that interfere with visual development,”
Yam’s group wrote. “Last, since times of near work and outdoor activities play stronger roles in myopia development and progression in more senior school years, the role of SHS exposure may become marginal, weakening the association between SHS exposure and myopia in older children.
Our findings suggest that earlier SHS avoidance is of greater importance in myopia management in schoolchildren.”
Causal Relationship Not Proved
In an invited commentary published in JAMA Network Open, Professor Ian Morgan from Australian National University, described the study as “the most precise evidence of an association between exposure to SHS and myopia in children currently available”.2
But he noted that because the study was cross-sectional it could not prove a causal relationship.
“The existence of a dose-response curve is consistent with a causal relationship but does not prove it… further longitudinal evidence would be particularly useful, but it is hard to imagine an ethics-approvable clinical trial in which children are randomised to receive SHS, so unless mendelian randomisation provides additional information, the issue of causality may be hard to resolve.”
The authors had suggested that eliminating SHS exposure is important for myopia prevention among children, particularly in families with young children.
However, Prof Morgan said “without in any way arguing against attempts to further lower smoking rates, this may not be the best way of addressing the myopia problem in Hong Kong”.
“The fact that Hong Kong now has one of the highest myopia prevalence rates in the world, but one of the lowest smoking rates, does not suggest that smoking, or exposure to SHS, plays a major causal role in relation to myopia,” he wrote.
“Hong Kong would be well-advised to look at what is happening in mainland China, just across the border, where President Xi made control of the myopia epidemic a major issue as far back as 2018, and where there have been major reforms to the school system and how schools operate.
“There are also many lessons to be drawn from Singapore and Taiwan,” Prof Morgan said.
References
1. Zhang, Y., Zhang, X.J., Yuan, N., et al., Analysis of secondhand smoke exposure and myopia among children aged 6 to 8 years in Hong Kong” JAMA Network Open 2023; DOI: 10.1001/jamanetworkopen.2023.13006.
2. Morgan, I.G., Exposure to secondhand smoke and myopia in children aged 6 to 8 years in Hong Kong” JAMA Network Open 2023; DOI: 10.1001/jamanetworkopen.2023.12995.
