Exposure to Secondhand Smoke Linked to Myopia in Kids

— However, cross-sectional study from Hong Kong does not show causality

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A photo of a woman smoking a cigarette while her son rides in the back seat

Secondhand smoke exposure was associated with myopia in kids, according to a cross-sectional study from Hong Kong.

After adjusting for several factors including age, sex, and parental myopia, exposure to secondhand smoke was associated with greater myopic refraction (β -0.09, 95% CI -0.14 to -0.03) and longer axial length (β 0.05, 95% CI 0.02-0.08) in children ages 6 to 8 years, reported Jason C. Yam, MPH, of the Chinese University of Hong Kong, and colleagues.

Kids who were exposed to secondhand smoke were also more likely to develop moderate (OR 1.30, 95% CI 1.06-1.59) and high myopia (OR 2.64, 95% CI 1.48-4.69), they noted in JAMA Network Open.

Every increase in exposure in units of 10 cigarettes per day was associated with greater myopic refraction (β -0.07, 95% CI -0.11 to -0.02) and axial length (β 0.04, 95% CI 0.01-0.06), as well as a higher likelihood of developing moderate (OR 1.23, 95% CI 1.05-1.44) and high myopia (OR 1.75, 95% CI 1.20-2.56) and earlier myopia onset (β -1.30, 95% CI -2.32 to -0.27).

Similar findings were shown in a study from Singapore, while conflicting findings were shown in studies from Egypt and the U.S., Yam and team said, adding that variables such as patient age, among other factors, may play a role in the conflicting findings.

"Younger children may be potentially exposed to more SHS [secondhand smoke] chemical components, as they have faster respiratory rates and breathe more air relative to their body mass," they 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 localized at the retina and other ocular tissues by nicotine, which may have dose-dependent effects."

In this study, exposure to secondhand smoke was associated with an earlier mean age at onset of myopia, with those exposed developing the condition at 72.8 months compared with 74.6 months among unexposed children (P=0.01). Each younger year of exposure was associated with a 0.07-D decrease in spherical equivalence (β 0.07, 95% CI 0.01-0.13) and a 0.05-mm increase in axial length (β -0.05, 95% CI -0.08 to -0.01).

"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."

Myopia is common worldwide, with a prevalence of 36% in U.S. children ages 5 to 17, but is even more common in Hong Kong. As of this year, approximately 26% of children ages 6 to 8 alone have the condition.

However, in an invited commentary, Ian G. Morgan, PhD, of the Australian National University in Canberra, expressed doubts on the study's findings, citing small sample sizes, among other factors.

"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," Morgan wrote. "For that reason, 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."

For this study, Yam and colleagues used data from the population-based Hong Kong Children Eye Study from March 2015 to September 2021. They included 12,630 participants from the Chinese University of Hong Kong Eye Center, 32.4% of whom were exposed to secondhand smoke. Mean age was 7.4, and 53.2% were boys.

Secondhand smoke exposure was evaluated with a validated questionnaire. All participants underwent comprehensive ophthalmic and physical examinations. The researchers adjusted for age, sex, parental myopia, body mass index, near-work time, outdoor time, and family income.

Yam and team noted that the cross-sectional design of their study prevented them from determining causality, which was a limitation. Other limitations included potentially underreported data on secondhand smoke exposure outside the home, and the possibility of exposure outside the time frame of 6 to 8 years of age.

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    Elizabeth Short is a staff writer for MedPage Today. She often covers pulmonology and allergy & immunology. Follow

Disclosures

This study was supported by grants from the General Research Fund, Research Grants Council, Hong Kong; the Collaborative Research Fund; the Health and Medical Research Fund, Hong Kong; the Chinese University of Hong Kong; the Innovation and Technology Fund; the UBS Optimus Foundation; the Centaline Myopia Fund; the National Natural Science Foundation of China; and the CUHK Jockey Club Children's Eye Care Programme and CUHK Jockey Club Myopia Prevention Programme.

Yam reported no conflicts of interest. A co-author reported relationships with Amgen, Bristol Myers Squibb, Pfizer, Janssen Pharmaceuticals, Bayer AG, GSK, Novartis AG, Takeda Pharmaceutical Company, the Hong Kong Research Grants Council, the Hong Kong Health and Medical Research Fund, the National Institute for Health Research in England, the European Commission, the National Health and Medical Research Council in Australia, Jacobson Medical (Hong Kong), and the World Health Organization.

Morgan reported no conflicts of interest.

Primary Source

JAMA Network Open

Source Reference: Zhang Y, et al "Analysis of secondhand smoke exposure and myopia among children aged 6 to 8 years in Hong Kong" JAMA Netw Open 2023; DOI: 10.1001/jamanetworkopen.2023.13006.

Secondary Source

JAMA Network Open

Source Reference: Morgan IG "Exposure to secondhand smoke and myopia in children aged 6 to 8 years in Hong Kong" JAMA Netw Open 2023; DOI: 10.1001/jamanetworkopen.2023.12995.