Vision and Smoking

 

UNDERSTANDING THE SCIENTIFIC EVIDENCES FOR THE HARMFUL EFFECTS OF SMOKE AND SECONDHAND SMOKE ON THE SYSTEM

Article published on the edition of June 2009 of the magazine Universo Visual (Visual Universe).

 

Smoking is not only recognized as a potential risk factor for several cardiovascular and breathing diseases, but it is also meaningful in the genesis of malignant diseases. While the majority of the malignant diseases may result from the association of genetic and environmental factors, the smoking habit is considered a direct cause of lung cancer and has strong association with prostate, breast and intestinal colon cancers.

From the cardiovascular point of view, smoking increases the serum levels of total cholesterol, LDL (Low Density Lipoprotein cholesterol, or “bad cholesterol) and triglycerides, and decreases the HDL level (High Density Lipoprotein cholesterol, the “good cholesterol”).

The dyslipidemic, atherosclerotic and thrombotic effects, strongly associated with smoking, absurdly increase the incidence of cardiovascular diseases, with an apparent dose-dependent effect on its connection with mortality due to cardiovascular diseases.

In cerebrovascular accidents (strokes) and in the myocardial infarction, the smoking habit is considered a direct risk factor. From the epidemiological point of view, it has been scientifically evidenced that men who have never smoked had 78% chance to live to be 73, while those who started smoking at the age of 20 and never stopped, had 42% chance.

In relation to eye diseases, smoking has been pointed out as a risk factor for Grave’s Ophthalmopathy, Glaucoma, Cataract and age-related Macular Degeneration, leading to irreversible blindness in many cases. There are also strong scientific evidences of its harmful effect on the ocular surface and tear film and, moreover, the dose-dependent effect of smoking on eye diseases.

