One of the medical myths we live with is the notion that eye disease arises as an inevitable consequence of growing older. This month's cover story concerns eyedrops given for glaucoma, a condition that, with cataracts, has been considered as inevitable in the over-50s as the sprouting of grey hairs. However, some fascinating new research underscores how much eye disease results from the stress of particular environmental insults and that age only figures because the damage is cumulative.
Debra A. Schaumberg and colleagues at Brigham and Women's Hospital in East Boston were impressed by studies showing a link between accumulated lead exposure and several chronic disorders of ageing, including hypertension and cognitive decline. Several other bits of evidence here and there, such as the presence of lead in the lenses of people with cataract, suggested that accumulated lead exposure might also increase the risk of cataracts - another disease caused by oxidative stress.
To test their hypothesis, they examined levels of lead in the thighbones and knees of 795 men over 60 and compared the findings against data from eye examinations. Bone lead levels are thought to be the most accurate measure of cumulative long-term exposure (Environ Health Perspect, 1998; 106: 1-8). Lead levels in blood are only indicative of recent exposure, with no added risk for degenerative illnesses like cataract.
Researchers also controlled for a number of risk factors, including cigarette-smoking and diabetes, and for intake of vitamin C, carotenoids and vitamin E, all nutrients known to be eye-protective. The 122 men who went on to develop cataracts were also those with the highest levels of lead in their thigh bones. Indeed, what the researchers discovered was that those with the highest levels of lead had nearly triple the risk of developing cataracts.
These results suggest that accumulated exposure to lead may be a huge unrecognised factor in cataracts, still the leading cause of blindness in the world. The authors of the study estimate that nearly half of all cases of cataract may be from lead.
The problem is that lead literally settles in our bones - some 95 per cent of our body's lead levels is present there - where it carries out an endless tango with other tissues of our bodies. Lead stored in cortical bone, for instance, stays there for at least 10 years, after which it circulates in the blood at extremely low levels, which are difficult to measure, and then interacts with other tissues.
In the case of the eye, lead can disrupt the eye's redox status (oxidation reduction) and epithelial cells, both necessary for clear eyesight (Toxicology, 1998; 130: 167-74) as well as the metabolism of glutathione - again needed for a healthy lens.
This study parallels lab research, which has discovered that lead is often present in the cataractous lenses of living things. It also highlights just how much of a problem lead still poses in the world. Although the US and UK, along with other industrialised countries, have reduced lead levels through non-leaded petrol and eliminating workplace exposure, even conservative medical quarters accept that most adults have substantial body burdens of lead (J Am Geriatr Soc, 2000; 48: 1501-6).
The real culprit for low-level, chronic lead exposure is very likely our own homes. More than three-quarters of all houses built before 1980 contain lead-based paint or lead waterpipes. Every last one of us adults probably has lead in our bones.
Preventing a premature decline may be mostly about taking a good look around to see what in our lives is making us get old.