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Incidence and prevalence of uveitis in Northern California: discussion by
Ophthalmology Volume 111, Number 3, March 2004 5. McCannel CA, Holland GN, Helm CJ, et al. Causes of uveitis in the general practice of ophthalmology: UCLA CommunityBased Uveitis Study Group. Am J Ophthalmol 1996;121:35– 46. 6. Last JM, Spasoff RA, Harris SS, Thuriaux MC, eds. A Dictionary of Epidemiology. 4th ed. New York: Oxford University Press; 2001:35. 7. Bloch-Michel E, Nussenblatt RB. International Uveitis Study Group recommendations for the evaluation of intraocular inflammatory disease [letter]. Am J Ophthalmol 1987;103: 234 –5. 8. Saari KM, Pa¨ ivo¨ nsalo-Heitanen T, Vaahtoranta-Lehtonen H, et al. Epidemiology of endogenous uveitis in south-western Finland. Acta Ophthalmol Scand 1995;73:345–9. 9. Miettinen R. Incidence of uveitis in Northern Finland. Acta Ophthalmol (Copenh) 1997;55:252– 60.
10. Freedman J. Incidence of uveitis in Bantu-speaking negroes of South Africa. Br J Ophthalmol 1974;58:595–9. 11. Dandona L, Dandona R, John RK, et al. Population based assessment of uveitis in an urban population in southern India. Br J Ophthalmol 2000;84:706 –9. 12. Hunter I, Greene SA, MacDonald TM, Morris AD. Prevalence and aetiology of hypothyroidism in the young. Arch Dis Child 2000;83:207–10. 13. James OF, Bhopal R, Howel D, et al. Primary biliary cirrhosis once rare, now common in the United Kingdom? Hepatology 1999;30:390 – 4. 14. Doufas AG, Mastorakos G, Chatziioannou S, et al. The predominant form of non-toxic goiter in Greece is now autoimmune thyroiditis. Eur J Endocrinol 1999;140:505–11.
Discussion by William G. Hodge, FRCSC, PhD Incidence and prevalence studies are inexpensive, underused, but very valuable studies for clinicians. They help us differentiate what does happen (incidence studies) from what can happen (case reports). Furthermore, incidences and prevalences can be compared between 2 groups. For example, case reports of ocular opportunistic infections associated with human immunodeficiency virus (HIV) were documented widely (what can happen), but a study published in this journal1 that compared the incidences of ocular infection among HIV-positive and HIV-negative patients demonstrated some surprising results (what did happen): only 3 infections (cytomegalovirus retinitis, herpes zoster, and Toxoplasmosis retinitis) occurred at substantially higher rates than in nonAIDS patients. Gritz and Wong attempt to use a population-based study to calculate the incidence and prevalence of uveitis in Northern California. The study design was a retrospective cohort, which is the best design for the money when studying incidence and prevalence. Although prospective cohort designs of populations have the advantage of slightly higher accuracy of variable collection (less misclassification bias), their expense is often absurd. The retrospective cohort design, with a well thought out database, will usually provide the same results at a fraction of the price. The authors were appropriately conservative with their definition of cases, so as not to inflate their numerators in their calculations. The calculation of their denominator was less clearly outlined. They used a 1-year study period from 6 Kaiser centers from communities on both sides of San Francisco Bay. Their results can be summarized as follows: the incidence of uveitis was approximately 3 times higher than those of other population studies that they cited. Most surprisingly, the highest rate of uveitis in their age-stratified analysis was in elderly patients. There are several possible reasons for the results of incidence and prevalence studies. The rates cited may be true. Alternatively, From the Cornea and Uveitis Services, Ocular Epidemiology, University of Ottawa Eye Institute, Ottawa, Canada. Address correspondence to William G. Hodge, Associate Professor of Ophthalmology, Cornea and Uveitis Services, Ocular Epidemiology, University of Ottawa Eye Institute, 501 Smyth Road, Ottawa, Ontario K1H 8L6, Canada. E-mail: [email protected].
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the numerator (the cases) or denominator (the population) may be incorrect. The population distribution may be skewed toward one age group, making the overall calculation misleading. Better diagnostic methods may increase the case yield when there is no true increase in the rate. There may be mechanisms in place that increase reporting of certain diseases without a true increase in the rate. Finally, new disease definitions may artificially change rates. In this study, the overall increase in rates relative to other studies is likely true. However, the highest rate among the elderly is perplexing, as it defies published reports2,3 and the common experience of uveitis specialists. In my opinion, the denominator in this group has likely been artificially reduced. Are there elderly patients in the denominator who might be covered in the Medicare system but not in these centers? Would veterans who would typically be served in a Veterans Affairs hospital not be served in these centers? Perhaps other reasons are possible, but it is suspicious that the rate in the elderly is so high. Finally, in all population-based studies the generalizability should be discussed. It is likely that the results of this study can be applied to Northern California and, perhaps, the entire state as well. The application to other areas in the United States will be questionable, and almost certainly, the study cannot be applied to other countries. Nevertheless, incidence and prevalence studies are a valuable resource for clinicians and health policy experts. Our profession will inch ahead further if more articles like this are published in our ophthalmology journals at the expense of fewer case reports and fewer case series. References 1. Hodge WG, Seiff SR, Margolis TP. Ocular opportunistic infection incidences among patients who are HIV positive compared to patients who are HIV negative. Ophthalmology 1998;105: 895–900. 2. Brophy S, Calin A. Ankylosing spondylitis: interaction between genes, joints, age at onset, and disease expression. J Rheumatol 2001;28:2151– 4. 3. Prieto JF, Dioz E, Gutierrez JM, et al. Pars planitis: epidemiology, treatment, and association with multiple sclerosis. Ocul Immunol Inflamm 2001;9:93–102.
