Action News vol 10 no 3, June 2010, ISSN 1324-6011
Incorporating Lead Aware Times ( ISSN 1440-4966) and Lead Advisory Service News (ISSN 1440-0561)
The Journal of The LEAD (Lead Education and Abatement Design) Group Inc.
Editor-in-Chief: Anne Roberts
How does lead exposure affect our eyes?
By Daisy Shu, Optometry student, University of New South Wales, Australia, Jan
Lead exposure is known to disrupt a myriad of body processes due to its toxicity to our vital organs, particularly our bones, heart, kidneys and nervous system1. However, there has been scarce research into its effects on vision, a fundamentally cognitive process. Due to the direct association between our eyes and the central nervous system (CNS), there is no doubt that the ability of lead to hinder the development of the nervous system will inevitably affect our vision. Studies have shown that lead exposure can result in a reduced sensitivity of rod photoreceptors2, blurred vision3 and irritated eyes4 as well as an increased susceptibility to cataract5 and optic neuritis6. Interestingly, although high concentrations of lead exposure are detrimental, a recent study7 has elucidated the beneficial effects of small amounts of lead in preventing and treating eye disease. [Photoreceptors: A nerve ending, cell, or group of cells specialized to sense or receive light. www.answer.com Rods are not sensitive to colour, unlike cones, but are many times more numerous than cones, and more sensitive to light.]
Scotopic visual deficits
Human vision is brought about by the photoreceptors of the retina, namely the rods and cones. Cones are responsible for vision under high light levels whilst scotopic vision, which is vision under dim lighting, is mediated by the rods. Fox and Katz2 conducted an electrophysiological study on rats, revealing long-term changes in the sensitivity of rods following low and moderate lead exposure (peak blood lead of 19 and 59 μg/dL, respectively)2. Electroretinographic (ERG) observations show that such alterations are present at the level of the retina. They found an increase in rod outer segment (ROS) calcium, a decrease in rhodopsin content (photopigment found in rods) per eye and reduced rod sensitivity in the dark adaptation function suggesting that rods are directly and selectively influenced by lead2. Evidently, lead can severely affect the ability of our eyes to function under dim light conditions, particularly in adapting to the dark.
The development of the CNS and retina occur during gestation in humans and hence lead exposure during this period can have a detrimental effect2. A study on rhesus monkeys by Bushnell et al8 revealed that lead exposure of 85 μg/dL during the first year of life impaired visual discrimination under dim lighting compared to their performance under bright light. Although the study was conducted on animals, the data has consequences for children exposed to high lead levels during development. It has been suggested that temporary blood lead levels in the vicinity of 200 μg/dL early in life and chronic exposure at 85 μg/dL subsequently can cause similar impairments of scotopic vision in humans8.
Lead intoxication induces a harmful, chronic impairment of the vision needed under dim lighting, a condition known as night blindness8 and it has been proposed that this may occur via damage originating from the brain. Rods are quite poorly represented at an area of the brain responsible for vision, called the visual cortex, as there is much less neural tissue dedicated to processing its information compared to the cone photoreceptors8. Since lead induces brain damage via demyelinisation8, which is the loss of the myelin sheath around nerve fibres, deficits in the visual system will be likely to appear first in rod-mediated vision.
Susceptibility to cataract
Cataract is a clouding of the crystalline lens of the eye, causing an obstruction in the passage of light to our retina. Schaumberg et al5 found that cumulative lead exposure can increase the risk of age-related cataract. They measured bone lead levels in the tibia and patella of a selection of men aged 60-93 years old (mean age of 69) from Boston5. It was found that men with the highest levels of lead in the tibia (7.78 ± 4.85 μg/dL) had a greater than 2.5-fold increased risk of cataract compared to men with the lowest tibia lead levels (4.49 ± 2.65 μg/dL)5. After controlling for age, the approximated attributable fraction of cataract in this population due to lead exposure was 42%5.
Lead has been found to be present in lenses with cataract in various studies9. It is thought that the invasion of lead into the lens may alter its redox status and cause conformational changes in protein, hence reducing lens clarity9. Lead is known to disrupt glutathione metabolism in the lens9 and raise the protein-bound glutathione and cysteine content5. Moreover, lead can hinder the biological balance of calcium in our system, that is, the calcium homeostasis, which is vital in maintaining lens transparency9. Evidently, many studies reveal that lead may be present at higher concentrations in cataractous lenses compared to transparent lenses5, 9-11. [redox: a reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction. The Free Dictionary]
Other visual symptoms
Since ancient times, lead poisoning has been found to cause damage to the visual system and even blindness in humans and animals6. These effects are collectively termed as “optic atrophy” or “blurred vision”, appearing only in cases of lead poisoning severe enough to cause brain damage5. The importation of lead in wine-making, cookery, and jewellery into Rome’s aristocracy circa 150 BC may have contributed to its ultimate ruin and decay6. Classical authorities on medicine at the time described symptoms of deteriorating eyesight due to optic neuritis, which is an inflammation of the optic nerve6. Moreover, lead was also found to potentially result in amaurosis which is the loss of sight due to disease of the optic nerve or brain without pathology of the eye itself6.
More recently, lead exposure has been implicated in ocular neuritis in children which rends them either visually impaired or permanently blind12. Gibson found that cases of optic neuritis were often accompanied by an increased intracranial pressure which seemed to directly irritate the optic nerve head13. Hence he coined the condition as “ocular plumbism” believing it to be due to swelling rather than inflammation13. Lead-induced blindness, albeit a now rare and often transient phenomena, can be quite startling, emphasizing the burden of lead on our well-being.
