Studies that include larger numbers of cases have narrower confidence intervals, that is, provide a more precise estimate of association or risk. If the estimated prevalence or incidence rate in the reference group is much lower than for the general population, odds ratios or risk ratios may be inflated and should be interpreted cautiously. Finally, it is important to consider the findings from a single study in the context of the totality of evidence, either qualitatively or quantitatively in a formal meta-analysis. Smoking is known to depress antioxidants and to alter choroidal blood flow. Risk ratios of 2.0 or greater were found for 40 to 45 pack-years or more.
Hypertension is believed to damage the choroidal vasculature.
Fewer data are available for evaluating possible associations of AMD with serum lipid levels and with atherosclerotic carotid artery disease. Findings to date are inconsistent. Odds ratios and risk ratios estimated in most studies have been 2.0 or greater. More importantly, smoking is a factor that is a modifiable risk factor, that is, under the control of the patient. Biologically plausible hypotheses have been advanced for these associations but the mechanisms, particularly for UV radiation, may be complex.