10. Freedman J. Incidence of uveitis in Bantu-speaking negroes of South Africa. Br J Ophthalmol 1974;58:595–9. 11. Dandona L, Dandona R, John RK, et al. Population based assessment of uveitis in an urban population in southern India. Br J Ophthalmol 2000;84:706 –9. 12. Hunter I, Greene SA, MacDonald TM, Morris AD. Prevalence and aetiology of hypothyroidism in the young. Arch Dis Child 2000;83:207–10. 13. James OF, Bhopal R, Howel D, et al. Primary biliary cirrhosis once rare, now common in the United Kingdom? Hepatology 1999;30:390 – 4. 14. Doufas AG, Mastorakos G, Chatziioannou S, et al. The predominant form of non-toxic goiter in Greece is now autoimmune thyroiditis. Eur J Endocrinol 1999;140:505–11.
Discussion by William G. Hodge, FRCSC, PhD Incidence and prevalence studies are inexpensive, underused, but very valuable studies for clinicians. They help us differentiate what does happen (incidence studies) from what can happen (case reports). Furthermore, incidences and prevalences can be compared between 2 groups. For example, case reports of ocular opportunistic infections associated with human immunodeficiency virus (HIV) were documented widely (what can happen), but a study published in this journal1 that compared the incidences of ocular infection among HIV-positive and HIV-negative patients demonstrated some surprising results (what did happen): only 3 infections (cytomegalovirus retinitis, herpes zoster, and Toxoplasmosis retinitis) occurred at substantially higher rates than in nonAIDS patients. Gritz and Wong attempt to use a population-based study to calculate the incidence and prevalence of uveitis in Northern California. The study design was a retrospective cohort, which is the best design for the money when studying incidence and prevalence. Although prospective cohort designs of populations have the advantage of slightly higher accuracy of variable collection (less misclassification bias), their expense is often absurd. The retrospective cohort design, with a well thought out database, will usually provide the same results at a fraction of the price. The authors were appropriately conservative with their definition of cases, so as not to inflate their numerators in their calculations. The calculation of their denominator was less clearly outlined. They used a 1-year study period from 6 Kaiser centers from communities on both sides of San Francisco Bay. Their results can be summarized as follows: the incidence of uveitis was approximately 3 times higher than those of other population studies that they cited. Most surprisingly, the highest rate of uveitis in their age-stratified analysis was in elderly patients. There are several possible reasons for the results of incidence and prevalence studies. The rates cited may be true. Alternatively, From the Cornea and Uveitis Services, Ocular Epidemiology, University of Ottawa Eye Institute, Ottawa, Canada. Address correspondence to William G. Hodge, Associate Professor of Ophthalmology, Cornea and Uveitis Services, Ocular Epidemiology, University of Ottawa Eye Institute, 501 Smyth Road, Ottawa, Ontario K1H 8L6, Canada. E-mail: [email protected].
500
the numerator (the cases) or denominator (the population) may be incorrect. The population distribution may be skewed toward one age group, making the overall calculation misleading. Better diagnostic methods may increase the case yield when there is no true increase in the rate. There may be mechanisms in place that increase reporting of certain diseases without a true increase in the rate. Finally, new disease definitions may artificially change rates. In this study, the overall increase in rates relative to other studies is likely true. However, the highest rate among the elderly is perplexing, as it defies published reports2,3 and the common experience of uveitis specialists. In my opinion, the denominator in this group has likely been artificially reduced. Are there elderly patients in the denominator who might be covered in the Medicare system but not in these centers? Would veterans who would typically be served in a Veterans Affairs hospital not be served in these centers? Perhaps other reasons are possible, but it is suspicious that the rate in the elderly is so high. Finally, in all population-based studies the generalizability should be discussed. It is likely that the results of this study can be applied to Northern California and, perhaps, the entire state as well. The application to other areas in the United States will be questionable, and almost certainly, the study cannot be applied to other countries. Nevertheless, incidence and prevalence studies are a valuable resource for clinicians and health policy experts. Our profession will inch ahead further if more articles like this are published in our ophthalmology journals at the expense of fewer case reports and fewer case series. References 1. Hodge WG, Seiff SR, Margolis TP. Ocular opportunistic infection incidences among patients who are HIV positive compared to patients who are HIV negative. Ophthalmology 1998;105: 895–900. 2. Brophy S, Calin A. Ankylosing spondylitis: interaction between genes, joints, age at onset, and disease expression. J Rheumatol 2001;28:2151– 4. 3. Prieto JF, Dioz E, Gutierrez JM, et al. Pars planitis: epidemiology, treatment, and association with multiple sclerosis. Ocul Immunol Inflamm 2001;9:93–102.