Grave’s Ophthalmopathy
In the ophthalmopathy or Grave’s Disease, which has a multifactorial nature, there is a clear association between genetic and environmental factors in its pathogenesis. High serum levels of thyroglobulin and low levels of TSH (thyroid-stimulating hormone) are found in long-term smokers. Results of scientific studies clearly point out that there are more smokers that develop ophthalmopathy associated to Graves than non-smokers. Not less surprising is the fact that smokers with active, moderate and severe Grave’s Ophthalmopathy present less favorable results in relation to the response to treatment through radiotherapy, or any other treatment, if compared to non-smokers.
Age-related Macular Degeneration (AMD)
Characterized by the presence of drusen and association of modifications in the retina pigment epithelium and in the macular area, the AMD is one of the main causes of vision loss in elderly patients. Such blindness results from the degeneration of the retina pigment epithelium and from damages to the photoreceptors. The main risk factors associated to AMD are old age, smoking habits (especially in exudative form), eating habits, exposure to ultraviolet radiation and hypercholesterolemia. There are more smokers among the patients with exudative AMD than non-smokers. In a study carried out by the Eye Disease Case-Control Study Group in the United States, in which 421 patients with neovascular AMD and over 55 years of age were compared to 615 people in a control group (compatible gender and age), it was verified that smokers and former smokers have greater risk of developing AMD than non-smokers. The smoking habit effect in relation to the AMD is also dose-dependent, and several theories have been advocated towards this malign relation. Firstly, smoking could promote the development and progression of subretinal neovascular membranes, which would lead to the formation of tubular capillary plexus in the coriocapillary, resulting in disciform macular degeneration. Secondly, smoking can exercise an atherosclerotic effect and hypoxic damage to the choroidal vasculature, with hypoxia, ischemia and microinfarctions in the macula. Thirdly, oxidants present in the cigarette or generated through the activation of phagocytic cells may increase the retina oxidative stress. Fourthly, smoking reduces the plasma antioxidant concentration, and as consequence, the retina antioxidants.
Glaucoma
Neuropathy associated with typical visual field loss, in which the ocular hypertension is the greatest risk factor, the glaucoma is associated with irreversible vision loss. According to recent studies, there is a weak correlation between the smoking habit and the glaucoma. However, most writers believe that the smoking habit may be one of the environmental factors that negatively affect the optical nerve, increasing the risk of glaucoma. In a study on aqueous humor dynamics, it was verified an increase of up to 5 mmHg in the intraocular pressure immediately after smoking. The suspicion is that selective vasoconstrictor agents may lead to an episcleral venous pressure, preventing the regular flow of the aqueous humor.
Caract
The greatest cause of blindness in the world, cataracts have a multifactorial aetiology and its formation mechanism is complex. The smoking habit is one out of the many risk factors, which include old age, trauma, chronic intraocular inflammation, ultraviolet radiation, diabetes mellitus, hypoparathyroidism, long-lasting use of corticosteroids and high body mass index. The cataracts have an epidemiological relation with the smoking habit in the dose-dependent and cumulative form. Curiously, the nuclear cataract has strong association with pipe smoking habit, an association even greater than the association with cigarette smoking. It is believed that the excess of smoke from the pipe may cause direct damage to the crystalline lens, whether through the direct entrance of the tobacco’s combustion and condensation products in the eyes or through the continuous temperature increase near the lens.
Ocular Surface Diseases
It is perceptible, even for a layperson, the condition of irritation, hyperemia, tearing associated with exposure to tobacco smoke. In active smokers, several modifications to the ocular surface have been described in the medical literature, such as the tear film break-up time reduction, changes in the tear film lipid layer, reduction of the tear basal production, reduction of the cornea and conjunctiva sensibility, reduction of the tear lysozyme ratio and development of conjunctival squamous metaplasia. Smokers that use contact lenses are subject to a greater risk of developing corneal infiltrates, if compared to non-smoker. In addition, smokers have four times greater the risk of developing ulcerative keratitis, regardless of the type of contact lenses they use. Patients with ocular allergy, especially children as secondhand smokers, have an increased growth of neutrophils, neutrophic factors and neurotrophins 3 and 4 in the tear, which worsens the symptoms of the disease and makes it chronic. In sum, we can affirm that, even though the diseases herein discussed have multifactorial nature, it is an excellent public health preventive measure to motivate people to quit smoking, as well as to promote measures aimed at avoiding secondhand smoke on non-smokers in public places.
Environmental Exposure to Smoke (Secondhand Smoke) and ocular diseases
In line with the world recent trend of preventing smoking in public environments, recently, the House of Representatives of the City of São Paulo approved a municipal law forbidding smoking in closed public environments, such as bars, restaurants, show places, offices and public buildings. Such measure, with meaningful economic and social impact, comes as a response to the medical consensus already established that the non-smokers exposure to tobacco smoke (secondhand smoke) may initiate and exacerbate diseases like lung cancer and cardiovascular diseases in adults, in addition to asthma in children. Such exposure may, moreover, lead to a reduction in the cognitive abilities in children exposed to smoke. Tobacco smoke is considered the most important pollutant in closed environments in the developed world. The environmental tobacco smoke composition, arising from a cigarette or cigar, has two origins: the tobacco that burns at the cigarette tip (sidestream smoke, or SS, and which corresponds to 80% of the smoke) and the tobacco exhaled by the smoker after being inhaled through the filter (mainstream smoke, or MS, which corresponds to 20% of the smoke). The smoke from the cigarette tip, or SS, is more toxic than that inhaled by the smoker (MS), and it is two to six times more cancerous and induces four times more to inflammations than the MS. There are 4 thousand substances in the composition of the tobacco smoke, most of them cancerous, such as the 1.3 butadiene, the benzene, benzopyrene, and polycyclic aromatic hydrocarbons. The most important and identified substances that induce to inflammations, found in the tobacco smoke are: acrolein, formaldehyde, and solvents like the styrene and phenol. Depending on the variety of tobacco used, there are also metals such as nickel, cadmium, arsenic and iron, which when inhaled in excessive and cumulative amounts are potentially toxic to humans. From the ophthalmological point of view, the relation between active smoking and macular degeneration, Grave’s Ophthalmopathy, cataract and ocular surface diseases, has already been established. However, the effect of the exposure to environmental tobacco smoke and eye diseases has very few studies. We will give details on some evidences associated to secondhand smoke and eye diseases. “From the ophthalmological point of view, the relation between active smoking and macular degeneration, Grave’s Ophthalmopathy, cataract and ocular surface diseases, has already been established. However, the effect of the exposure to environmental tobacco smoke and eye diseases has very few studies.”
Refraction Errors and Strabismus
Stone et al., in a crossed sectional study, have demonstrated the meaningful association between childhood refraction error (high Farsightedness) and parental smoking (whether one or both parents). In the same study, it is shown that the parents’ smoking habits during pregnancy were associated to the child farsightedness and strabismus.

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