March 2009

 

OPHTHALMOLOGY NEWS

 

The ocular truth about secondhand smoke


by Matt Young EyeWorld Contributing Editor

 

 

Clearly, smoking isn’t good for eye health. It has been shown to be a risk factor for age-related macular degeneration, after all. But what about secondhand smoke (SS), also known as environmental tobacco smoke (ETS)? Could others’ smoking somehow damage our eyes? A new comprehensive literature review published online in August 2008 in the British Journal of Ophthalmology is inconclusive overall but finds some evidence to support such a notion. Take ocular surface disorders, for example. Research is inconclusive—but interesting—about the association between ETS and ocular surface disorders. In one study in patients with atopic keratoconjunctivitis (AKC), for example, passive smoking increased levels of neurotrophic growth factor, brain-derived neurotrophic factor, and neurotrophins 3 and 4 in tears of patients with AKC but not in normal participants. “It has been suggested that these neurotrophins may play a role modulating allergic responses,” reported the literature review author Noemi Lois, M.D., Ph.D., Department of Ophthalmology, University of Aberdeen, Foresterhill, Aberdeen, United Kingdom.

There also are some potential associations between ETS and cataract, refractive error, uveitis, and maybe even other ocular problems.

Ocular surface disease

Rats exposed to cigarette smoke (two hours a day over 60 days) have developed conjunctival and sub-epithelial stromal thinning. Further, the number of microvilli—important for stabilizing tear film—in conjunctival epithelium was reduced. In other research, an ETS irritant index was developed from a questionnaire. When a relatively large population was surveyed with respect to that index, 82% reported eye irritation. And “a large cross sectional study of over 14,500 adolescent children in France showed that ETS exposure increased the risk of rhinoconjunctivitis by 20%,” Dr. Lois reported. “These findings were supported in a very large Korean study of nearly 43,000 children.

ETS exposure increased the risk of developing allergic rhino-conjunctivitis both in 6-12 year olds and 12-15 year olds. The well validated ISAAC questionnaire was used, the results showing a consistent approximately 20% increased risk of developing allergic conjunctivitis associated with ETS exposure in children.”

Refractive error

In one study, researchers found a statistically significant link between parental smoking and refractive error in children.

Children of smoking parents actually had lower myopia prevalence, which is perhaps not a bad thing. But they also had more hyperopic mean refraction whether or not parental smoking was at home or outside. “In chicks nicotinic antagonists inhibited experimental myopia which contrasts with the above findings in humans but which could be explained by possible up-regulation and subsequent desensitisation of ocular nicotinic receptors by nicotine,” Dr. Lois wrote. “The association between refractive error and ETS exposure could also be explained on the basis of an effect of tobacco in other neurotransmitters, such as dopamine and aminobutyric acid (GABA).”

Cataract

When it comes to cataract, evidence of a link in some research is weak, but not others. “The Blue Mountains Eye Study, a large population-based crosssectional study, found that passive smoking was not statistically significantly associated with the development of cataract,” Dr. Lois reported. But pipe smoking was strongly associated with nuclear cataract. “Given that pipe smokers are more likely to produce SS smoke and leave the smoking source nearer the eye for longer periods of time, it is possible that pipe smoking may have a greater impact as a causative factor in the occurrence of cataract,” Dr. Lois noted. “If this were the case, ETS could potentially affect the lens in a similar manner.”

The link here remains tenuous at best. “Long-term experimental studies in rats addressing the effect of ETS exposure to the lens (exposure to SS cigarette smoke for six hours per day, seven days a week, for two years) failed to find an association between the development of cataract and exposure,” Dr. Lois reported.

Uveitis

Finally, although there are no studies that have researched a link between ETS and uveitis, Dr. Lois finds the association to be “plausible.” “Acute anterior uveitis can be induced experimentally, in animal models, by injection of the pro-inflammatory molecule endotoxin in a site remote from the eye,” Dr. Lois reported. “Given that tobacco and the filter of a cigarette contain endotoxin, it is plausible that cigarette smoking and ETS could be associated with the development of anterior uveitis or could influence the severity, recurrence rate and/or response to treatment of this disease.”

Meanwhile, William Trattler, M.D., director, Cornea, Center for Excellence in Eye Care, Miami, said Dr. Lois’ research is commendable because it’s useful to see how secondhand smoke impacts eye disease. “It makes sense that smoke, which can be an irritant, may produce a change on the ocular surface,” Dr. Trattler said. “Smoke is a chemical.”

Dr. Trattler added that it might be useful to ask a potential dry eye patient during patient history taking if he or she is a smoker, he said. Nonetheless, Dr. Trattler said the research to date still appears inconclusive about the relationship between secondhand smoke and eye disease. “Certainly there are some nice hypotheses but there’s still a lot to be looked at,” he said. For instance, one reason smokers themselves might be at increased risk for cataract could be related to being outside more to smoke (rather than the smoke itself), Dr. Trattler suggested.

Editors’ note: Dr. Lois has no financial interests related to this study. Dr. Trattler has no financial interests related to his comments.

Contact information

Lois: 00-44-1224-314-672, noemilois@aol.com
Trattler: 305-598-2020, wtrattler@earthlink.net

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