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A criteria-based review of the literature.The literature on the epidemiology of low back pain is accumulating, but for the most part studies are restricted to high-income countries, which comprise less than 15% of the world's population. Little is known about the epidemiology of low back pain in the rest of the world.To address the imbalance in the literature and to review the relatively few studies on the epidemiology of low back pain in low- and middle-income countries. Rates from these studies are contrasted with rates from selected high-income countries. In reviewing the literature, a hypothesis is tested: low back pain rates are higher in low-income countries than in high-income countries, not only because hard physical labor is more prevalent in low-income countries, but also because, unlike high-income countries, hard physical labor for older workers in low-income countries often is unavoidable.Among other sources, articles for the review come from a search of the MEDLINE bibliographic database, with "back pain" and individual low- and middle-income countries entered as key words. To avoid recall biases, findings specifically on point prevalence are reviewed.Within the categories of low-income and high-income countries, low back pain rates vary twofold or more. In comparisons between these categories of countries, rates on the whole are higher among the general populations of selected high-income countries than among rural low-income populations; specifically, rates are 2-4 times higher among Swedish, German, and Belgium general populations than among Nigerian, southern Chinese, Indonesian, and Filipino farmers. Within low-income countries, rates are higher among urban populations than among rural populations and still higher among workers in particular worksites, referred to as "enclosed workshops."The disparity in low back pain rates within categories of countries, high-income and low-income, calls attention to the high proportion of studies on the epidemiology of low back pain that are methodologically questionable. Recommendations are offered to improve the methodologic quality of this type of study. Conclusions may be drawn from comparisons between studies, although, in the absence of set methodologic standards, they are tentative. The considerably lower rates among populations of low-income farmers compared with rates of the affluent populations of selected northern European countries indicate that, contrary to the hypothesis proposed here, hard physical labor itself is not necessarily related to low back pain. The higher rates in urban low-income populations as compared with rates in rural low-income populations and the sharply higher rates among workers in enclosed workshops of low-income countries suggest a disturbing trend: low back pain prevalence may be on the rise among vast numbers of workers as urbanization and rapid industrialization proceed.