Cross-sectional findings from Hong Kong suggest exposure to secondhand smoke presented a dose-dependent association with myopia among children aged 6 to 8 years.
Exposure to secondhand smoke in children was associated with a greater likelihood of developing moderate and high myopia, as well as greater myopic refraction and longer axial length, according to new research.1
Results from the cross-sectional analysis in Hong Kong suggested the association with myopia development was significantly greater in those with higher secondhand smoke exposure and among children of younger age.
“Our study showed that children with secondhand smoke exposure exhibited an onset of myopia approximately 2 months earlier than children without exposure, which echoes our speculated clinical significance of accumulated differences in spherical equivalence and axial length as discussed earlier,” wrote the investigative team, led by Jason C. Yam, MPH, and Chi Pui Pang, DPhil, from the ophthalmology and visual sciences department at Hong Kong Eye Hospital, Chinese University of Hong Kong.
Genetic and environmental factors contribute to myopia development and nicotine exposure may be a potential risk factor. For children, exposure to nicotine is significantly higher in those with secondhand smoke exposure, with up to 40% of children worldwide exposed to secondhand smoke. Prior epidemiological studies are varied on the association between secondhand smoke exposure and myopia and thus warrant further exploration.2
The current analysis aimed to address the gaps in research by further examining the association between secondhand smoke exposure and myopia. Participants aged 6 to 8 years were recruited from the Hong Kong Children Eye Study between March 2015 - September 2021 and all participated in comprehensive ophthalmic and physical examinations. Exclusions included those with prior eye trauma, congenital malformations, ocular diseases, history of ocular surgery, and inability to complete ocular examinations.
Validated questionnaires were used to evaluate secondhand smoke exposure, examining the smoking behavior of parents and other family members to determine a child’s collective exposure to secondhand smoke. The non-exposure group comprised children whose family members did not smoke or only smoked away from their home and the exposure group comprised children living with ≥1 smoker in the family. The analyses of the association between secondhand smoke exposure and ocular parameters (spherical equivalent and axial length) were-eye based and data from both eyes were included.
The analyses included a total of 12,630 children (mean age, 7.37 years; 53.2% boys), who completed the ocular and physical examinations, in addition to the questionnaires. Due to familial smoking status, there were 4092 children (32.4%) in the secondhand smoke exposure group and 8538 children (67.6%) in the non-exposure group. Both groups were similar in age, sex, and time spent outdoors, but children in the exposure group had greater body mass index (BMI), birth weight, and near-work time.
After adjustments for age, sex, parental myopia, BMI, near-work time, outdoor time, and family income, the analysis showed secondhand smoke exposure 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. Those with secondhand smoke exposure were 1.30 (95% CI, 1.06 - 1.59) and 2.64 (95% CI, 1.48 - 4.69) times more likely to develop moderate and high myopia, respectively, compared with the non-exposure group.
Investigators found significant interactions between age and secondhand smoke exposure, suggesting the association of secondhand smoke exposure with spherical equivalence and axial length was amplified in younger children. Each younger year of exposure to secondhand smoke was associated with a -0.07D 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).
Results showed exposure to secondhand smoke was associated with an earlier mean age at myopia onset (72.8 vs. 74.6 months; P = .01). Moreover, every 10 cigarettes smoked in the family per day was associated with a 1-month advance in myopia onset (β, -1.30; 95% CI, -2.32 to -0.27), according to the analysis.
The investigative team noted the limitations of the analysis included the extent of smoking exposure, as the analysis focused on current smoking quantity, but not duration. They indicated that retrospective smoking history may not be reliable over a longer period of time.
“However, using pack-years by multiplying the daily cigarettes’ packs by the years a child has been exposed, similar results emerged that more pack-years were associated with greater myopic refraction and axial length elongation.”