Other visual symptoms have been documented, including strabismus and double vision noticed in a child with lead poisoning in a public health report on Queensland children14. [“Strabismus: a condition in which the eyes do not point in the same direction. It can also be referred to as a tropia or squint.” www.answer.com]
It has been suggested that the increased intracranial pressure induced by lead exposure can also cause paralysis of the external recti [straight] muscles involved in eye movement12. This may contribute to strabismus, and consequently, double vision, due to the lack of fixation of both eyes on a target.
Tetraethyl lead is a compound more commonly encountered in occupational conditions where it is used as an anti-knock compound in petroleum, including leaded Aviation Gasoline or AvGas3. Exposure to tetraethyl lead can cause symptoms of redness and pain in the eyes, as well as blurred vision3. Moreover, it can irritate the eyes and result in a potential loss of vision4.
Vision-related cognitive deficits
Various studies suggest that cumulative lead exposure is related to many chronic disorders of aging, including cognitive decline5. A study conducted by Shih et al measured tibia lead levels using 109Cd-induced K shell X-ray fluorescence (XRF)15. Subjects were then required to complete a series of tests including those related to hand-eye coordination, visual memory and visuoconstruction15. It was found that higher tibia lead levels significantly correlated with poorer vision-related cognitive functioning15. [“Visuoconstruction abilities involve the coordination of fine motor skills with visuospatial abilities, usually in the reproduction of geometric figures. This domain looks not only at the individual's aptitude for copying a figure, but how well planned and organized that figure is. Individuals who have difficulties with visuoconstruction and spatial abilities often struggle with daily tasks such as arithmetic, driving, and writing.” www.advancedpsy.com/visuoconstruction_abilities-page-25.html]
Prevention of lead-induced damage to the eyes
Lead contamination is widespread in our environment, primarily due to leaded petroleum and lead-based paint, causing practically every adult to have amassed some degree of lead in their system5. In industrial settings, avoiding lead exposure of pregnant women, adolescents and children is particularly crucial3. Other preventative measures include avoidance of generation of mists3 [e.g. liquid droplets of an acid in the sulphuric acid works of a lead smelter], adherence to strict hygiene rules and implementation of eye wash fountains in the immediate work area4. Eye protection such as splash and impact resistant goggles as well as face shields is also necessary4. It is advised that contact lenses should not be worn when working with tetraethyl lead4.
Prevention of the lead-induced visual disorders remains an important public health goal, and can only be achieved by reducing the distribution of lead in our environment. Through public health campaigns and the enforcement of stringent lead contamination policies, the burden of lead can be significantly reduced.
Cataract is commonly treated by surgery which involves removing and replacing the opaque crystalline lens with a plastic intraocular lens (IOL). In Australia, expenditures for cataract surgery cover the largest single line item in the Medicare budget5, revealing the serious financial burden which lead has contributed to.
Other causes of cataract: “There is some evidence that long-term exposure to sunlight, tobacco, and heavy alcohol consumption may be associated with cataract formation. Some research suggests that people who have a low dietary intake of fruits and vegetables, vitamin C and E and betacarotene are also at higher risk of the disease. Systemic diseases such as diabetes and vascular disease may increase the risk of cataract development, as may eye injury or the use of some medications, including corticosteroids.” www.health.gov.au/internet/eyehealth/publishing.nsf/Content/commonprob
Treatment of irritated and red eyes includes the application of lubricant eyedrops.
Beneficial effects of lead
During Egyptian antiquity, lead-based compounds were used in cosmetics, particularly in kohl, used to outline eyelids7. Analytical chemistry has revealed that two lead chlorides, laurionite and phosgenite, were manufactured to form fine powders found in eye lotions and makeup7. Contrary to modern beliefs of the toxicity of lead, the study suggested that “submicromolar concentrations” of lead ions may play a role in promoting the action of our immunological defences7.
By studying the consequences of a “submicromolar concentration” of lead on human keratinocytes, lead ions were found to increase the “oxidative stress response7.” Lead ions stimulated the production of nitric oxide (NO), an essential messenger in our innate immune system which communicates with immune cells such as macrophages and enhances the capillary blood flow for more white blood cells to migrate to the site of infection7.
It was suggested that such small dosages could prevent diseases such as bacterial conjunctivitis7. However, bacterial conjunctivitis is usually self-limiting and current treatment involves antibiotic eye drops and ointments. Although lead does amplify the immune response, it is only effective in small amounts, so careful stoichiometric analysis should be implemented if it is to serve as a potential therapeutic agent.
Although progress has been made in limiting lead exposure in industrialized countries, most individuals have already accrued a considerable body burden of lead5. Further research into the effects of lead on vision is particularly beneficial, especially considering the recent finding of accumulated lead exposure as an unrecognised risk factor for cataract5. As the leading cause of global blindness and visual impairment16. [As proof of this, the national eye health awareness campaign of the Australian Dept of Health and Aging, quoted from above under “other causes of cataract”, does not mention lead exposure as a cause of cataract. ]
Research suggests that reduction of lead exposure could help decrease the global burden of cataract. Future investigations into the potential of decreasing the risk of cataract may involve prompt treatment of lead poisoning using chelation therapy. Furthermore, the selective rod deficits resulting from lead exposure during gestation and in perinatal development6 should help raise greater concern for the need to limit foetal and childhood lead poisoning.
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